Evolution and Ecology

Evolution Overview

What is Evolution?

Definition. Evolution is the cumulative change in the heritable characteristics of a population over successive generations, driven by mechanisms such as natural selection, genetic drift, gene flow, and mutation.

Evolution operates on populations, not individuals. An individual organism does not evolve during its lifetime; rather, the genetic composition of the population shifts across generations. The unit of evolution is the population; the unit of selection is the individual (or, in some cases, the gene).

Evidence for Evolution

Type of Evidence	Description	Example
Fossil record	Sequential appearance of organisms in rock layers; transitional forms show intermediate traits	Archaeopteryx (reptile-bird transition); Tiktaalik (fish-tetrapod transition)
Comparative anatomy	Structural similarities across species indicating common ancestry	Pentadactyl limb in humans, bats, whales, cats
Molecular biology	DNA and protein sequence similarities reflect evolutionary relationships	Cytochrome c is nearly identical across all eukaryotes
Biogeography	Geographic distribution of species matches continental drift patterns	Marsupials in Australia vs placentals elsewhere
Embryology	Embryos of related species resemble each other in early developmental stages	Pharyngeal pouches in fish, chick, and human embryos

Lamarckism vs Darwinism

Feature	Lamarckism (Inheritance of Acquired Characteristics)	Darwinism (Natural Selection)
Proposed by	Jean-Baptiste Lamarck (1809)	Charles Darwin (1859)
Mechanism	Use and disuse of organs; traits acquired during lifetime are inherited	Variation exists naturally; favourable traits are selected for
Heritability of acquired traits	Yes -- traits gained during life are passed on	No -- only genetically determined traits are inherited
Direction of change	Driven by organism's needs and efforts	Driven by environmental pressures selecting from existing variation
Evidence	Discredited; no mechanism for inheritance of acquired traits	Strongly supported by genetics, fossil record, molecular biology
Example	Giraffes stretch necks to reach leaves, longer necks inherited	Giraffes with naturally longer necks survive and reproduce more

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Lamarckism is not accepted by modern science. However, the field of epigenetics has revealed that environmental factors can influence gene expression across generations without changing the DNA sequence itself. This is sometimes mistakenly conflated with Lamarckism, but epigenetic changes are reversible and do not create new alleles.

Natural Selection

Darwin's Theory of Natural Selection

Darwin's theory rests on five key observations and inferences:

Variation: Individuals within a population exhibit heritable variation in traits.
Overproduction: Populations produce more offspring than the environment can support.
Competition: There is competition for limited resources (food, mates, space).
Differential survival and reproduction: Individuals with traits better suited to the environment have a higher probability of surviving and reproducing.
Accumulation: Favourable traits accumulate in the population over generations.

Definition. Natural selection is the process by which organisms with traits that enhance survival and reproduction in a given environment tend to leave more offspring than those without such traits, causing the favourable traits to increase in frequency over time.

Conditions for Natural Selection

For natural selection to operate, four conditions must be met:

Variation: There must be phenotypic differences among individuals in the population.
Heritability: The variation must have a genetic basis so it can be passed to offspring.
Differential fitness: Different phenotypes must confer different probabilities of survival and/or reproduction.
Selection pressure: An environmental factor must create a consistent fitness differential (e.g., predation, climate, resource availability).

Types of Natural Selection

Type	Description	Graph Shape	Example
Stabilising selection	Favouring intermediate phenotypes; extreme phenotypes are selected against	Narrowing, centred	Human birth weight (very low and very high have higher mortality)
Directional selection	Favouring one extreme phenotype; the mean shifts in one direction	Shift along axis	Peppered moth (dark form favoured during industrial pollution)
Disruptive selection	Favouring both extremes; intermediate phenotypes are selected against	Two peaks	African seedcracker finch (large and small beaks favoured, intermediate beaks disadvantageous)

Classic Examples

Peppered moth (Biston betularia):

Before the Industrial Revolution in England, the light-coloured form was common on lichen-covered trees. During industrialisation, soot darkened tree bark, making light moths more visible to predators. The dark (melanic) form increased in frequency. After clean-air legislation reduced pollution, the light form rebounded. This demonstrates directional selection driven by changing environmental conditions.

Antibiotic resistance:

When a bacterial population is exposed to an antibiotic, most bacteria die. However, any bacteria carrying a resistance allele (from a prior mutation) survive, reproduce, and pass on the resistance gene. The next generation is predominantly resistant. This is directional selection and a major public health concern.

Sickle cell anaemia and malaria:

The sickle cell allele (HbS) is recessive. Homozygous HbS/HbS individuals suffer sickle cell disease. Homozygous HbA/HbA individuals are normal. Heterozygous HbA/HbS individuals have sickle cell trait with mild or no symptoms and are resistant to malaria. In regions where malaria is endemic, the heterozygote has a selective advantage -- this is heterozygote advantage (balanced polymorphism). The allele is maintained in the population at higher frequency than would be expected if it were purely deleterious.

warning

A common misconception is that antibiotic resistance develops because bacteria "need" to survive. Resistance arises from random pre-existing mutations; the antibiotic simply selects for resistant individuals. The mutation occurs regardless of the antibiotic's presence.

Genetics of Evolution

Hardy-Weinberg Equilibrium

The Hardy-Weinberg principle describes a theoretical population in which allele frequencies remain constant from generation to generation in the absence of evolutionary forces.

$p^2 + 2pq + q^2 = 1$

$p + q = 1$

Where:

$p$ = frequency of the dominant allele
$q$ = frequency of the recessive allele
$p^2$ = frequency of homozygous dominant genotype
$2pq$ = frequency of heterozygous genotype
$q^2$ = frequency of homozygous recessive genotype

Conditions for Hardy-Weinberg Equilibrium

For allele frequencies to remain constant, all of the following must hold:

No mutations -- no new alleles are introduced
Random mating -- individuals mate without regard to genotype
No natural selection -- all genotypes have equal fitness
Extremely large population size -- genetic drift is negligible
No gene flow -- no migration into or out of the population

In reality, no natural population satisfies all five conditions. HWE serves as a null model: if observed genotype frequencies deviate from HWE predictions, one or more evolutionary forces are at work.

Allele Frequency Calculations

Worked calculation:

In a population, 16% of individuals exhibit a recessive disorder (homozygous recessive, $q^2 = 0.16$ ).

$q = \sqrt{0.16} = 0.4$

$p = 1 - 0.4 = 0.6$

$p^2 = (0.6)^2 = 0.36$

$2pq = 2(0.6)(0.4) = 0.48$

Genotype frequencies: 36% homozygous dominant, 48% heterozygous, 16% homozygous recessive.

Allele frequency from a sample:

Given a population of 500 individuals with genotypes: 200 AA, 200 Aa, 100 aa.

Total alleles = $500 \times 2 = 1000$

Number of A alleles = $(200 \times 2) + 200 = 600$

Number of a alleles = $(100 \times 2) + 200 = 400$

$p = \frac{600}{1000} = 0.6$

$q = \frac{400}{1000} = 0.4$

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In DSE exams, HWE calculations are almost always tested. Remember: you can always determine $q$ from $q^2$ (the homozygous recessive frequency), and then $p$ from $p = 1 - q$ . The heterozygote frequency $2pq$ is the one most students miscalculate -- do not assume it equals $p + q$ .

Genetic Drift

Definition. Genetic drift is the random fluctuation of allele frequencies in a population due to chance events, having a more pronounced effect in small populations.

Founder effect:

A small group of individuals breaks off from a larger population to establish a new colony. The new population may have allele frequencies very different from the original. The Amish population in Pennsylvania has a high incidence of Ellis-van Creveld syndrome (a form of dwarfism) because several of the original founders carried the recessive allele.

Bottleneck effect:

A population is drastically reduced in size by an event (natural disaster, hunting, habitat destruction). The surviving individuals may not represent the genetic diversity of the original population. When the population recovers, genetic diversity is reduced. Cheetahs exhibit extremely low genetic diversity due to a historical bottleneck.

Gene Flow

Definition. Gene flow (migration) is the movement of alleles between populations through the movement of individuals or gametes (e.g., pollen transfer). Gene flow tends to reduce genetic differences between populations and can counteract the effects of natural selection and genetic drift.

Mutations as Raw Material

Mutations are the ultimate source of new genetic variation. Without mutations, there would be no new alleles for natural selection to act upon.

Point mutations: A single nucleotide change (substitution, insertion, deletion)
Chromosomal mutations: Deletions, duplications, inversions, translocations
Mutation rate: Typically $10^{-5}$ to $10^{-6}$ per gene per generation
Most mutations are neutral or harmful; a small fraction are beneficial and may be selected for

Speciation

Definition. Speciation is the formation of new species through the evolution of reproductive isolation between populations.

Allopatric speciation (geographic isolation):

Populations are physically separated by a geographic barrier (mountain range, river, ocean). Each population evolves independently through natural selection, genetic drift, and mutation. Over time, reproductive isolation mechanisms accumulate. This is the most common mode of speciation.

Sympatric speciation (same geographic area):

New species arise without geographic separation. Common mechanisms include:

Polyploidy: Common in plants. An error in cell division results in an organism with extra sets of chromosomes (e.g., tetraploid, 4n). The polyploid individual can only reproduce with other polyploids, creating instant reproductive isolation.
Ecological speciation: Different populations exploit different ecological niches within the same area (e.g., different host plants for insects).
Sexual selection: Different mating preferences or displays lead to reproductive isolation.

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Polyploidy is extremely rare in animals but is a major driver of speciation in plants. Approximately 30-70% of flowering plant species are polyploid. In the DSE, questions about speciation typically focus on allopatric speciation, but you should be able to explain polyploidy as a mechanism of sympatric speciation in plants.

Evidence for Evolution (Detailed)

Homologous vs Analogous Structures

Feature	Homologous Structures	Analogous Structures
Definition	Structures derived from a common ancestor	Structures with similar function but different evolutionary origins
Origin	Same embryonic origin	Different embryonic origin
Anatomical similarity	Similar underlying anatomy	May look very different internally
Example	Forelimb of human, bat wing, whale flipper (pentadactyl limb)	Wing of a bird vs wing of an insect
Evolutionary significance	Indicates divergent evolution	Indicates convergent evolution

Divergent evolution: Related species evolve different traits from a common ancestor (homologous structures).

Convergent evolution: Unrelated species evolve similar traits independently due to similar environmental pressures (analogous structures). Examples: streamlined body shape in sharks (fish) and dolphins (mammals); camera-type eye in cephalopods and vertebrates.

Vestigial Organs

Definition. Vestigial organs are remnants of structures that were functional in ancestral organisms but have lost their original function in the course of evolution.

Examples:

Human appendix: remnant of a large caecum used for digesting cellulose in herbivorous ancestors
Pelvic bones in whales and snakes: remnants of walking ancestors
Wings of flightless birds (ostrich, kiwi): no longer used for flight
Human tailbone (coccyx): remnant of a tail

Molecular Clocks

The molecular clock hypothesis states that mutations accumulate in DNA at a roughly constant rate over time. By comparing the number of sequence differences between two species in a neutral gene (one not subject to natural selection), researchers can estimate the time since the two species diverged from a common ancestor.

DNA/protein sequence comparison:

More closely related species share a higher percentage of DNA sequence
Humans and chimpanzees share approximately 98-99% of their DNA
Cytochrome c amino acid sequence differences correlate with evolutionary distance
The universal genetic code itself is evidence of common ancestry

Transitional Fossils

Transitional fossils show intermediate characteristics between ancestral and descendant groups:

Tiktaalik (375 million years ago): Fish-like with limb-like fins; intermediate between fish and tetrapods
Archaeopteryx (150 million years ago): Feathered dinosaur with teeth and a long bony tail; intermediate between reptiles and birds
Ambulocetus (50 million years ago): Amphibious whale ancestor with functional legs; intermediate between land mammals and modern whales
Australopithecus afarensis (3.9-2.9 million years ago): Bipedal ape-like hominin; intermediate between apes and humans

Comparative Embryology

Early embryos of vertebrates show remarkable similarity:

Pharyngeal pouches (gill slits in fish, become parts of the ear and throat in mammals)
Post-anal tail (present in all vertebrate embryos; reduced in adult humans)
Notochord (develops into vertebral column in most vertebrates)
Similar patterns of limb bud development

These similarities reflect shared developmental pathways inherited from a common ancestor.

Human Evolution

Primate Characteristics

Primates share several derived characteristics:

Opposable thumbs (or opposable big toe in some species) for grasping
Forward-facing eyes providing stereoscopic (3D) vision
Large brain relative to body size
Flat nails instead of claws on most digits
Reduced snout and enhanced sense of vision over olfaction
Long gestation period and usually single offspring
Extended period of parental care

Hominid Timeline

Definition. Hominids (hominins) are the group consisting of modern humans and all species more closely related to humans than to chimpanzees.

Species	Time Period	Key Characteristics
Sahelanthropus tchadensis	~7 million years ago	Possible earliest hominin; small brain; possibly bipedal
Ardipithecus ramidus	~4.4 million years ago	Bipedal on ground but still adapted for climbing
Australopithecus afarensis	3.9-2.9 million years ago	Bipedal walking; ape-like face and brain (~400-500 cm cubed); "Lucy" is a famous specimen
Australopithecus africanus	3-2 million years ago	Similar to A. afarensis; slightly larger brain
Homo habilis	2.4-1.4 million years ago	First species in genus Homo; larger brain (~600 cm cubed); simple stone tools (Oldowan)
Homo erectus	1.9 million - 110,000 years ago	Much larger brain (~900 cm cubed); more sophisticated tools (Acheulean); first to migrate out of Africa; use of fire
Homo neanderthalensis	~400,000 - 40,000 years ago	Large brain (~1,500 cm cubed); robust build; lived in Europe and western Asia; buried dead
Homo sapiens	~300,000 years ago - present	Large brain (~1,350 cm cubed); gracile build; complex language; symbolic thought; art and culture

Key Trends in Human Evolution

Increase in brain size: From ~400 cm cubed in Australopithecus to ~1,350 cm cubed in H. sapiens
Bipedalism: Adaptation of the pelvis, spine, and legs for upright walking (developed before large brain size)
Reduction in jaw size and tooth size: Related to changes in diet and tool use
Increase in manual dexterity: More precise thumb movements for tool making
Development of language and culture: Associated with brain reorganisation (not just size increase)

Out of Africa Theory

The "Recent African Origin" (Out of Africa) model proposes that modern humans (Homo sapiens) evolved in Africa approximately 300,000 years ago and subsequently migrated to other parts of the world, replacing existing hominin populations (such as Neanderthals in Europe).

Key evidence:

Greatest genetic diversity is found in African populations (consistent with Africa being the origin)
Fossil record shows the earliest H. sapiens fossils in Africa (Jebel Irhoud, Morocco, ~300,000 years ago)
Genetic analyses show that all non-African populations descend from a migration wave that left Africa approximately 60,000-70,000 years ago
Neanderthal DNA makes up approximately 1-2% of the genome of non-African modern humans (indicating limited interbreeding)

Mitochondrial Eve

Mitochondrial DNA (mtDNA) is inherited exclusively through the maternal line (no recombination). By comparing mtDNA sequences from diverse human populations, researchers estimated that the most recent common maternal ancestor of all living humans lived approximately 150,000-200,000 years ago in Africa. This individual is called "Mitochondrial Eve" -- not the only woman alive at the time, but the only one whose matrilineal line has survived unbroken to the present day.

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Mitochondrial Eve is a statistical concept, not a single individual who was the only human female alive. Many other women lived at the same time, but their matrilineal lines happened to die out at some point. The Y-chromosomal Adam (the most recent common paternal ancestor) lived approximately 200,000-300,000 years ago, and the two individuals were not contemporaries.

Ecology Basics

Levels of Organisation

Ecology studies interactions at multiple hierarchical levels:

Level	Definition	Example
Organism	A single individual	One oak tree
Population	All individuals of the same species in a given area	All oak trees in a forest
Community	All populations of different species living and interacting in an area	Oak trees, squirrels, fungi, bacteria in a forest
Ecosystem	Community plus the abiotic (non-living) environment	Forest plus soil, water, sunlight, climate
Biome	A large region with characteristic climate and communities	Tropical rainforest, tundra
Biosphere	All ecosystems on Earth; the sum of all living things and their environments	Earth

Biotic vs Abiotic Factors

Biotic Factors (living)	Abiotic Factors (non-living)
Predation	Temperature
Competition	Light intensity
Symbiosis	Water availability
Disease	Soil composition (pH, minerals)
Food availability	Oxygen concentration
Decomposers	Wind speed
Pollinators	Salinity

Niche Concept

Definition. A niche is the role and position a species has in its environment, including all interactions with biotic and abiotic factors. It encompasses how a species meets its needs for food and shelter, how it survives, and how it reproduces.

Fundamental niche: The full range of environmental conditions and resources a species can theoretically use in the absence of competitors or other limiting factors.
Realised niche: The actual range of conditions and resources a species uses in the presence of competitors and other species. The realised niche is always a subset of (or equal to) the fundamental niche.

Competitive Exclusion Principle

Definition. The competitive exclusion principle states that two species cannot coexist indefinitely in the same habitat if they occupy identical niches. One species will eventually outcompete the other, leading to the exclusion of the inferior competitor.

When two similar species coexist, they typically exhibit resource partitioning -- they divide the available resources to reduce direct competition (e.g., different feeding heights, different activity times, different food sizes).

Population Ecology

Population Growth Models

Exponential growth (J-shaped curve):

When resources are unlimited, a population grows exponentially:

$\frac{dN}{dt} = rN$

Where $N$ = population size, $r$ = intrinsic growth rate, $t$ = time.

Characteristics: rapid, accelerating growth; population doubles at regular intervals; not sustainable long-term.

Logistic growth (S-shaped curve):

As resources become limiting, growth rate slows and the population stabilises at the carrying capacity:

$\frac{dN}{dt} = rN\left(\frac{K - N}{K}\right)$

Where $K$ = carrying capacity.

Characteristics: initial exponential growth, deceleration as $N$ approaches $K$ , stabilisation at or near $K$ .

Carrying Capacity

Definition. Carrying capacity ( $K$ ) is the maximum population size that an environment can sustain indefinitely, given the available resources (food, water, shelter, etc.), without degrading the environment.

Carrying capacity is not fixed -- it can change due to:

Seasonal changes in resource availability
Environmental changes (drought, fire, climate change)
Technological advances (agriculture, medicine)
Species interactions (new predators, competitors, diseases)

Density-Dependent vs Density-Independent Factors

Feature	Density-Dependent Factors	Density-Independent Factors
Effect	Intensity varies with population density	Intensity is independent of population density
Role	Regulate population size, maintain it near $K$	Cause population fluctuations, often catastrophically
Examples	Competition for resources, predation, disease, parasitism, territoriality	Natural disasters (floods, earthquakes, fires), extreme weather, pollution
Graph effect	Growth rate decreases as density increases	Growth rate can drop suddenly regardless of density

r-Selection vs K-Selection

Feature	r-Selected Species	K-Selected Species
Environment	Unstable, unpredictable	Stable, predictable
Population size	Fluctuates wildly	Relatively stable, near $K$
Reproduction	Many offspring, small body size	Few offspring, large body size
Parental care	Little or none	Extensive
Maturation time	Short	Long
Lifespan	Short	Long
Competitive ability	Low	High
Examples	Bacteria, insects, weeds, annual plants	Elephants, whales, primates, oak trees

Survivorship Curves

Three idealised types:

Type I: High survival rate during early and middle life; rapid decline in late life (e.g., humans, large mammals, many perennial plants). Typically associated with K-selection.
Type II: Constant mortality rate throughout life (e.g., many birds, some reptiles). A straight diagonal line on a log-survivorship plot.
Type III: Very high early mortality rate; individuals that survive the early period have a good chance of long life (e.g., many fish, marine invertebrates, annual plants, most insects). Typically associated with r-selection.

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Real survivorship curves rarely match idealised types exactly. Many species show intermediate patterns. When interpreting DSE exam questions, look for the general shape rather than trying to force a perfect classification.

Community Ecology

Interspecific Interactions

Interaction	Effect on Species A	Effect on Species B	Example
Predation (+/-)	Beneficial	Harmful	Lion hunting zebra
Competition (-/-)	Harmful	Harmful	Two plant species competing for light
Mutualism (+/+)	Beneficial	Beneficial	Bees pollinating flowers; mycorrhizae on plant roots
Commensalism (+/0)	Beneficial	Neutral	Barnacles on a whale
Parasitism (+/-)	Beneficial	Harmful	Tapeworm in a human intestine; ticks on a dog

Predation

Predation is a major selective force that drives adaptations in both predators and prey through coevolution:

Predator adaptations: Speed, camouflage (ambush), claws, teeth, venom, pack hunting, enhanced senses
Prey adaptations: Camouflage (crypsis), warning colouration (aposematism), mimicry (Batesian and Mullerian), physical defences (spines, shells, toxins), behavioural defences (herding, alarm calls, playing dead), speed

Mimicry types:

Batesian mimicry: A harmless species mimics the appearance of a harmful one (e.g., hoverfly mimicking a wasp)
Mullerian mimicry: Two or more harmful species evolve to resemble each other (e.g., several species of brightly coloured poisonous frogs)

Competition

Intraspecific competition: Between individuals of the same species (e.g., male deer fighting for mates; plants of the same species competing for light)
Interspecific competition: Between individuals of different species (e.g., lions and hyenas competing for prey)

The outcome of interspecific competition depends on resource overlap and competitive ability. Over time, natural selection favours traits that reduce competition (character displacement).

Ecological Succession

Definition. Ecological succession is the sequential, directional process of change in the species composition and community structure of an ecosystem over time.

Primary succession:

Occurs on bare, lifeless substrate where no soil exists (e.g., newly formed volcanic rock, sand dunes, land exposed by retreating glacier)
Pioneer species: lichens, mosses, algae -- can colonise bare rock, begin soil formation
Intermediate species: grasses, herbs, shrubs -- improve soil, create microhabitats
Climax community: relatively stable, self-sustaining community (e.g., forest); species composition remains relatively constant
Process is slow: may take hundreds to thousands of years

Secondary succession:

Occurs in an area where an existing community has been disturbed but soil remains intact (e.g., after a forest fire, abandoned farmland, deforested area)
Faster than primary succession because soil and some organisms already exist
Pioneer species: grasses and fast-growing herbs
Eventually returns to a community resembling the original (but not necessarily identical)

Keystone Species

Definition. A keystone species is a species whose impact on its community or ecosystem is disproportionately large relative to its abundance. Removal of a keystone species causes significant changes in community structure and can lead to loss of biodiversity.

Examples:

Sea otters: prey on sea urchins; without otters, sea urchins overgraze kelp forests, destroying the habitat
Beavers: create wetland habitats that support many other species through dam building
Starfish (Pisaster): prey on mussels; without starfish, mussels outcompete other intertidal species

Trophic Cascades

Definition. A trophic cascade is an indirect ecological effect where changes in the abundance of predators at the top of a food chain cause ripple effects through lower trophic levels, altering the entire ecosystem structure.

Classic example: In Yellowstone National Park, the removal of wolves led to overpopulation of elk, which overgrazed willow and aspen, degrading riparian habitats and reducing beaver populations. The reintroduction of wolves in 1995 reduced elk numbers, allowed vegetation to recover, and restored beaver populations and the wetland habitats they create.

Ecosystem Ecology

Energy Flow

Energy enters ecosystems through photosynthesis and flows through trophic levels. Key principles:

Energy flow is unidirectional (unlike nutrient cycling)
Energy is lost at each trophic level (primarily as heat through respiration)
The Sun is the ultimate energy source for nearly all ecosystems

Trophic Levels

Trophic Level	Description	Example
Producer (autotroph)	Organisms that produce organic compounds from inorganic sources via photosynthesis or chemosynthesis	Plants, algae, cyanobacteria
Primary consumer (herbivore)	Eats producers	Rabbit, grasshopper, zooplankton
Secondary consumer (carnivore)	Eats primary consumers	Fox, frog, small fish
Tertiary consumer	Eats secondary consumers	Snake, large fish
Quaternary consumer (apex predator)	Top of the food chain; no natural predators	Eagle, shark, lion
Decomposer (detritivore)	Breaks down dead organic matter, recycling nutrients	Bacteria, fungi, earthworms

Ecological Pyramids

Type	What it shows	Typical pattern	Inverted cases
Pyramid of numbers	Number of organisms at each trophic level	Decreases with each level (broad base)	Can be inverted: one tree supports many herbivorous insects
Pyramid of biomass	Total dry mass of organisms at each trophic level	Decreases with each level	Can be inverted in aquatic ecosystems: phytoplankton biomass < zooplankton biomass (rapid turnover of phytoplankton)
Pyramid of energy	Total energy content at each trophic level	Always decreases with each level (never inverted)	Never inverted; reflects the laws of thermodynamics

The 10% Rule

Approximately 10% of the energy at one trophic level is transferred to the next. The remaining 90% is lost through:

Respiration (converted to heat)
Excretion (waste products)
Undigested material (faeces)
Maintenance and movement

This rule explains why food chains are typically limited to 4-5 trophic levels -- there is insufficient energy to sustain higher levels.

$\mathrm{Energy available at level } n = 0.1^n \times \mathrm{Energy at producer level}$

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info depending on the ecosystem and the organisms involved. Cold-blooded organisms are more energy-efficient than warm-blooded ones. In DSE calculations, use 10% unless the question specifies otherwise.

Nutrient Cycles

Nutrients cycle between biotic and abiotic components of ecosystems. Unlike energy, nutrients are recycled.

Carbon cycle:

Key processes:

Photosynthesis: $\mathrm{CO}_2$ converted to organic compounds by producers
Respiration: organic compounds broken down, releasing $\mathrm{CO}_2$
Combustion: burning fossil fuels releases stored carbon as $\mathrm{CO}_2$
Decomposition: decomposers break down dead organic matter, releasing $\mathrm{CO}_2$
Ocean absorption: oceans absorb $\mathrm{CO}_2$ from the atmosphere (dissolved $\mathrm{CO}_2$ , bicarbonate, carbonate)
Sedimentation: dead marine organisms form limestone and fossil fuels over geological time
Deforestation: reduces $\mathrm{CO}_2$ uptake by photosynthesis; burning releases stored carbon

Nitrogen cycle:

Key processes:

Nitrogen fixation: atmospheric $\mathrm{N}_2$ (unreactive) converted to ammonia ( $\mathrm{NH}_3$ ) by nitrogen-fixing bacteria (e.g., Rhizobium in root nodules of legumes) or lightning
Nitrification: ammonia converted to nitrite ( $\mathrm{NO}_2^-$ ) by Nitrosomonas, then to nitrate ( $\mathrm{NO}_3^-$ ) by Nitrobacter
Absorption: plants absorb nitrate through roots
Assimilation: plants incorporate nitrogen into amino acids and proteins
Feeding: animals obtain nitrogen by eating plants or other animals
Decomposition / ammonification: decomposers break down dead organisms and waste, releasing ammonia
Denitrification: denitrifying bacteria convert nitrate back to $\mathrm{N}_2$ , returning it to the atmosphere (e.g., Pseudomonas)

Phosphorus cycle:

Phosphorus has no significant gaseous phase (unlike carbon and nitrogen)
Weathering of rocks releases phosphate ions ( $\mathrm{PO}_4^{3-}$ ) into soil and water
Plants absorb phosphate; animals obtain it through food chains
Decomposition returns phosphorus to soil
Sedimentation: phosphorus settles to the ocean floor and forms sedimentary rock over geological time
Geological uplift returns phosphorus-containing rock to the surface (very slow process)

Water cycle:

Key processes:

Evaporation: water changes from liquid to vapour from oceans, lakes, rivers
Transpiration: water vapour released from plant leaves through stomata
Condensation: water vapour cools and forms clouds
Precipitation: water returns to Earth's surface as rain, snow, hail
Runoff: water flows over land into rivers, lakes, and oceans
Infiltration / percolation: water soaks into the ground, becoming groundwater

Primary Productivity

Gross Primary Productivity (GPP): Total amount of organic matter produced by photosynthesis per unit area per unit time
Net Primary Productivity (NPP): GPP minus the energy used by producers for respiration (R):

$\mathrm{NPP} = \mathrm{GPP} - R$

NPP represents the energy available to consumers (herbivores and decomposers). NPP is a measure of how much biomass is available for the rest of the food chain.

$\mathrm{NPP} = \mathrm{energy stored in plant biomass}$

Biomes

Major Terrestrial Biomes

Biome	Climate	Precipitation	Temperature	Key Vegetation	Key Adaptations
Tropical rainforest	Hot and wet year-round	2000-10000 mm/year	25-28 degrees C year-round	Broadleaf evergreen trees, epiphytes, dense understorey	Buttress roots, drip tips on leaves, thin bark, rapid nutrient cycling
Temperate deciduous forest	Four distinct seasons	750-1500 mm/year	-30 to 30 degrees C	Deciduous trees (oak, beech, maple) that shed leaves in winter	Leaf fall to reduce water loss; thick bark; dormant period
Grassland / savanna	Seasonal rainfall	250-750 mm/year	Warm summers, cold winters (temperate) or warm year-round (tropical)	Grasses, scattered trees (savanna), herbs	Deep root systems, grazing tolerance, fire adaptations
Desert	Very dry	<250 mm/year	Extreme diurnal range; hot deserts >40 degrees C day, cool nights	Sparse vegetation: cacti, succulents, deep-rooted shrubs	Water storage (succulence), reduced leaf surface area, thick cuticle, CAM photosynthesis, nocturnal behaviour
Taiga (boreal forest)	Long, cold winters; short, warm summers	400-1000 mm/year	-40 to 20 degrees C	Coniferous trees (pine, spruce, fir), lichens, mosses	Needle-like leaves (reduce water loss, shed snow), cone shape (shed snow), thick bark, evergreen
Tundra	Extremely cold; short growing season	150-250 mm/year	-40 to 10 degrees C; permafrost	Lichens, mosses, dwarf shrubs, grasses	Low-growing form (avoid wind), shallow roots (permafrost), dense hairs, dark pigmentation (absorb heat)

Aquatic Biomes

Marine biomes:

Open ocean (pelagic zone): Low nutrient levels, low productivity; phytoplankton are primary producers
Coral reefs: High biodiversity; found in warm, shallow, clear tropical waters; built by coral polyps (cnidarians with symbiotic algae)
Estuaries: Where rivers meet the sea; brackish water (mix of fresh and salt); high productivity; nursery for many species
Intertidal zone: Area between high and low tide; organisms must tolerate wave action, desiccation, and temperature changes

Freshwater biomes:

Lakes and ponds: Standing water; thermal stratification (epilimnion, thermocline, hypolimnion) in temperate lakes
Rivers and streams: Flowing water; organisms adapted to currents (streamlined bodies, suckers); water is well-oxygenated
Wetlands: Land saturated with water (marshes, swamps, bogs); high productivity; important for water filtration and flood control

info

info precipitation plotted by month). Key features to identify: annual temperature range, wet and dry seasons, total precipitation. Match these to the biome descriptions above.

Human Impact

Deforestation

Clearing of forests for agriculture, logging, urbanisation, and infrastructure
Tropical rainforests are being cleared at approximately 10 million hectares per year
Consequences: habitat loss, biodiversity decline, soil erosion, increased $\mathrm{CO}_2$ in atmosphere (reduced photosynthesis + burning), disruption of water cycle, loss of indigenous cultures

Habitat Fragmentation

Definition. Habitat fragmentation is the process by which large, continuous habitats are broken into smaller, isolated patches.

Consequences:

Reduced total habitat area
Isolated populations with reduced gene flow (increased inbreeding, reduced genetic diversity)
Edge effects: conditions at habitat edges differ from interior (more light, wind, temperature extremes, predators)
Populations in small fragments are more vulnerable to extinction (island biogeography theory)

Pollution

Eutrophication:

Excessive nutrient input (nitrogen and phosphorus from agricultural fertilisers, sewage) into water bodies:

Nutrient enrichment causes algal bloom (rapid growth of algae)
Algal layer blocks light, killing submerged aquatic plants
Algae die and decomposers break them down, consuming oxygen through respiration
Dissolved oxygen levels drop (hypoxia or anoxia)
Fish and other aquatic organisms die

Bioaccumulation vs Biomagnification:

Feature	Bioaccumulation	Biomagnification
Definition	Accumulation of a substance within a single organism over its lifetime	Increasing concentration of a substance at each successive trophic level
Scope	Within one organism	Across the food chain
Example	Heavy metals accumulating in fish liver	DDT concentration increasing from plankton to zooplankton to small fish to large fish to birds of prey

Substances that biomagnify are typically:

Fat-soluble (not easily excreted)
Persistent in the environment (slow to degrade)
Examples: DDT, PCBs, mercury, lead

Climate Change

Burning fossil fuels releases $\mathrm{CO}_2$ , methane ( $\mathrm{CH}_4$ ), and nitrous oxide ( $\mathrm{N}_2\mathrm{O}$ ) -- greenhouse gases
Greenhouse gases trap infrared radiation (heat) in the atmosphere, increasing global temperatures
Pre-industrial $\mathrm{CO}_2$ concentration: ~280 ppm; current: >420 ppm
Global average temperature has risen approximately 1.1 degrees C above pre-industrial levels

Consequences:

Rising sea levels (thermal expansion of water + melting ice caps)
More frequent and severe extreme weather events (hurricanes, droughts, floods)
Shifts in species distributions and ranges
Ocean acidification (dissolved $\mathrm{CO}_2$ forms carbonic acid, lowering pH)
Disruption of phenology (timing of biological events: flowering, migration, breeding)
Coral bleaching (warm water causes corals to expel symbiotic algae)

Biodiversity Loss

Current extinction rate is estimated to be 100-1000 times higher than the natural background rate, leading many scientists to call the current era the "sixth mass extinction."

Major causes (often remembered by the acronym HIPPO):

Habitat destruction and fragmentation
Invasive species (outcompete, prey on, or introduce diseases to native species)
Population growth (human population increase driving resource consumption)
Pollution
Overexploitation (overfishing, hunting, poaching)

Conservation Strategies

In situ conservation: Protecting species in their natural habitats.

National parks and nature reserves
Marine protected areas
Wildlife corridors connecting fragmented habitats
Legal protection of endangered species and their habitats

Ex situ conservation: Protecting species outside their natural habitats.

Botanical gardens and seed banks (e.g., Millennium Seed Bank)
Zoos and captive breeding programmes
Tissue culture and cryopreservation of genetic material

International agreements:

CITES (Convention on International Trade in Endangered Species): Regulates international trade in wild animals and plants to prevent overexploitation
Convention on Biological Diversity (CBD): International treaty with three goals: conservation, sustainable use, fair sharing of benefits from genetic resources
Ramsar Convention: Protection of wetlands of international importance

warning

warning bioaccumulation is within an individual; biomagnification is across trophic levels. Both terms can appear in the same question.

DSE Exam Focus

Common Question Types

Explain the process of natural selection using a specific example. Structure your answer: identify variation, state the selection pressure, describe differential survival/reproduction, explain the change in allele frequency over generations.
Calculate allele and genotype frequencies using Hardy-Weinberg. Always start by identifying the homozygous recessive frequency ( $q^2$ ), then calculate $q$ , then $p$ , then all genotype frequencies.
Interpret ecological data from graphs and tables. Look for trends, correlations, and causal relationships. Identify independent and dependent variables.
Compare and contrast related concepts. Use structured tables or point-by-point comparisons. Always include both similarities and differences.
Explain the impact of human activities on ecosystems. Be specific about mechanisms (e.g., how eutrophication actually kills fish, not just that it does).

Calculation Techniques

Energy transfer calculations:

If producers have 10,000 kJ of energy:

Primary consumers: $10,000 \times 0.10 = 1,000$ kJ
Secondary consumers: $1,000 \times 0.10 = 100$ kJ
Tertiary consumers: $100 \times 0.10 = 10$ kJ

HWE calculations (step by step):

Identify $q^2$ from the homozygous recessive frequency
Take the square root to get $q$
Calculate $p = 1 - q$
Calculate $p^2$ , $2pq$ , and verify $p^2 + 2pq + q^2 = 1$

Population growth:

Doubling time can be estimated using the rule of 70:

$\mathrm{Doubling time} \approx \frac{70}{r \times 100}$

Where $r$ is the per capita growth rate expressed as a decimal.

Data Interpretation

When presented with data in DSE exams:

Read axis labels and units carefully
Identify the type of relationship (direct, inverse, no correlation)
Look for anomalies and consider biological explanations
Calculate percentages and ratios when asked
Distinguish between correlation and causation
Consider confounding variables

Experimental Design

Key elements the DSE expects in ecology and evolution experimental design questions:

Hypothesis: A clear, testable statement (e.g., "Darker-coloured moths will have higher survival rates on polluted bark than light-coloured moths")
Variables: Identify independent, dependent, and controlled variables
Sample size: Large enough to be statistically meaningful; mention replication
Control: A baseline for comparison (e.g., unpolluted area)
Method: Clear, step-by-step procedure
Data collection: Quantitative data where possible
Data analysis: Statistical tests (chi-squared test for allele frequencies, t-test for comparing means)
Evaluation: Limitations, sources of error, improvements

info

The chi-squared test is commonly used in DSE to determine whether observed genotype frequencies differ significantly from Hardy-Weinberg expected frequencies:

$\chi^2 = \sum \frac{(O - E)^2}{E}$

Where $O$ = observed frequency and $E$ = expected frequency. Compare the calculated value to the critical value at the appropriate degrees of freedom ( $df = \mathrm{number of categories} - 1$ ) and significance level (typically $p = 0.05$ ).

Worked Examples

Worked Example 1: Hardy-Weinberg and Sickle Cell

In a population of 10,000 people in a malaria-endemic region, 160 people have sickle cell disease (HbS/HbS).

(a) Calculate the frequency of the HbS allele.

(b) Calculate the number of heterozygous carriers (HbA/HbS) in the population.

(c) Explain why the HbS allele frequency is higher in this region than in non-malarial regions.

If you get this wrong, revise: Genetics of Evolution -- Hardy-Weinberg Equilibrium

Solution

Sickle cell disease is homozygous recessive, so:

$q^2 = \frac{160}{10000} = 0.016$

$q = \sqrt{0.016} = 0.1265$

Frequency of HbS = 0.1265 (or 12.65%)

$p = 1 - 0.1265 = 0.8735$

$2pq = 2(0.8735)(0.1265) = 0.2210$

Number of heterozygotes = $0.2210 \times 10000 = 2210$ people

In malaria-endemic regions, heterozygous individuals (HbA/HbS) have resistance to malaria, giving them a selective advantage over homozygous normal individuals (susceptible to malaria) and homozygous recessive individuals (sickle cell disease). This is heterozygote advantage (balanced polymorphism), maintaining the HbS allele at higher frequency.

Worked Example 2: Ecological Pyramid Calculations

A grassland ecosystem has the following energy values at each trophic level:

Producers: 50,000 kJ/m $^2$ /year
Primary consumers: 5,000 kJ/m $^2$ /year
Secondary consumers: 400 kJ/m $^2$ /year

(a) Calculate the percentage energy transfer from producers to primary consumers.

(b) Calculate the percentage energy transfer from primary consumers to secondary consumers.

(c) Explain why a fifth trophic level is unlikely in this ecosystem.

If you get this wrong, revise: Ecosystem Ecology -- The 10% Rule; Ecological Pyramids

Solution

(a) $\mathrm{Transfer efficiency} = \frac{5000}{50000} \times 100\% = 10\%$

(b) $\mathrm{Transfer efficiency} = \frac{400}{5000} \times 100\% = 8\%$

(c) At the secondary consumer level, only 400 kJ/m $^2$ /year is available. A tertiary consumer would receive approximately 10% of this: 40 kJ/m $^2$ /year. A quaternary consumer (fifth trophic level) would receive only about 4 kJ/m $^2$ /year, which is insufficient to sustain a viable population. This demonstrates the energy limitation on food chain length.

Worked Example 3: Population Growth

A bacterial population starts with 100 cells and has a growth rate of $r = 0.5$ per hour under ideal conditions.

(a) Calculate the population size after 6 hours (exponential growth).

(b) If the carrying capacity is 5,000 cells, calculate the population size after 6 hours using the logistic growth model.

If you get this wrong, revise: Population Ecology -- Population Growth Models; Carrying Capacity

Solution

(a) $N = N_0 \times e^{rt} = 100 \times e^{0.5 \times 6} = 100 \times e^3 = 100 \times 20.09 = 2009$ cells

(b) Using the logistic equation:

$N = \frac{K}{1 + \left(\frac{K - N_0}{N_0}\right) e^{-rt}} = \frac{5000}{1 + \left(\frac{5000 - 100}{100}\right) e^{-0.5 \times 6}}$

$N = \frac{5000}{1 + 49 \times e^{-3}} = \frac{5000}{1 + 49 \times 0.0498} = \frac{5000}{1 + 2.44} = \frac{5000}{3.44} = 1453 \mathrm{ cells}$

The logistic model predicts fewer cells (1,453 vs 2,009) because it accounts for the limiting effect of carrying capacity -- as the population grows, resources become scarcer and the growth rate decreases.

Worked Example 4: Chi-Squared Test for Hardy-Weinberg

A population of 1,000 plants is surveyed for flower colour. Red flowers (RR) are dominant, white flowers (rr) are recessive. The observed numbers are: Red (RR and Rr): 840; White (rr): 160.

(a) Calculate the expected numbers under Hardy-Weinberg equilibrium.

(b) Interpret the result.

If you get this wrong, revise: Genetics of Evolution -- Hardy-Weinberg Equilibrium; DSE Exam Focus

Solution

$q^2 = \frac{160}{1000} = 0.16, \quad q = 0.4, \quad p = 0.6$

Expected frequencies: $p^2 = 0.36$ , $2pq = 0.48$ , $q^2 = 0.16$

Expected numbers: RR = $0.36 \times 1000 = 360$ ; Rr = $0.48 \times 1000 = 480$ ; rr = $0.16 \times 1000 = 160$

Red (RR + Rr): $360 + 480 = 840$ ; White (rr): 160

$\chi^2 = \frac{(840 - 840)^2}{840} + \frac{(160 - 160)^2}{160} = 0$

The chi-squared value is 0, which is less than the critical value of 3.84 (for $df = 1$ at $p = 0.05$ ). There is no significant difference between observed and expected frequencies. The population is in Hardy-Weinberg equilibrium for this trait.

Worked Example 5: Eutrophication Data Interpretation

The following data show dissolved oxygen (DO) levels in a lake at different distances from a point where agricultural fertiliser runoff enters:

Distance from runoff source (km)	DO concentration (mg/L)
0.5	2.1
1.0	3.4
2.0	5.8
5.0	7.9
10.0	8.5
15.0	8.6

(a) Describe the trend and explain the biological processes responsible.

(b) A DO level below 4 mg/L is harmful to most fish. Over what distance range would fish be adversely affected?

(c) Suggest two strategies to reduce the impact.

If you get this wrong, revise: Human Impact -- Pollution (Eutrophication)

Solution

(a) DO increases with distance from the runoff source. At the source, high nutrient concentration stimulates rapid algal growth (algal bloom). When algae die, decomposers break them down through aerobic respiration, consuming dissolved oxygen and causing hypoxia. As distance increases, nutrient concentration decreases (dilution), algal growth is less excessive, and DO levels recover.

(b) DO is below 4 mg/L at 0.5 km (2.1 mg/L) and 1.0 km (3.4 mg/L). At 2.0 km, DO is 5.8 mg/L (above threshold). Fish would be adversely affected within approximately 0-1.5 km of the runoff source.

(c) 1. Create buffer zones (riparian zones): plant vegetation strips along the lake edge to absorb nutrients before they reach the water. 2. Regulate fertiliser application: restrict the amount and timing of fertiliser use on nearby farmland (e.g., no application before heavy rain; use slow-release fertilisers).

Summary Table

Topic	Key Concept	DSE Relevance
Natural selection	Variation, differential survival/reproduction, allele frequency change	Core concept; appears in structured and essay questions
Hardy-Weinberg	$p^2 + 2pq + q^2 = 1$ ; null model for population genetics	Calculation questions; chi-squared test
Evidence for evolution	Fossils, comparative anatomy, molecular biology, embryology	Data interpretation; explain how evidence supports evolution
Speciation	Allopatric, sympatric, polyploidy; reproductive isolation	Structured questions on mechanisms
Ecology levels	Organism to biosphere; biotic/abiotic factors	Foundation for all ecology questions
Population ecology	Exponential/logistic growth, carrying capacity, survivorship curves	Graph interpretation; calculations
Community ecology	Interactions, succession, keystone species	Explain cascading effects; interpret data
Ecosystem ecology	Energy flow, pyramids, nutrient cycles	Calculations (10% rule); cycle descriptions
Biomes	Climate, vegetation, adaptations	Climate graph interpretation; adaptation questions
Human impact	Pollution, climate change, biodiversity loss, conservation	Data interpretation; evaluate strategies

Exam Tips

In natural selection questions, always identify the specific selection pressure and explain how it creates differential survival and reproduction. Generic answers without a concrete example score poorly.
For Hardy-Weinberg, always state your working: show $q^2$ , then $q$ , then $p$ , then the genotype frequencies. Examiners award marks for the process.
When describing nutrient cycles, name the specific processes (nitrification, denitrification, etc.) and the organisms involved (specific bacteria names earn extra marks).
In ecology questions, use quantitative data whenever possible (e.g., "the DO level dropped to 2.1 mg/L" rather than "the DO level was very low").
For conservation questions, distinguish clearly between in situ and ex situ strategies, and be specific about examples.
When explaining eutrophication, always describe the complete sequence: nutrient input, algal bloom, death and decomposition, oxygen depletion, organism death.
Remember that energy flow is unidirectional and always decreasing, while nutrient cycles are cyclic.

Problem Set

Problem 1: In a population of butterflies, 64% have white wings (recessive, ww) and 36% have coloured wings (dominant, W*). Calculate the allele frequencies and the proportion of heterozygotes.

If you get this wrong, revise: Genetics of Evolution -- Allele Frequency Calculations

Solution

$q^2 = 0.64$ , so $q = \sqrt{0.64} = 0.8$ .

$p = 1 - 0.8 = 0.2$ .

$2pq = 2(0.2)(0.8) = 0.32$ .

The proportion of heterozygotes (Ww) is 0.32, or 32%.

Note that the heterozygote frequency (32%) is higher than the homozygous dominant frequency ( $p^2 = 0.04$ , or 4%), even though coloured butterflies are less common. This is a common result when one allele is rare.

Problem 2: Explain how Darwin's theory of natural selection accounts for the development of antibiotic resistance in bacteria. Use the terms "variation," "selection pressure," "differential survival," and "inheritance" in your answer.

If you get this wrong, revise: Natural Selection -- Darwin's Theory; Classic Examples (Antibiotic resistance)

Solution

Variation: Within any bacterial population, random mutations occur during DNA replication. Some bacteria may acquire a mutation that confers resistance to a particular antibiotic (e.g., a gene for beta-lactamase enzyme).
Selection pressure: When an antibiotic is administered, it creates a strong selection pressure. The antibiotic kills or inhibits susceptible bacteria.
Differential survival: Bacteria carrying the resistance allele survive the antibiotic treatment and continue to reproduce, while susceptible bacteria die. The resistant bacteria have higher fitness in the presence of the antibiotic.
Inheritance: The resistance gene is passed to offspring during binary fission (vertical transmission) and can also be transferred to other bacteria via plasmids during conjugation (horizontal transmission). Over generations, the resistant allele increases in frequency.

The key point is that the mutation arises randomly, not because the bacteria "need" it. The antibiotic merely selects for pre-existing resistant individuals.

Problem 3: A volcanic island is formed in the ocean. Describe the process of primary succession that would occur on this island over thousands of years.

If you get this wrong, revise: Community Ecology -- Ecological Succession

Solution

Pioneer species: Lichens colonise the bare volcanic rock first. They require no soil, absorb water and minerals directly from the rock, and can tolerate extreme conditions.
Soil formation: Lichens secrete acids that slowly break down the rock surface, beginning soil formation. Dead lichens contribute organic matter.
Mosses and liverworts: As a thin soil layer accumulates, mosses colonise. They further contribute organic matter when they die, deepening the soil.
Grasses and herbs: As soil deepens, grasses and herbaceous plants establish. Their roots help bind soil and retain water.
Shrubs and small trees: As soil becomes deeper and more nutrient-rich, woody plants establish.
Climax community: After hundreds to thousands of years, a relatively stable community (e.g., tropical forest, depending on climate) becomes established.

Problem 4: A food chain consists of grass $\to$ rabbit $\to$ fox. If the grass contains 20,000 kJ of energy, calculate the energy available to the fox. Explain the energy losses at each trophic level.

If you get this wrong, revise: Ecosystem Ecology -- The 10% Rule; Trophic Levels

Solution

Using the 10% rule:

Grass (producer): 20,000 kJ
Rabbit (primary consumer): $20,000 \times 0.10 = 2,000$ kJ
Fox (secondary consumer): $2,000 \times 0.10 = 200$ kJ

Energy losses at each level are due to: respiration (heat), excretion (waste), undigested material (faeces), and energy used for movement and maintenance. The 90% loss at each step explains why food chains rarely exceed 4-5 trophic levels.

Problem 5: Distinguish between homologous and analogous structures, providing one example of each. Explain the evolutionary significance of each type.

If you get this wrong, revise: Evidence for Evolution (Detailed) -- Homologous vs Analogous Structures

Solution

Homologous structures: Derived from a common ancestor; similar underlying anatomy but may serve different functions. Example: the pentadactyl limb in humans (grasping), bats (flying), and whales (swimming) -- all share the same basic bone arrangement. Significance: indicates divergent evolution from a common ancestor.

Analogous structures: Similar function but different evolutionary origins. Example: the wing of a bird (modified forelimb with bones) and the wing of an insect (exoskeleton extension with no bones). Significance: indicates convergent evolution -- unrelated species evolve similar traits independently due to similar environmental pressures.

Problem 6: Two species of finch live on the same island. One has a large beak and eats hard seeds; the other has a small beak and eats small seeds. If a drought causes only large, hard seeds to be available, predict what will happen to each population over several generations. Explain your reasoning.

If you get this wrong, revise: Natural Selection -- Types of Natural Selection

Solution

The large-beaked finch population will likely increase because:

The available food (hard seeds) matches their beak adaptation
They have higher survival and reproductive success
This is directional selection favouring the large beak phenotype

The small-beaked finch population will likely decrease because:

They cannot efficiently crack hard seeds
They face increased competition and reduced food availability
Many individuals will starve or fail to reproduce

If the drought persists long enough, the small-beaked species may go extinct on this island (competitive exclusion). If some individuals with slightly larger beaks exist due to genetic variation, the population may evolve larger beaks over generations. This demonstrates natural selection acting on heritable variation in response to an environmental change.

Problem 7: Describe the nitrogen cycle, naming the specific processes and the bacteria involved at each stage.

If you get this wrong, revise: Ecosystem Ecology -- Nutrient Cycles (Nitrogen cycle)

Solution

Nitrogen fixation: Atmospheric N $_2$ converted to NH $_3$ by nitrogen-fixing bacteria (Rhizobium in root nodules of legumes, Azotobacter free-living in soil, cyanobacteria) or lightning.
Nitrification: NH $_3$ converted to NO $_2^-$ by Nitrosomonas, then to NO $_3^-$ by Nitrobacter.
Absorption: Plants absorb NO $_3^-$ through their roots.
Assimilation: Plants incorporate nitrogen into amino acids and proteins.
Feeding: Animals obtain nitrogen by eating plants or other animals.
Ammonification: Decomposer bacteria break down dead organisms and waste, releasing NH $_3$ .
Denitrification: Denitrifying bacteria (Pseudomonas) convert NO $_3^-$ back to N $_2$ under anaerobic conditions, returning it to the atmosphere.

Problem 8: Explain the process of allopatric speciation using an example of a river changing course and dividing a population of beetles.

If you get this wrong, revise: Genetics of Evolution -- Speciation

Solution

Geographic isolation: The river changing course creates a barrier, physically separating the beetle population into two subpopulations.
No gene flow: The two subpopulations cannot interbreed across the river.
Different selection pressures: Each bank may have different environmental conditions (vegetation, predators, soil), favouring different traits.
Independent evolution: Natural selection, mutation, and genetic drift act independently on each population, causing allele frequencies to diverge.
Accumulation of differences: Over many generations, genetic and phenotypic differences accumulate.
Reproductive isolation: Even if the river dries up, accumulated differences (behavioural, mechanical, or genetic) may prevent successful interbreeding. At this point, two separate species exist.

Problem 9: Explain the difference between bioaccumulation and biomagnification, using DDT as an example.

If you get this wrong, revise: Human Impact -- Pollution (Bioaccumulation vs Biomagnification)

Solution

Bioaccumulation is the accumulation of a substance within a single organism over its lifetime. For example, DDT accumulates in the fatty tissues of an individual fish as it continuously absorbs DDT from contaminated water and food, faster than it can excrete it.

Biomagnification is the increasing concentration of a substance at each successive trophic level in a food chain. For example, DDT concentration increases from plankton (low) to zooplankton to small fish to large fish to birds of prey (highest). At each trophic level, organisms consume many organisms from the level below, accumulating all the DDT those prey organisms contained.

Key distinction: bioaccumulation is within one organism; biomagnification is across the food chain. Both can occur simultaneously.

Problem 10: A student measures Simpson's Diversity Index for two ponds. Pond A: D = 0.85. Pond B: D = 0.35. The student concludes that Pond A is healthier. Evaluate this conclusion and suggest what additional information would strengthen it.

If you get this wrong, revise: Biodiversity Measurement -- Simpson's Diversity Index

Solution

The conclusion is partially supported. A higher Simpson's Diversity Index (Pond A: D = 0.85) indicates greater biodiversity (more species, more evenly distributed) compared to Pond B (D = 0.35, which suggests one or a few species dominate). Higher biodiversity generally correlates with a healthier, more stable ecosystem.

However, additional information would strengthen the conclusion:

Abiotic factors: Water pH, temperature, dissolved oxygen, pollutant levels
Specific species present: Presence of indicator species (e.g., stonefly larvae indicate clean water)
Temporal data: Measurements over time to show trends (declining biodiversity in Pond B?)
Physical habitat: Pond size, vegetation cover, surrounding land use

Without this context, it is possible that Pond A has high diversity but is polluted with tolerant species, or that Pond B naturally has low diversity due to its size or location.

tip

tip Ready to test your understanding of Evolution and Ecology? The diagnostic test contains the hardest questions within the DSE specification for this topic, each with a full worked solution.

Unit tests probe edge cases and common misconceptions. Integration tests combine Evolution and Ecology with other biology topics to test synthesis under exam conditions.

See Diagnostic Guide for instructions on self-marking and building a personal test matrix.

Population Ecology

Population Growth Models

Exponential growth (J-shaped curve):

Under ideal conditions (unlimited resources, no predators, no disease), populations grow exponentially:

$\frac{dN}{dt} = rN$

Where $N$ = population size, $r$ = intrinsic rate of natural increase, $t$ = time.

Exponential growth produces a J-shaped curve. This phase is characteristic of populations introduced to a new environment with abundant resources.

Logistic growth (S-shaped curve):

In reality, resources are limited. Population growth slows as it approaches the carrying capacity ( $K$ ):

$\frac{dN}{dt} = rN\left(\frac{K - N}{K}\right)$

Where $K$ = carrying capacity (maximum population size the environment can sustain).

The logistic growth curve has three phases:

Lag phase: Slow growth while the population adjusts to the environment
Exponential phase: Rapid growth (approaching exponential)
Stationary phase: Growth rate declines to zero as the population reaches carrying capacity; birth rate approximately equals death rate

Factors Affecting Population Size

Factor Type	Description
Abiotic factors	Temperature, water availability, light, pH, mineral nutrients, salinity
Biotic factors	Food availability, predation, competition, disease, parasitism
Density-dependent	Factors whose effect increases with population density (e.g., competition for resources, disease transmission, predation)
Density-independent	Factors whose effect is independent of population density (e.g., natural disasters, temperature extremes, seasonal changes)

Carrying Capacity

The carrying capacity ( $K$ ) of an environment is the maximum population size that can be sustained indefinitely by the available resources.

Factors determining carrying capacity:

Food availability (primary limiting factor in most ecosystems)
Water supply
Space/shelter availability
Disease prevalence (increases with density)
Competition intensity (increases with density)

Human Population Growth

The human population has grown exponentially over the past 200 years due to:

Advances in medicine and public health (reduced death rate, especially infant mortality)
Improved agricultural productivity (Green Revolution)
Improved sanitation and clean water supply
Antibiotics and vaccines

Demographic transition model:

Stage	Birth Rate	Death Rate	Population Growth	Description
1	High	High	Low	Pre-industrial; high birth and death rates; population stable
2	High	Falling	Rapidly increasing	Industrialising; death rate falls due to medicine/sanitation
3	Falling	Low	Increasing but slowing	Birth rate falls due to contraception, education, urbanisation
4	Low	Low	Low (stable)	Post-industrial; both rates are low; population is stable

Ecological Relationships

Symbiosis

Relationship	Effect on A	Effect on B	Example
Mutualism	+	+	Lichen (fungus + alga); mycorrhizae (fungus + plant roots); coral + zooxanthellae
Commensalism	+	0	Barnacles on a whale (barnacles benefit from transport; whale is unaffected)
Parasitism	+	-	Tapeworm in human intestine; Plasmodium in red blood cells; mistletoe on a tree
Predation	+	-	Lion and zebra; owl and mouse; shark and fish
Herbivory	+	-	Rabbit and grass; caterpillar and leaf
Competition	-	-	Two species competing for the same resource (e.g., two species of Paramecium)

Interspecific vs Intraspecific Competition

Type	Description
Intraspecific	Competition between individuals of the SAME species (e.g., oak trees competing for light)
Interspecific	Competition between individuals of DIFFERENT species (e.g., oak trees and maple trees competing for light)

Competitive exclusion principle: Two species cannot occupy exactly the same ecological niche in the same habitat. If they compete for the same limited resource, one species will outcompete the other, leading to the competitive exclusion of the weaker competitor. The two species can coexist only if they occupy slightly different niches (resource partitioning).

Resource partitioning: When two species share a habitat, they may divide the resources to reduce competition. For example, different species of warblers feed on insects at different heights in the same tree canopy.

Biomes

Major Biomes

Biome	Climate	Vegetation	Fauna	Soil
Tropical rainforest	Hot and wet all year; high rainfall (2000+ mm/year); high humidity; little seasonal variation	Dense, multi-layered canopy (emergent, canopy, understorey, forest floor); high biodiversity; broad-leaved evergreen trees; epiphytes, lianas, ferns	Highest biodiversity of any biome; monkeys, toucans, jaguars, tree frogs, butterflies, ants, snakes	Nutrient-poor (rapid decomposition and nutrient uptake by plants); thin topsoil
Temperate deciduous forest	Moderate rainfall (750-1500 mm/year); four distinct seasons (warm summers, cold winters)	Deciduous trees (oak, beech, maple) that lose leaves in autumn; understorey of shrubs and spring-flowering plants	Deer, squirrels, foxes, owls, woodpeckers, insects	Fertile (rich in organic matter from leaf litter)
Coniferous forest (taiga)	Cold winters; moderate rainfall (400-1000 mm/year); short growing season	Evergreen coniferous trees (pine, spruce, fir, larch); needle-like leaves (reduce water loss and snow damage); dense canopy reduces understorey	Moose, bears, wolves, lynx, beavers, owls, crossbills	Acidic; low in nutrients; thick layer of pine needles slows decomposition
Savanna (tropical grassland)	Hot all year; distinct wet and dry seasons; moderate rainfall (500-1500 mm/year); seasonal droughts	Tall grasses with scattered trees (baobab, acacia); frequent fires maintain the grassland	Large herbivores (zebras, wildebeest, gazelles, elephants); predators (lions, cheetahs, hyenas)	Thin; frequent fires prevent deep soil development
Desert	Very hot days; cold nights; very low rainfall (<250 mm/year); high evaporation rates	Sparse vegetation; succulents (cacti); deep root systems; water-storing tissues; small or absent leaves (reduce transpiration)	Camels, snakes, lizards, scorpions, kangaroo rats, fennec foxes	Low organic matter; sandy or rocky; saline
Tundra	Very cold; short cool summers; low rainfall (150-250 mm/year); permafrost (permanently frozen subsoil)	Low-growing vegetation: mosses, lichens, sedges, dwarf shrubs; no trees (permafrost prevents deep root growth)	Caribou (reindeer), arctic foxes, polar bears (near coast), snowy owls, lemmings, migratory birds	Permafrost; thin active layer thaws in summer; low in nutrients
Temperate grassland	Moderate rainfall (500-900 mm/year); hot summers, cold winters	Dominated by grasses; few trees (drought and fire prevent tree growth); deep root systems	Bison, antelope, prairie dogs, coyotes, hawks, grasshoppers	Deep, fertile topsoil (mollisols); high organic matter from grass roots
Freshwater	Variable; depends on depth, flow rate, latitude	Aquatic plants (reed beds, water lilies, algae, phytoplankton)	Fish, amphibians, invertebrates (dragonfly larvae, water beetles), water birds	Sediment at bottom; nutrient-rich or oligotrophic
Marine	Stable temperature (oceans buffer temperature changes); high salinity	Phytoplankton (basis of marine food web); seaweed; kelp forests; coral reefs (in warm, shallow tropical waters)	Fish, whales, dolphins, sharks, crustaceans, molluscs, coral polyps	Sediment on ocean floor; rocky or sandy coasts

Interpreting Climate Graphs

Climate graphs show both temperature (line graph) and precipitation (bar chart) for a given location over a 12-month period.

Biome clue	What to look for on the climate graph
Tropical rainforest	High temperature all year (>20 degrees C); high rainfall all year (>150 mm/month)
Savanna	High temperature all year; distinct wet season (high rainfall) and dry season (very low rainfall)
Desert	High temperature; very low rainfall all year (<20 mm/month)
Temperate grassland	Moderate temperature range; moderate, fairly even rainfall; may have a slight drought in summer
Temperate deciduous forest	Moderate temperature range with four seasons; moderate, fairly even rainfall throughout the year
Coniferous forest	Long cold winters; short cool summers; moderate, fairly even rainfall
Tundra	Very low temperatures all year (may be below 0 degrees C for much of the year); low rainfall

Ecological Techniques and Data Analysis

Measuring Abundance

Method	Description	Most Suitable For
Density	Count of individuals per unit area (e.g., per m $^2$ )	Small, stationary or slow-moving organisms (plants, limpets, barnacles)
Frequency	Percentage of quadrats in which a species is present (NOT a count of individuals)	Plants that are difficult to count individually (e.g., grass)
Percentage cover	Estimate of the proportion of ground within a quadrat that is covered by a species (using a gridded quadrat for accuracy)	Plants (especially when individual plants are hard to distinguish, e.g., grass, moss)

Standard Deviation

Standard deviation is a measure of the spread of data around the mean. It is used to assess the reliability of data:

$s = \sqrt{\frac{\sum (x - \bar{x})^2}{n - 1}}$

Where:

$s$ = standard deviation
$x$ = each individual value
$\bar{x}$ = mean
$n$ = number of values

Interpretation	Description
Large standard deviation	Data points are widely spread from the mean; lower reliability; more variation in the data
Small standard deviation	Data points are close to the mean; higher reliability; less variation in the data
Overlap of standard deviation bars	If standard deviation bars on a bar chart overlap, the difference between the means is NOT statistically significant; the difference could be due to random variation
No overlap of standard deviation bars	The difference between the means IS likely to be statistically significant (though a proper statistical test such as a t-test is needed to confirm this)

Student's t-test

Used to determine whether the difference between the means of two sets of data is statistically significant.

Null hypothesis ( $H_0$ ): There is no significant difference between the two means
If the calculated t-value exceeds the critical value (at $p < 0.05$ and the appropriate degrees of freedom), the null hypothesis is REJECTED and the difference is considered statistically significant
If the calculated t-value is less than the critical value, the null hypothesis is ACCEPTED and the difference is NOT statistically significant

Common Pitfalls

Biomes are determined by CLIMATE (temperature and rainfall), not by latitude alone. Two locations at the same latitude can have very different biomes if their climate differs (e.g., due to ocean currents, altitude, or rain shadow effects)
Tropical rainforest soils are NOT fertile. Despite the lush vegetation, the soil is nutrient-poor because nutrients are rapidly taken up by plants and locked in the biomass. When the forest is cleared for agriculture, the soil quickly becomes infertile because the nutrient cycle is disrupted
Permafrost prevents tree growth in the tundra, NOT cold temperatures alone. Coniferous forests grow at similar latitudes to tundra but where the soil is not permanently frozen; the deep root systems of trees cannot penetrate the permafrost layer**
Frequency is the percentage of quadrats containing a species, NOT the number of individuals. Students often confuse frequency with density. Frequency tells you how widely distributed a species is, not how abundant it is**
Standard deviation measures the SPREAD of data, not the accuracy of the mean. A small standard deviation means the data points are close to the mean (consistent/reliable), but it does not tell you whether the mean itself is correct**

danger

Confusing Lamarckism with Darwinian evolution: Lamarck proposed that acquired characteristics are inherited (e.g., a giraffe stretches its neck and passes the longer neck to offspring). Darwin proposed natural selection of random mutations. DSE exams frequently test this distinction. Only Darwinian evolution is accepted by modern biology.
Misunderstanding what "fitness" means in evolution: Evolutionary fitness refers to an organism's ability to SURVIVE and REPRODUCE in its environment, not physical strength or intelligence. A trait that improves reproductive success (even if it seems disadvantageous in isolation) increases fitness. A peacock's tail reduces survival but increases mating success.
Confusing ecological niches with habitats: A habitat is where an organism lives (a physical place). A niche is the ROLE an organism plays in its ecosystem -- how it obtains food, interacts with other species, and reproduces. Two species can share a habitat but cannot occupy the exact same niche (competitive exclusion principle).
Assuming natural selection always leads to "better" organisms: Natural selection produces organisms that are BETTER ADAPTED to their CURRENT environment, not objectively "better" or "more advanced." If the environment changes, previously advantageous traits can become disadvantageous. Evolution has no direction or goal.

Nutrient Cycles in Detail

The Nitrogen Cycle

Nitrogen is essential for the synthesis of amino acids (proteins), nucleotides (DNA, RNA), and ATP. Although nitrogen gas (N $_2$ ) makes up approximately 78% of the atmosphere, it is unusable by most organisms because the triple bond in N $_2$ is very strong and requires a lot of energy to break.

Process	Description	Organisms Involved
Nitrogen fixation	Conversion of atmospheric N $_2$ into ammonia (NH $_3$ ) or ammonium ions (NH $_4^+$ )	Nitrogen-fixing bacteria: Rhizobium (symbiotic, in root nodules of legumes); Azotobacter (free-living in soil); blue-green algae (cyanobacteria); lightning (converts N $_2$ to NOx, which washes into soil as nitrate)
Nitrification	Conversion of ammonium ions (NH $_4^+$ ) into nitrite (NO $_2^-$ ) and then nitrate (NO $_3^-$ )	Nitrifying bacteria: Nitrosomonas (NH $_4^+$ $\rightarrow$ NO $_2^-$ ); Nitrobacter (NO $_2^-$ $\rightarrow$ NO $_3^-$ ); these are aerobic bacteria (require oxygen)
Absorption by plants	Plants absorb nitrate (NO $_3^-$ ) and ammonium (NH $_4^+$ ) ions from the soil through their roots (active transport)	Plants
Assimilation	Plants incorporate nitrogen into amino acids, proteins, nucleotides, and other organic compounds	Plants; animals obtain nitrogen by eating plants or other animals
Ammonification	Decomposition of organic nitrogen (proteins, urea, nucleic acids) in dead organisms and waste into ammonium (NH $_4^+$ )	Decomposer bacteria and fungi
Denitrification	Conversion of nitrate (NO $_3^-$ ) back into nitrogen gas (N $_2$ ), which returns to the atmosphere	Denitrifying bacteria: Pseudomonas denitrificans, Thiobacillus denitrificans; these are anaerobic bacteria (found in waterlogged, oxygen-poor soils)

The Carbon Cycle

Process	Description	Carbon Form
Photosynthesis	Plants, algae, and some bacteria convert CO $_2$ and water into glucose and oxygen using light energy	CO $_2$ in atmosphere $\rightarrow$ glucose in organisms
Respiration	All organisms break down glucose (and other organic molecules) to release energy, producing CO $_2$ as a waste product	Glucose $\rightarrow$ CO $_2$ released to atmosphere
Combustion	Burning of fossil fuels (coal, oil, natural gas) and biomass releases CO $_2$ that was previously locked in organic matter	Organic carbon $\rightarrow$ CO $_2$ released to atmosphere
Decomposition	Decomposers break down dead organic matter, releasing CO $_2$ through respiration	Organic carbon $\rightarrow$ CO $_2$ released to atmosphere
Feeding	Carbon passes from one trophic level to the next through food chains	Carbon moves through the food web
Fossilisation	Organic matter is buried and compressed over millions of years, forming fossil fuels (coal, oil, natural gas)	Carbon locked in fossil fuels
Ocean absorption	CO $_2$ dissolves in ocean water; some is used by marine organisms for photosynthesis and shell formation	CO $_2$ in atmosphere $\rightarrow$ dissolved CO $_2$ and carbonate ions in oceans

The Phosphorus Cycle

Unlike the carbon and nitrogen cycles, the phosphorus cycle has NO significant gaseous phase. Phosphorus cycles mainly between the land and oceans.

Process	Description
Weathering	Phosphorus is released from rocks (phosphate minerals such as apatite) by weathering (physical breakdown and chemical dissolution by weak acids in rainwater)
Absorption by plants	Plants absorb phosphate ions (PO $_4^{3-}$ ) from the soil through their roots
Assimilation	Plants incorporate phosphorus into organic molecules (DNA, RNA, ATP, phospholipids); animals obtain phosphorus by eating plants
Decomposition and excretion	Decomposers break down dead organisms and release phosphate back into the soil; animals excrete phosphate in urine
Sedimentation	Phosphate is washed into rivers and eventually reaches the oceans, where it accumulates in marine sediments
Geological uplift	Over millions of years, marine sediments are uplifted by geological processes, exposing phosphate-containing rocks on land -- completing the cycle

Ecological Relationships

Symbiosis

Type	Description	Effect on A	Effect on B	Example
Mutualism	Both organisms benefit	+	+	Lichens (fungus + algae); Rhizobium in root nodules; mycorrhizae; clownfish and sea anemone; cleaner fish and larger fish
Commensalism	One organism benefits; the other is unaffected	+	0	Barnacles on whales; epiphytic orchids on trees; remora fish and sharks
Parasitism	One organism (parasite) benefits at the expense of the other (host)	+	-	Tapeworms in human intestine; Plasmodium (malaria) in red blood cells; fleas on mammals; mistletoe on trees

Competition

Type	Description	Example
Intraspecific competition	Competition between individuals of the SAME species for the same limited resources (food, water, mates, territory, light)	Trees in a forest competing for light; male deer competing for mates during the rut
Interspecific competition	Competition between individuals of DIFFERENT species for the same limited resources	Lions and hyenas competing for prey; different plant species competing for soil minerals and light

Competitive exclusion principle (Gause's principle): Two species cannot coexist in the same habitat if they occupy exactly the same ecological niche. One species will outcompete the other, leading to the exclusion of the weaker competitor.

Resource partitioning: To avoid competitive exclusion, similar species evolve to use slightly different resources or exploit the same resource at different times or in different ways.

Example	Resource Partitioning Mechanism
Darwin's finches on the Galapagos Islands	Different beak sizes and shapes allow different finch species to eat different food types (seeds of different sizes, insects, cactus flowers)
Warblers in a coniferous forest	Five species of warbler feed in different parts of the same tree (one at the top, one in the middle, one near the trunk, etc.)
Lions and leopards in African savanna	Lions hunt larger prey in groups during the day; leopards hunt smaller prey alone at night

Predation

Predation has significant effects on population dynamics and community structure:

Regulation of prey population size: Predators keep prey populations below carrying capacity
Evolution of adaptations: Predator-prey relationships drive the evolution of adaptations in both species (e.g., speed, camouflage, warning colouration)
Trophic cascades: Changes in the population of a top predator can have cascading effects through the entire food web

Adaptation	Predator Example	Prey Example
Camouflage (crypsis)	Tiger stripes; owl feather pattern	Stick insects; leaf-tailed geckos; Arctic hare (white in winter)
Warning colouration (aposematism)	--	Poison dart frogs; wasps; ladybirds (bright colours warn predators of toxicity or unpalatability)
Mimicry -- Batesian	--	Hoverfly (looks like a wasp but is harmless); king snake (looks like coral snake)
Mimicry -- Mullerian	--	Several species of unpalatable butterflies that look similar (all benefit from shared warning signal)
Chemical defences	--	Skunks (spray); bombardier beetle (hot chemical spray); poison dart frogs (batrachotoxin)
Physical defences	--	Porcupine (quills); tortoise (shell); armadillo (armour)
Group defence	Wolves (pack hunting)	Musk oxen (circle around young); sardines (schooling)

Human Impact on Ecosystems

Eutrophication

Eutrophication is the excessive enrichment of water bodies with nutrients (especially nitrates and phosphates), leading to excessive algal growth.

Stage	Description
1. Nutrient input	Nitrates (from fertilisers, sewage) and phosphates (from detergents, fertilisers) enter rivers and lakes
2. Algal bloom	The increased nutrient concentration stimulates rapid growth of algae (algal bloom) on the surface of the water
3. Light blockage	The dense layer of algae on the surface blocks sunlight from reaching aquatic plants deeper in the water
4. Plant death	Submerged aquatic plants die because they cannot photosynthesise without sufficient light
5. Decomposition	Bacteria decompose the dead plants and algae; this process uses up dissolved oxygen in the water
6. Oxygen depletion	Dissolved oxygen levels drop (hypoxia or anoxia); fish and other aquatic organisms die from oxygen deprivation
7. Further decomposition	Dead fish and organisms are decomposed by bacteria, further depleting oxygen levels; the ecosystem is severely degraded

Deforestation

Impact	Description
Loss of biodiversity	Habitat destruction leads to species extinction; tropical rainforests contain over 50% of all terrestrial species
Soil erosion	Tree roots stabilise soil; without trees, topsoil is washed away by rain, leading to loss of fertile land and sedimentation in rivers
Increased CO $_2$	Trees absorb CO $_2$ through photosynthesis; deforestation reduces this carbon sink, contributing to global warming
Disruption of water cycle	Trees play a crucial role in transpiration, which contributes to cloud formation and rainfall; deforestation reduces local rainfall
Flooding	With fewer trees to intercept rainfall and absorb water, surface run-off increases, leading to more frequent and severe flooding
Climate change	Deforestation in tropical regions is a major contributor to global greenhouse gas emissions (approximately 10-15% of total)

Common Pitfalls

The nitrogen cycle involves LIVING organisms (bacteria) to convert nitrogen between forms. Plants CANNOT absorb atmospheric N $_2$ directly; they can only absorb nitrate (NO $_3^-$ ) and ammonium (NH $_4^+$ ) ions
Nitrification is an aerobic process (requires oxygen); denitrification is an anaerobic process (occurs in oxygen-poor conditions). This is why waterlogged soils (e.g., rice paddies) can lead to nitrogen loss through denitrification
Decomposers are essential for nutrient cycling. Without decomposers, nutrients would remain locked in dead organisms and waste, and plants would eventually run out of mineral ions
Eutrophication is caused by excess NUTRIENTS, not toxic chemicals. The problem is too many nutrients (nitrates and phosphates), not pollution in the conventional sense**
Batesian mimicry benefits the mimic (harmless species looks like harmful species); Mullerian mimicry benefits all species involved (all are harmful/unpalatable). Students often confuse these two types of mimicry**

Evolution Overview​

What is Evolution?​

Evidence for Evolution​

Lamarckism vs Darwinism​

Natural Selection​

Darwin's Theory of Natural Selection​

Conditions for Natural Selection​

Types of Natural Selection​

Classic Examples​

Genetics of Evolution​

Hardy-Weinberg Equilibrium​

Conditions for Hardy-Weinberg Equilibrium​

Allele Frequency Calculations​

Genetic Drift​

Gene Flow​

Mutations as Raw Material​

Speciation​

Evidence for Evolution (Detailed)​

Homologous vs Analogous Structures​

Vestigial Organs​

Molecular Clocks​

Transitional Fossils​

Comparative Embryology​

Human Evolution​

Primate Characteristics​

Hominid Timeline​

Key Trends in Human Evolution​

Out of Africa Theory​

Mitochondrial Eve​

Ecology Basics​

Levels of Organisation​

Biotic vs Abiotic Factors​

Niche Concept​

Competitive Exclusion Principle​

Population Ecology​

Population Growth Models​

Carrying Capacity​

Density-Dependent vs Density-Independent Factors​

r-Selection vs K-Selection​

Survivorship Curves​

Community Ecology​

Interspecific Interactions​

Predation​

Competition​

Ecological Succession​

Keystone Species​

Trophic Cascades​

Ecosystem Ecology​

Energy Flow​

Trophic Levels​

Ecological Pyramids​

The 10% Rule​

Nutrient Cycles​

Primary Productivity​

Biomes​

Major Terrestrial Biomes​

Aquatic Biomes​

Human Impact​

Deforestation​

Habitat Fragmentation​

Pollution​

Climate Change​

Biodiversity Loss​

Conservation Strategies​

DSE Exam Focus​

Common Question Types​

Calculation Techniques​

Data Interpretation​

Experimental Design​

Worked Examples​

Worked Example 1: Hardy-Weinberg and Sickle Cell​

Worked Example 2: Ecological Pyramid Calculations​

Worked Example 3: Population Growth​

Worked Example 4: Chi-Squared Test for Hardy-Weinberg​

Worked Example 5: Eutrophication Data Interpretation​

Summary Table​

Exam Tips​

Problem Set​

Population Ecology​

Population Growth Models​

Evolution Overview

What is Evolution?

Evidence for Evolution

Lamarckism vs Darwinism

Natural Selection

Darwin's Theory of Natural Selection

Conditions for Natural Selection

Types of Natural Selection

Classic Examples

Genetics of Evolution

Hardy-Weinberg Equilibrium

Conditions for Hardy-Weinberg Equilibrium

Allele Frequency Calculations

Genetic Drift

Gene Flow

Mutations as Raw Material

Speciation

Evidence for Evolution (Detailed)

Homologous vs Analogous Structures

Vestigial Organs

Molecular Clocks

Transitional Fossils

Comparative Embryology

Human Evolution

Primate Characteristics

Hominid Timeline

Key Trends in Human Evolution

Out of Africa Theory

Mitochondrial Eve

Ecology Basics

Levels of Organisation

Biotic vs Abiotic Factors

Niche Concept

Competitive Exclusion Principle

Population Ecology

Population Growth Models

Carrying Capacity

Density-Dependent vs Density-Independent Factors

r-Selection vs K-Selection

Survivorship Curves

Community Ecology

Interspecific Interactions

Predation

Competition

Ecological Succession

Keystone Species

Trophic Cascades

Ecosystem Ecology

Energy Flow

Trophic Levels

Ecological Pyramids

The 10% Rule

Nutrient Cycles

Primary Productivity

Biomes

Major Terrestrial Biomes

Aquatic Biomes

Human Impact

Deforestation

Habitat Fragmentation

Pollution

Climate Change

Biodiversity Loss

Conservation Strategies

DSE Exam Focus

Common Question Types

Calculation Techniques

Data Interpretation

Experimental Design

Worked Examples

Worked Example 1: Hardy-Weinberg and Sickle Cell

Worked Example 2: Ecological Pyramid Calculations

Worked Example 3: Population Growth

Worked Example 4: Chi-Squared Test for Hardy-Weinberg

Worked Example 5: Eutrophication Data Interpretation

Summary Table

Exam Tips

Problem Set

Population Ecology

Population Growth Models