Evolution and Ecology | DSE - Wyatt's Notes

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, the Gene).

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

1. Variation: Individuals within a population exhibit heritable variation in traits.
2. Overproduction: Populations produce more offspring than the environment can support.
3. Competition: There is competition for limited resources (food, mates, space).
4. Differential survival and reproduction: Individuals with traits better suited to the environment have a higher probability of surviving and reproducing.
5. 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.

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).

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.

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

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

1. No mutations — no new alleles are introduced
2. Random mating — individuals mate without regard to genotype
3. No natural selection — all genotypes have equal fitness
4. Extremely large population size — genetic drift is negligible
5. 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.

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 = $(100 \times 2) + 200 = 400$

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

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

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.

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 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: $10^{-5}$ to $10^{-6}$ per gene per generation
• Most mutations are neutral or harmful; a small fraction are beneficial and may be selected for

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.

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.

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

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 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

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.

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 single offspring
• Extended period of parental care

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

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

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 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.

Ecology studies interactions at multiple hierarchical levels:

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.

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 exhibit resource partitioning — they divide The available resources to reduce direct competition (e.g., different feeding heights, different Activity times, different food sizes).

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 $K$.

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)

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). 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). associated with r-selection.

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)

• 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).

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)

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

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.

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

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 limited to 4-5 trophic levels — there is Insufficient energy to sustain higher levels.

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

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

• 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.

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

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

• 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

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)

Eutrophication:

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

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

Bioaccumulation vs Biomagnification:

Substances that biomagnify are :

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

• 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)

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)

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

1. 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.

2. 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.

3. Interpret ecological data from graphs and tables. Look for trends, correlations, and causal relationships. Identify independent and dependent variables.

4. Compare and contrast related concepts. Use structured tables or point-by-point comparisons. Always include both similarities and differences.

5. Explain the impact of human activities on ecosystems. Be specific about mechanisms (e.g., how eutrophication actually kills fish, not just that it does).

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):

1. Identify $q^2$ from the homozygous recessive frequency
2. Take the square root to get $q$
3. Calculate $p = 1 - q$
4. 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.

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

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

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

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

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

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

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

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

(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)

Life’s web of interactions: Ecology is like studying a city’s social network — species interact through food chains, competition, and symbiosis, and these interactions determine community structure.

Why it matters: From conservation to climate change, ecology explains how ecosystems function and how human activities affect them.

The key insight: Energy flows through ecosystems in one direction (sun → producers → consumers), but nutrients cycle endlessly.

flowchart TD
    A[1_Ecology] --> B[Key Concepts]
    A --> C[Core Principles]
    A --> D[Practical Applications]
    B --> E[Fundamental definitions]
    C --> F[Design patterns]
    D --> G[Real-world usage]

• 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 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 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

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)

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

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

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

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

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)

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

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)

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

• Ecology
• Biodiversity and Conservation
• DSE Biology