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DSE Biology Diagnostic: Evolution and Ecology

Unit Test 1: Natural Selection and Evidence for Evolution​

Question

(a) Describe the process of natural selection as proposed by Darwin, using the development of antibiotic resistance in bacteria as an example. [5 marks]

(b) Fossil evidence provides one line of evidence for evolution. Describe two other types of evidence that support the theory of evolution. [4 marks]

(c) A student claims that evolution is "just a theory" and therefore not well-supported. Evaluate this claim by distinguishing between the everyday use of "theory" and the scientific use of "theory." [2 marks]

Worked Solution

(a) Natural selection (using antibiotic resistance as an example):

  1. Variation: Within a population of bacteria, there is genetic variation due to random mutations. Some bacteria possess alleles that make them resistant to a particular antibiotic, while others do not.

  2. Selection pressure: When an antibiotic is applied to treat a bacterial infection, it creates a strong selection pressure. Bacteria that are susceptible to the antibiotic are killed, while resistant bacteria survive.

  3. Reproduction: The surviving resistant bacteria reproduce (by binary fission), passing on their resistant alleles to their offspring.

  4. Increase in frequency of advantageous allele: Over generations, the proportion of resistant bacteria in the population increases while the proportion of susceptible bacteria decreases.

  5. Evolution: The population evolves -- the frequency of the antibiotic resistance allele in the gene pool has increased over time. This is natural selection in action: organisms with advantageous traits are more likely to survive and reproduce.

(b) Two other types of evidence for evolution:

  1. Comparative anatomy (homologous structures): Structures in different species that share a similar underlying anatomy (same basic bone arrangement) despite different functions are called homologous structures. For example, the pentadactyl limb (five-digit limb) is found in the human arm, the wing of a bat, the flipper of a whale, and the leg of a horse. These suggest these species share a common ancestor; the structures have been modified by natural selection for different functions.

  2. Molecular evidence (DNA/protein comparison): Comparing the DNA sequences or protein sequences (e.g. cytochrome c) of different species shows that more closely related species have more similar DNA/protein sequences. The degree of similarity reflects evolutionary relatedness and can be used to construct phylogenetic trees.

(Alternative: embryological evidence -- early embryos of vertebrates are very similar; biogeographical evidence -- species on oceanic islands are more similar to nearby mainland species than to species on distant islands with similar environments.)

(c) In everyday language, "theory" means a guess or speculation (e.g. "I have a theory about who stole the cookies").

In science, a "theory" is a well-substantiated, comprehensive explanation supported by a large body of evidence, observations, and experiments. The theory of evolution by natural selection is one of the most well-supported scientific theories, with evidence from fossils, genetics, comparative anatomy, molecular biology, and direct observation (e.g. antibiotic resistance). It is not a guess or speculation.

Unit Test 2: Ecological Pyramids and Energy Flow​

Question

(a) Explain why there is a loss of energy at each trophic level in an ecosystem. State the approximate percentage of energy that is transferred from one trophic level to the next. [3 marks]

(b) A pyramid of numbers for a forest ecosystem may appear inverted (wider at the top), whereas a pyramid of biomass for the same ecosystem is always a normal pyramid (wider at the base). Explain this difference. [4 marks]

(c) Explain why a pyramid of energy is never inverted. [2 marks]

Worked Solution

(a) Energy is lost at each trophic level because:

  1. Not all biomass at one level is consumed by the next level (some organisms die and are decomposed).
  2. Some energy is lost as heat through respiration (used for metabolic processes such as movement, maintaining body temperature).
  3. Some energy is lost in undigested material (faeces/egestion) and excretory products (urea).
  4. Some energy is used for growth and reproduction that is not passed on when consumed.

Approximately 10% of the energy at one trophic level is transferred to the next (the 10% rule). The remaining approximately 90% is lost as described above.

(b) Pyramid of numbers in a forest: A single tree (producer) supports thousands of insects (primary consumers), which may support fewer birds (secondary consumers). If you count individual organisms, there may be very few trees but many insects, giving an inverted pyramid. This happens because the pyramid of numbers does not account for the size of organisms.

Pyramid of biomass: Biomass measures the total mass of living material at each trophic level. The total biomass of the trees (producers) is much greater than the total biomass of the insects (primary consumers), even though there are fewer individual trees. Biomass accounts for the size difference, so the pyramid is a normal pyramid (larger at the base).

(c) A pyramid of energy is never inverted because energy always flows in one direction through an ecosystem (from the Sun to producers, then to consumers). Energy cannot be created; it can only be transferred and transformed. At each level, energy is lost (mainly as heat through respiration), so the total energy available at each successive level is always less than at the previous level. There is no situation in which a higher trophic level has more energy than the level below it.

Unit Test 3: The Carbon Cycle and Nitrogen Cycle​

Question

(a) Describe the role of photosynthesis and respiration in the carbon cycle. [3 marks]

(b) Describe the role of decomposers in the carbon cycle. [2 marks]

(c) In the nitrogen cycle, describe the roles of (i) nitrogen-fixing bacteria, (ii) nitrifying bacteria, and (iii) denitrifying bacteria. [6 marks]

Worked Solution

(a) Photosynthesis: Green plants and algae absorb carbon dioxide (CO2CO_{2}) from the atmosphere (or dissolved CO2CO_{2} from water) and incorporate it into organic molecules (glucose, then other organic compounds) through the process of photosynthesis. This converts inorganic carbon into organic carbon, moving carbon from the atmosphere/biosphere into living organisms.

Respiration: All living organisms carry out respiration, breaking down organic molecules (e.g. glucose) to release energy (ATP). As a by-product, CO2CO_{2} is released back into the atmosphere (or into water). This converts organic carbon back into inorganic carbon, returning carbon to the atmosphere.

(b) Decomposers (bacteria and fungi) break down dead organic matter (dead plants, animals, and waste products) through extracellular digestion. They secrete enzymes onto dead material, digesting it and absorbing the soluble products. In doing so, they release CO2CO_{2} back into the atmosphere through their own respiration. Decomposers are essential for recycling carbon (and other elements) from dead organisms back into the ecosystem.

(c) Roles in the nitrogen cycle:

(i) Nitrogen-fixing bacteria: These bacteria convert atmospheric nitrogen gas (N2N_{2}), which is inert and unavailable to most organisms, into ammonium ions (NH4+NH_{4}^{+}) or other nitrogen-containing compounds. This is called nitrogen fixation. Some nitrogen-fixing bacteria live freely in the soil (e.g. Azotobacter), while others live in root nodules of leguminous plants (e.g. Rhizobium), forming a mutualistic relationship.

(ii) Nitrifying bacteria: These bacteria convert ammonium ions (NH4+NH_{4}^{+}) (from decomposition and nitrogen fixation) first into nitrite ions (NO2−NO_{2}^{-}) and then into nitrate ions (NO3−NO_{3}^{-}). This two-step process is called nitrification. Nitrates are the form of nitrogen most readily absorbed by plant roots. Examples: Nitrosomonas (ammonia to nitrite) and Nitrobacter (nitrite to nitrate).

(iii) Denitrifying bacteria: These bacteria convert nitrate ions (NO3−NO_{3}^{-}) in the soil back into nitrogen gas (N2N_{2}), which is released into the atmosphere. This process is called denitrification and occurs in waterlogged, anaerobic (oxygen-poor) soil conditions. Denitrifying bacteria reduce the amount of nitrogen available to plants.

Integration Test 1: Eutrophication​

Question

A farmer applies excess nitrate fertiliser to a field adjacent to a lake. Over several weeks, the following changes are observed in the lake:

  1. Algal population increases dramatically.
  2. Submerged aquatic plants die.
  3. Fish population decreases.
  4. Water becomes turbid and foul-smelling.

(a) Explain the sequence of events from the application of excess fertiliser to the death of submerged aquatic plants, using the term eutrophication. [5 marks]

(b) Explain why the fish population decreases. [3 marks]

(c) Suggest two farming practices that could reduce the risk of eutrophication, and explain how each practice helps. [4 marks]

Worked Solution

(a) Sequence of events (eutrophication):

  1. Leaching: Excess nitrate fertiliser that is not absorbed by crops is leached (washed) from the soil by rainwater into nearby water bodies (the lake), increasing the nitrate concentration in the water.

  2. Algal bloom: The increased nitrate concentration acts as a nutrient (limiting factor), causing a rapid increase in the growth of algae -- an algal bloom. Algae grow and reproduce rapidly on the surface of the lake.

  3. Light blockage: The dense layer of algae on the surface blocks sunlight from reaching the deeper parts of the lake. Submerged aquatic plants can no longer carry out sufficient photosynthesis due to lack of light.

  4. Death of aquatic plants: Unable to photosynthesise, the submerged plants die. They are broken down by decomposers.

  5. Increased decomposer activity: The dead algae (when they die) and dead plants provide a large amount of organic material for decomposers (bacteria and fungi). Decomposer populations increase dramatically.

  6. Oxygen depletion: The large population of decomposers carries out aerobic respiration, consuming large amounts of dissolved oxygen from the water. The dissolved oxygen concentration drops to very low levels (hypoxia).

(b) The fish population decreases because:

The low dissolved oxygen concentration caused by decomposer respiration means that fish and other aquatic organisms cannot respire adequately. Many fish species require a minimum dissolved oxygen level to survive. As oxygen levels drop:

  • Fish may suffocate and die.
  • Sensitive species die first; more tolerant species may survive temporarily.
  • Dead fish are decomposed, further increasing decomposer activity and oxygen consumption, creating a positive feedback loop that accelerates oxygen depletion.

(c) Two farming practices:

  1. Controlled application of fertiliser: Apply fertiliser at the correct rate, time, and method to ensure crops absorb most of it. Use soil testing to determine the exact amount needed, avoiding excess. This reduces the amount of nitrate available for leaching.

  2. Creating buffer zones: Plant strips of vegetation (trees, grass) between the field and the water body. Buffer zones absorb and filter runoff water before it reaches the lake, trapping nitrates and preventing them from entering the water body.

(Alternative: using slow-release fertilisers; applying fertiliser when rain is not expected; crop rotation with legumes to naturally fix nitrogen, reducing the need for artificial fertiliser.)

Integration Test 2: Speciation and Population Dynamics​

Question

(a) Describe the process of allopatric speciation, explaining the role of geographical isolation and natural selection. [5 marks]

(b) A population of beetles on an island has an initial size of 500. The birth rate is 0.4 per individual per year and the death rate is 0.2 per individual per year. Calculate the population size after one year, assuming no immigration or emigration. [2 marks]

(c) The carrying capacity of the island for these beetles is 2000. Explain what is meant by carrying capacity and describe what would happen to the population growth rate as the population approaches the carrying capacity. [3 marks]

Worked Solution

(a) Allopatric speciation:

  1. Geographical isolation: A physical barrier (e.g. a mountain range, a river, an ocean) separates a population of a species into two or more geographically isolated populations. This prevents interbreeding between the populations (no gene flow).

  2. Different selection pressures: The two isolated populations experience different environmental conditions (e.g. different climate, food sources, predators). Different selection pressures act on each population.

  3. Natural selection: In each population, natural selection favours different alleles and traits that are advantageous in their respective environments. Over many generations, the allele frequencies in each population change in different directions.

  4. Accumulation of differences: Over a long period, the two populations accumulate so many genetic differences that even if they were brought back together, they could no longer interbreed to produce fertile offspring. They have become reproductively isolated.

  5. New species: The two populations are now classified as separate species. This process is called allopatric speciation (speciation that occurs due to geographical separation).

(b) Population change: Birth rate = 0.4 per individual per year; Death rate = 0.2 per individual per year.

Net growth rate = birth rate - death rate = 0.4−0.2=0.20.4 - 0.2 = 0.2 per individual per year.

Population increase in one year = 500×0.2=100500 \times 0.2 = 100 individuals.

Population after one year = 500+100=600500 + 100 = 600 beetles.

(c) Carrying capacity is the maximum population size of a species that an environment can sustain indefinitely, given the available resources (food, water, shelter, etc.) without degrading the environment.

As the population approaches the carrying capacity:

  • Resources become limiting: Food, space, and other resources become scarcer as more individuals compete for them.
  • Growth rate decreases: The population growth rate slows down and eventually stabilises at approximately zero when the population reaches the carrying capacity. The population fluctuates around the carrying capacity rather than continuing to grow exponentially.
  • This pattern produces a sigmoidal (S-shaped) population growth curve: rapid growth initially (exponential phase), followed by a slowing of growth (deceleration phase), and finally a stable phase where births approximately equal deaths.