Convergent, Divergent & Punctuated Equilibrium

Q1, Sample Band 6 response (7 marks), annotated

The Theory of Evolution by Natural Selection explains both convergent and divergent evolution as outcomes of the same mechanism acting in different environmental contexts. [1, frames one mechanism, two patterns]

Convergent evolution occurs when unrelated species independently evolve similar features because they live in similar environments and face similar selection pressures. [1, defines convergent evolution] Divergent evolution occurs when related species, descended from a common ancestor, become increasingly different as natural selection adapts them to different environments or niches. [1, defines divergent evolution]

The structures involved differ accordingly. Analogous structures have similar function but different evolutionary origins and are produced by convergent evolution; for example, the streamlined bodies of sharks (fish) and dolphins (mammals). [1, analogous linked to convergence with named example] Homologous structures have the same evolutionary origin but possibly different functions and are produced by divergent evolution; for example, the pentadactyl limb shared by humans, whales, bats and cats, or the different beaks of Darwin's finches. [1, homologous linked to divergence with named example]

The same mechanism produces both patterns. When unrelated populations face the same selection pressure (for example, the demands of fast swimming), natural selection favours similar advantageous variations independently in each lineage, so they converge. [1, explains convergence via natural selection] When populations of a single ancestor face different selection pressures in different niches, natural selection favours different variations in each population, so they diverge, and rapid divergence into many niches is adaptive radiation. The direction (convergence or divergence) depends entirely on whether the selection pressures the lineages face are similar or different. [1, explains divergence and ties direction to environment]

Marking criteria.

• 1 mark Frames both patterns as products of one mechanism.
• 1 mark Correct definition of convergent evolution.
• 1 mark Correct definition of divergent evolution.
• 1 mark Analogous structures defined and linked to convergence with a named example.
• 1 mark Homologous structures defined and linked to divergence with a named example.
• 1 mark Explains how natural selection produces convergence (same pressures select similar traits independently).
• 1 mark Explains how natural selection produces divergence (different pressures select different traits), tying direction to the environment.

Q2, Sample Band 6 response (8 marks), annotated

The radiation of Lake Victoria cichlids is best described as divergent evolution, specifically adaptive radiation. [1, explicit judgement]

Divergent evolution occurs when related species descended from a common ancestor become increasingly different as they adapt to different niches. Adaptive radiation is the rapid divergence of one ancestor into many species filling different niches. [1, defines adaptive radiation and divergent evolution]

When the single ancestral cichlid entered Lake Victoria, the lake offered many empty ecological niches with different available foods (algae, molluscs, the scales of other fish) and conditions. Because each niche presented a different selection pressure, the populations exploiting them were subjected to different environments. [1, role of open niches and differing selection pressures]

Step by step, within each population there was natural variation, for example, in mouth and jaw morphology. In each niche, natural selection favoured the variants best suited to the locally available food: algae-scraping mouths where algae dominated, crushing jaws where molluscs were common, and so on. These individuals survived and reproduced more successfully, so the favourable traits became more common in each population. [1, step-by-step natural selection] Over many generations the populations accumulated different adaptations and became reproductively isolated, eventually forming separate species. [1, divergence to separate species over generations]

This is divergent rather than convergent evolution because all the species share a single common ancestor and have become more different from one another, which is the defining pattern of divergence. The shared body plan and modified structures are homologous (same origin), not analogous. Convergent evolution would instead involve unrelated lineages becoming more similar. [1, evaluates and justifies divergent over convergent]

The speed and scale, approximately 500 species in around 15,000 years, makes this the sharpest known demonstration of adaptive radiation: a single ancestor entering an environment with many open niches, with divergent selection pressure in each niche driving many independent evolutionary trajectories. [1, links scale/speed to adaptive radiation] In conclusion, the cichlid radiation is unambiguously divergent evolution produced by natural selection acting differently in each niche. [1, justified conclusion with precise vocabulary]

Marking criteria (8 marks).

• 1 mark Explicit judgement that this is divergent evolution / adaptive radiation.
• 1 mark Defines adaptive radiation and divergent evolution.
• 1 mark Identifies the role of open niches and differing selection pressures.
• 1 mark Explains natural selection step by step (variation, differential survival, trait frequency change).
• 1 mark Explains divergence into separate species over generations.
• 1 mark Justifies divergent (common ancestor, becoming more different; homologous) over convergent.
• 1 mark Links the scale and speed to adaptive radiation.
• 1 mark Reaches a justified conclusion using precise lesson vocabulary.

Q3, Sample Band 6 response (6 marks)

The claim is flawed in both of its parts. [1, overall evaluative judgement]

Part one (sharks and dolphins must be closely related). This is incorrect. Sharks are fish and dolphins are mammals, so they are not closely related. Their similar streamlined bodies are the result of convergent evolution, both lineages independently evolved similar features in response to the same selection pressure (efficient fast swimming in water). [1, applies convergent evolution to reject close relationship] The shared body shape is made of analogous structures, similar function but different evolutionary origins, so it is evidence of similar selection pressures, not of shared recent ancestry. Shared ancestry is shown by homologous structures, not by surface resemblance. [1, distinguishes analogous from homologous as the correct test of relatedness]

Part two (gradualism must be completely wrong). This is also incorrect. Long periods of stasis in the fossil record support punctuated equilibrium, but they do not disprove gradualism. [1, stasis supports punctuated equilibrium without disproving gradualism] The two models are not mutually exclusive: the fossil record shows gradual transitions in some lineages and long stasis followed by rapid change in others. Both describe different tempos of the same underlying process of evolution by natural selection. [1, explains the models are not mutually exclusive]

Defensible reformulation: “Sharks and dolphins look similar because of convergent evolution to the same aquatic selection pressures (analogous structures), not because they are closely related; relatedness is shown by homologous structures. Long stasis in the fossil record supports punctuated equilibrium, but gradualism and punctuated equilibrium are not mutually exclusive, both patterns occur in different lineages.” [1, biologically accurate reformulation]

Marking criteria.

• 1 mark States an overall evaluative judgement that the claim is flawed.
• 1 mark Uses convergent evolution to reject the close-relationship conclusion for sharks and dolphins.
• 1 mark Distinguishes analogous from homologous structures as the correct evidence of relatedness.
• 1 mark Explains that stasis supports punctuated equilibrium but does not disprove gradualism.
• 1 mark Explains that the two models are not mutually exclusive (both seen in the fossil record).
• 1 mark Reformulates the claim into a biologically accurate statement.