Divergent evolution and adaptive radiation
Divergent evolution
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Divergent evolution is the process by which populations sharing a common ancestor become increasingly different over time.
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It happens whenever isolated populations experience different selection pressures or diverge through drift — so every speciation event is an instance of divergence.
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The evidence for divergence is homologous structures:
- A homologous structure has the same underlying anatomy and embryonic origin in different species, but may perform different functions.
- The classic case is the pentadactyl limb — the five-boned limb plan of tetrapods. The same arrangement of humerus, radius, ulna, carpals and digits appears in a human arm, a bat wing, a whale flipper and a horse leg.
- The functions are completely different — grasping, flying, swimming, running — yet the structure is the same, which only makes sense if all were inherited from a common ancestor and then modified.
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Homology is evidence of shared ancestry. Similar structure despite different function is exactly what descent with modification predicts, and it is difficult to explain any other way.
Adaptive radiation
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Adaptive radiation is the rapid divergence of one ancestral species into many species, each adapted to a different niche.
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It is divergent evolution happening quickly and repeatedly from a single starting point.
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Three conditions make it likely, and questions usually supply at least one:
- Ecological opportunity — many unoccupied niches are available, so there is little competition from established species.
- Access to those niches — the ancestor reaches them, typically by colonising new territory.
- Isolation between the diverging populations, so gene flow does not mix them back together.
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The commonest settings are therefore:
- Island archipelagos, where each island isolates a population and offers unoccupied niches.
- After a mass extinction, when many niches are suddenly vacant.
- After a key innovation — a new trait such as flight or seeds that opens a whole set of previously unavailable niches.
Adaptive radiation in New Zealand
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New Zealand is an outstanding example, because it combined long isolation, no land mammals, and a geologically active, varied landscape.
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Empty niches. With no terrestrial mammals other than bats, niches occupied elsewhere by mammals were vacant. They were filled instead by:
- Birds — moa as large browsers, takahē as a grazer, kiwi as a ground-foraging insectivore probing soil, kākāpō as a flightless herbivore.
- Invertebrates — wētā filling roles taken by rodents elsewhere, some reaching very large body size.
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Plant radiations. Veronica (the NZ hebes) radiated into around 100 species, from coastal to alpine, shrub to tree. Coprosma and the alpine daisies show the same pattern.
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Why flightlessness recurred. Flight is metabolically expensive and its main benefit is escape from predators. With no mammalian predators, that benefit was small, so selection favoured losing flight and diverting the energy to growth and reproduction. Flightlessness evolved independently, several times, in unrelated NZ birds.
Selective advantage
- In a radiation, the advantage is reduced competition through specialisation:
- An individual using a resource that few others exploit obtains more food per unit of effort than one competing for a contested resource.
- More energy assimilated means more allocated to reproduction, so it leaves more offspring.
- Over generations each population becomes more specialised to its own niche, and the specialisation reinforces the isolation.
- This is the same logic as resource partitioning in AS91603, operating over evolutionary rather than ecological time.
Worked Example
Worked Example
An island chain formed by volcanic activity over the last 5 million years is colonised by a single species of small bird.
- The chain now holds 9 closely related species, each on one or two islands.
- Beak shapes differ markedly: some are short and deep (seed-crushing), some long and slender (nectar-feeding), some fine and pointed (insect-gathering).
- DNA analysis shows all 9 are more closely related to each other than to any mainland species.
- The oldest island has the most species; the youngest has one.
- No mammals, and few other bird species, are present.
Explain the pattern shown and how it arose.
Answer:
Identifying the pattern. This is an adaptive radiation — the rapid divergence of one ancestral species into many, each adapted to a different niche. It is a case of divergent evolution, since all 9 species share a common ancestor and have become increasingly different.
Why we can be confident they share one ancestor. The DNA evidence is decisive: all 9 are more closely related to each other than to any mainland species. If they had arrived as 9 separate colonisations, each would be most closely related to its own mainland relative. A single ancestral colonist is the only explanation consistent with the genetic data.
How the radiation occurred.
1. Ecological opportunity. The islands were newly formed and biologically empty. With no mammals and few other birds, a great many niches — seed-eating, nectar-feeding, insect-gathering — were unoccupied. A colonist faced almost no competition, so populations could exploit resources that would be contested on the mainland.
2. Repeated isolation. The chain provides multiple islands separated by sea. Birds colonising a new island were geographically isolated from those on other islands, so gene flow stopped between them. This is allopatric speciation, and the chain supplies the barrier repeatedly.
3. Divergent selection. Each island differs in the food available. On an island where hard seeds predominate, birds with deeper, stronger beaks crack them more efficiently, obtain more energy, and leave more offspring — so beak depth increases over generations. Where nectar predominates, longer, more slender beaks are favoured instead. Because each population is isolated, each adaptation persists rather than being diluted by gene flow.
4. Drift. Each colonisation began with few individuals, so the founder effect made each new population genetically unrepresentative from the start, and continuing drift in small populations added further divergence.
5. Reproductive isolation. Over generations the accumulated differences produced intrinsic isolating mechanisms. Beak shape itself contributes, since it affects feeding and often song and courtship, so birds increasingly mate only with similar individuals.
Why the age pattern supports this. The oldest island has the most species and the youngest has one. This is exactly what a radiation predicts: species accumulate over time, through repeated cycles of colonisation, isolation and divergence. An older island has had more time for those cycles. If the species had arrived independently from the mainland, there would be no reason for number to track island age — so this observation discriminates between the two hypotheses rather than merely fitting one.
The selective advantage driving it. Each specialisation is favoured because it reduces competition. A bird with a beak suited to a resource few others exploit obtains more food per unit of foraging effort, so it has more energy for egg production and chick provisioning and leaves more surviving offspring than a generalist competing for contested food.