Convergent evolution and co-evolution
Convergent evolution
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Convergent evolution is the process by which unrelated species independently evolve similar features, because they experience similar selection pressures.
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It is the opposite of divergence:
- Divergence — one ancestor, becoming more different.
- Convergence — different ancestors, becoming more similar.
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The evidence for convergence is analogous structures:
- An analogous structure performs the same function in different species but has a different underlying anatomy and embryonic origin.
- The wing of a bird, a bat and an insect all achieve flight, but a bird wing is supported by a modified forelimb with feathers, a bat wing by elongated finger bones with a skin membrane, and an insect wing by a chitinous outgrowth with no bones at all.
- Same job, completely different construction — so the similarity was not inherited.
Homologous or analogous? The distinction that matters
| Homologous | Analogous | |
|---|---|---|
| Structure | Same underlying anatomy | Different underlying anatomy |
| Function | May differ | Same |
| Origin | Inherited from a common ancestor | Evolved independently |
| Evidence of | Divergent evolution, shared ancestry | Convergent evolution, similar selection |
- The test is always internal structure and embryonic development, never outward appearance.
- This matters for classification: species must be grouped by homologous features, because only those reflect ancestry. Grouping by analogous features would put dolphins with sharks.
Why convergence happens
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Convergence occurs because there are a limited number of workable solutions to a given physical problem.
- Any fast-swimming predator faces the same physics of drag, so a streamlined, fusiform body with fins is favoured — in sharks (fish), ichthyosaurs (reptiles) and dolphins (mammals), three unrelated groups.
- Any organism storing water in a hot dry place faces the same problems, so cacti (Americas) and euphorbias (Africa) independently evolved thick water-storing stems, spines and reduced leaves.
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The physical constraint, not shared ancestry, produces the similarity.
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Convergence in New Zealand:
- Flightlessness evolved independently in several unrelated NZ bird lineages — kiwi, kākāpō, takahē, weka, moa — under the same selection pressure of costly flight and absent mammalian predators.
- Wētā converge on the ecological role of small rodents elsewhere, occupying a similar niche without any relationship to mammals.
- The divaricating growth form appears in around 60 NZ plant species from many unrelated families — a convergent response to the same browsing pressure from moa.
Co-evolution
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Co-evolution is the process by which two species exert reciprocal selection pressure on each other, so that each evolves in response to the other.
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The requirement is that the relationship runs both ways: a change in species A alters selection on species B, and the resulting change in B alters selection on A again.
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Co-evolution arises from close ecological relationships, so it applies to the relationships covered in AS91603:
- Mutualism — plants and their pollinators. A flower's shape, colour, scent and nectar reward evolve in response to its pollinator, and the pollinator's mouthparts, sensory abilities and behaviour evolve in response to the flower.
- Exploitation — predator and prey, or parasite and host, in an evolutionary arms race. Improved prey defence selects for improved predator ability, which selects for improved defence again.
- Herbivory — plants evolve chemical defences, herbivores evolve detoxifying enzymes, plants evolve new compounds.
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New Zealand examples:
- Flax (harakeke) and tūī: the flower's tube length, sturdy perch-like structure and copious nectar suit a bird pollinator, while nectar-feeding birds have brush-tipped tongues suited to such flowers.
- Dactylanthus and the short-tailed bat — ground-level, strongly scented flowers matched to a ground-foraging, night-active pollinator.
- Divaricating shrubs and moa — plant structure and browser feeding method each shaping the other.
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Co-evolution creates dependency, which is why it matters for conservation: if one partner is lost, the other may have no alternative, as with the large-fruited trees now relying on the kererū alone.
Selective advantage
- Convergence. The advantage belongs to the trait in its environment: a streamlined body reduces drag, so the animal expends less energy per distance travelled and catches more prey, leaving more offspring. The same advantage exists for any lineage facing the same physics, which is precisely why the trait evolves repeatedly.
- Co-evolution in mutualism. Each partner gains: a flower matched to a reliable pollinator wastes less pollen and nectar, achieving more successful fertilisations; a pollinator matched to a rewarding flower obtains more energy per visit.
- Co-evolution in exploitation. Each side's advantage is at the other's expense — better defence means surviving to reproduce, better attack means more food — which is why the arms race continues rather than settling.
Worked Example
Worked Example
Two burrowing mammals are compared: one from Australia, one from Africa. They are not closely related — one is a marsupial, one a placental.
- Both have cylindrical bodies, greatly reduced eyes, short dense fur, and enlarged front limbs with strong claws.
- Their forelimb bones show the same set of elements — humerus, radius, ulna, carpals, digits — but in very different proportions, and the digging surface is formed from different bones in each.
- DNA analysis shows they are in different mammalian lineages that separated over 100 million years ago.
Explain the evolutionary process shown, and explain whether the limbs are homologous or analogous.
Answer:
Identifying the process: convergent evolution.
The two species are unrelated, separating over 100 million years ago, yet they have independently evolved very similar features. This is convergent evolution — unrelated species evolving similar features because they experience similar selection pressures.
The shared selection pressure is the burrowing niche, and each feature is explained by it:
- A cylindrical body minimises cross-sectional area, so less soil must be displaced and less energy is used per metre of tunnel.
- Reduced eyes are favoured because vision is useless underground. Eyes are metabolically costly and vulnerable to damage and infection from soil, so individuals investing less in them save energy and suffer less injury.
- Short dense fur does not catch or drag in soil, reducing friction and preventing soil packing against the skin.
- Enlarged forelimbs with strong claws generate the force needed to excavate.
Individuals with each of these features dug more efficiently, so they expended less energy, reached more food and were exposed to predators for less time. They therefore survived and reproduced more, and the alleles increased in frequency — independently in each lineage, because the same physical problem favoured the same solution.
Are the limbs homologous or analogous?
This is the interesting part, because the answer is both, at different levels — and saying so precisely is what distinguishes a strong answer.
The limbs are homologous as limbs. Both are built from the same set of bones — humerus, radius, ulna, carpals, digits — because both are tetrapod forelimbs inherited from a shared tetrapod ancestor. That common inheritance is why the same elements appear at all.
The digging adaptations are analogous. The proportions differ markedly and the digging surface is formed from different bones in each species. So the specific modification for digging was not inherited from a common ancestor — it evolved separately in each lineage. Same function, different construction: analogous.
Why this distinction matters. The two levels support different conclusions:
- The homologous limb skeleton is evidence of common ancestry — it tells us both are tetrapods, and it is the kind of feature classification must be based on.
- The analogous digging structures are evidence of convergence — they tell us about the environment, not about relatedness.
If we classified these animals by their digging features, we would wrongly place them as close relatives. Classification must therefore use homologous features only, since only those reflect descent. That is exactly why the DNA evidence is decisive here: it settles relatedness independently of the misleading external similarity.