Patterns of evolution: divergence, convergence and radiation

Divergent evolution

• In divergent evolution, populations of one ancestral species are exposed to different selection pressures, so they change in different directions and become more different over time.
• It usually follows a split in the population (by a barrier or a new niche), so the gene pools can no longer mix and each adapts to its own environment.
• Divergence leaves homologous structures: the same underlying structure, from the shared ancestor, modified for different jobs, such as the five-fingered limb in a human arm, a bat wing and a whale flipper.
• Kea and kākā are close relatives from one ancestor: kea adapted to the cold, open alpine zone and kākā to lowland forest, so they differ in behaviour, diet and plumage.

Convergent evolution

• In convergent evolution, unrelated species living in similar environments face similar selection pressures, so natural selection favours similar features in each.
• The similar features are analogous structures: they do the same job but are built differently and come from different ancestors, like the wings of birds and insects.
• Sharks (fish) and dolphins (mammals) both have streamlined bodies and fins, because water resists fast movement in the same way for both, yet a dolphin's skeleton, lungs and flipper bones are those of a mammal.
• Kiwi fill a niche taken by small mammals elsewhere: they are nocturnal, forage on the ground by smell, have nostrils at the tip of the bill and whisker-like feathers, and have a lower body temperature than most birds.
• Convergence can mislead: moa and emu were once grouped together because both are flightless, but DNA shows flightlessness evolved separately in different ratite lineages.

Adaptive radiation

• Adaptive radiation is rapid divergent evolution of one ancestral species into many species, each adapted to a different niche.
• It is triggered when many niches are open at once: a founder reaches islands with few competitors, new habitats form, or a mass extinction removes the competition (as mammals radiated after the dinosaurs died out 66 million years ago).
• Each group that moves into a new niche faces its own selection pressures, often with small populations and little gene flow, so divergence and speciation are fast.
• New Zealand examples include hebes (about a hundred species), mountain daisies (Celmisia), moa (nine species) and Kikihia cicadas, many of which radiated as the Southern Alps rose over the last few million years and created alpine habitats that had not existed before.

Co-evolution

• Co-evolution is when two species are each a selection pressure on the other, so a change in one selects for a change in the other, over many generations.
• It can be mutualistic (flowers and pollinators), or an arms race (predator and prey, parasite and host), where each improvement in one is answered by the other.
• Pua o te reinga (Dactylanthus, the wood rose) is a parasitic plant that flowers at ground level with strong-smelling, nectar-rich flowers, and is pollinated mainly by the short-tailed bat (pekapeka-tou-poto), unusual among bats in folding its wings to forage on the ground.
• Many New Zealand shrubs grow a tangled, small-leaved divaricating form when young and lose it above about 3 metres, the height a moa could reach; one hypothesis is that this evolved as a defence against moa browsing.

Try it: name the pattern

Gradualism and punctuated equilibrium

• Gradualism is the idea that species change slowly and steadily, by small changes building up over long periods, so the fossil record should show many intermediate forms.
• Punctuated equilibrium, proposed by Eldredge and Gould in 1972, is the idea that species stay much the same for long periods (stasis) and change in short bursts, usually when a small population splits off and speciates.
• Under punctuated equilibrium, few intermediate fossils are expected, because the change happens quickly in a small, local population that rarely fossilises.
• Polyploidy is an extreme punctuated event: a new species appears in one generation.
• Both patterns occur; which one fits a lineage depends on how stable its environment was and how the population split, and a gap in the fossil record can also make gradual change look sudden.

Try it: gradualism or punctuated equilibrium?

What Achieved, Merit and Excellence look like

• Achieved defines the pattern and gives an example; Merit explains how selection produces it in the context; Excellence compares it with the pattern it could be confused with, using evidence to justify the choice.