Speciation and the Rate of Evolution
Explain how reduced gene flow can let populations diverge and how reproductive isolation can produce new species. Compare evidence from living populations and fossils, and explain why evolutionary rates vary.
What a learner can do afterwards
- Describes a sequence from one population to two species and names the isolating step.
- Says what evidence would show that two similar-looking populations are now separate species.
- Gives one reason why evolution can be fast in one period and slow in another.
1 · Read
For sexually reproducing organisms, a biological species groups populations that can normally interbreed and produce fertile offspring. One route to speciation starts when a barrier stops gene flow, the exchange of genes through breeding. Mutation, selection, and chance can make the populations differ. If reproductive barriers develop, the groups may become separate species.
A river or mountain can be a geographic barrier. Different mating signals or breeding seasons can be reproductive barriers, as can hybrids that cannot survive or reproduce. In living populations, breeding patterns and DNA help identify species. Two sterile individuals alone prove little. For ancient fossils, breeding cannot be tested and DNA is usually unavailable, so scientists compare anatomy and other fossil evidence.
Imagine a river that squirrels cannot cross. Their populations diverge over generations. Different breeding seasons then prevent mating even if they meet again. Geographic separation has been followed by reproductive isolation; separation alone did not guarantee speciation.
New food sources or habitats can favour rapid adaptation and speciation, as in island finches. Stable pressures can favour similar traits for long periods. Neither islands nor continents have a fixed evolutionary speed. After mass extinctions, survivors may diversify into available ecological roles, called niches. Fossils record both gradual change and faster bursts.
Reduced gene flow can let populations diverge. Reproductive isolation can produce new species, but neither isolation nor time guarantees a split.
Rewritten readings were written for this dataset.
More to read
How populations become separate species
A barrier can reduce gene exchange between populations. Mutations, natural selection and chance genetic changes may increase their differences. Different mating signals or breeding times can limit mating between groups. Reduced hybrid survival or fertility can also restrict gene flow. Separation alone need not produce species: populations may reunite and interbreed.
Why evolution changes pace
Evolution proceeds at different speeds. Changes in available food may favour different beaks among island finches, while stable pressures may preserve similar traits over long periods. Sharks continue evolving even when they retain ancient features. Fossils show slow shifts as well as quicker bursts. Following extinction, new opportunities may help surviving groups diversify, although recovery takes time.
Selection, mixing and species boundaries
Different surroundings may favour different heritable features. Migration of genes between populations mixes them, whereas selection in different directions increases differences. Strong barriers to reproduction may produce separate species. Researchers examine DNA and reproduction in living groups. Fossil comparisons usually depend on structure, geological age and place of discovery, since breeding tests are impossible and DNA is usually unavailable.
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