Speciation happens when populations of an existing species become reproductively isolated, diverge genetically and eventually can no longer interbreed to produce fertile offspring. It therefore splits one species into two or more species.
The Mechanism
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A population contains heritable variation, produced by mutation and sexual reproduction.
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A barrier reduces or prevents gene flow, which is the transfer of alleles between populations through reproduction. For example, the formation of a river may geographically isolate two populations.
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The isolated populations experience different selection pressures, such as climate, food supply, predators or competition.
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Individuals with advantageous heritable traits have greater fitness, meaning greater reproductive success. Natural selection therefore changes the populations in different ways.
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Mutations, natural selection and genetic drift cause differences in allele frequencies to accumulate over many generations.
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Eventually, reproductive barriers prevent successful interbreeding even if the populations meet again. Speciation is then complete.
The IB example is the separation of bonobos and common chimpanzees by the Congo River. Geographic isolation restricted gene flow, while differential selection contributed to divergence.
Case | How isolation occurs | IB level |
|---|---|---|
Geographic isolation | A physical barrier separates populations, allowing divergence under different selection pressures | SL and HL mechanism |
Allopatric speciation | Speciation occurs in geographically separated populations | HL terminology |
Sympatric speciation | Reproductive isolation develops without geographic separation, for example through polyploidy in plants | HL extension |
A common misconception is that organisms deliberately adapt because they need to become a new species. This is incorrect: mutations are random, individuals do not evolve, and natural selection changes the heritable characteristics of populations over generations.
Exam Technique
For an explain question, present a linked sequence: isolation → reduced gene flow → different selection pressures → differential survival and reproduction → genetic divergence → reproductive isolation. Do not state that geographic separation alone creates a new species; the decisive outcome is reproductive isolation.
