Allopatric speciation
Laura Armstrong
Teacher
Contents
Recall Questions
This topic requires prior knowledge of speciation from GCSE Biology. You can test your knowledge on this below.
What is meant by 'speciation'?
The formation of a new species from a pre-existing species.
What must occur for speciation to happen?
Populations must become reproductively isolated so they can no longer interbreed.
How can geographic barriers affect species?
They can separate populations, preventing gene flow and leading to genetic differences.
Topic Explainer Video
What Is Allopatric Speciation?
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Allopatric speciation occurs when a population is split by a geographical barrier (e.g. river, mountain, desert), leading to reproductive isolation.
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Over time, the separated populations evolve independently, leading to the formation of new species.
The Process of Allopatric Speciation – Step-by-Step
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Geographical Isolation
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A physical barrier separates a population into two or more isolated groups.
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The groups become reproductively isolated, there is no gene flow between the groups.
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Different Environmental Conditions
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Each group experiences different selection pressures (e.g. climate, predators, food sources).
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Natural selection favours different alleles in each population.
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Genetic Divergence
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Mutations occur independently in each population.
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Over generations, allele frequencies change in each group due to natural selection and genetic drift.
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New species formed
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Eventually, genetic changes mean members of the two groups can no longer interbreed to create fertile offspring.
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They are now different species = speciation
Classic Example: Darwin’s Finches
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Finches on the Galápagos Islands became geographically isolated.
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This led to reproductive isolation.
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Different environments with different food availability led to selection for different beak shapes.
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Over time, allele frequencies changed.
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The finches on different islands were no longer able to interbreed to produce fertile offspring → speciation.
Key Terms
- Allopatric speciation: The formation of a new species due to geographical isolation.
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Reproductive isolation: When individuals of different populations can no longer interbreed/ no gene flow.
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Natural selection: The process by which advantageous alleles increase in frequency in a population due to increased survival chances and reproduction.
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Genetic drift: Random changes in allele frequencies, especially in small populations.
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Gene pool: All the alleles in a population.
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Exam Tip
In long-mark questions, many students correctly mention a barrier, but forget to explain how different selection pressures and lack of gene flow lead to reproductive isolation. Always explain: (1) separation, (2) reproductive isolation (3) different selection pressures, (4) allele frequency change, (5) new species formation.
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Describe how allopatric speciation can lead to the formation of new species. (5 marks)
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A geographical barrier separates a population into two isolated groups.
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No gene flow occurs between the populations / they become reproductively isolated.
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Each group experiences different selection pressures in their environments.
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Different mutations arise and natural selection favours different alleles in each group.
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Over time, allele frequencies change.
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Individuals can no longer interbreed to produce fertile offspring.
Practice Question
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!