zarahh09
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How does geographic isolation give a rise in new species
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simxne_
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- When populations become geographically isolated, they cannot interbreed with each other/will have separate gene pools.
- They will be exposed to different selection pressures, resulting in natural selection* taking place.
- Natural selection:
  • Either mutations will occur independently within a population or directional selection will occur (a single extreme phenotype is favoured due to the selection pressure).
  • Selected organisms will survive and reproduce/pass on their alleles to their offspring. This will lead to a change or an increase in the allele frequency.

This leads to differences in the gene pools of the separated populations, changing the phenotype frequencies. Eventually, individuals from different populations will be unable to interbreed and produce fertile offspring = reproductively isolated. There are now separate species.

*Instead of natural selection, genetic drift can also occur - organisms reproducing and passing on alleles is due to chance rather than environmental factors/selection pressures. Genetic drift usually occurs within smaller populations.

I'm not sure if this is a great explanation, but hope it makes sense and is helpful anyway.
Last edited by simxne_; 2 months ago
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OxFossil
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(Original post by simxne_)
- When populations become geographically isolated, they cannot interbreed with each other/will have separate gene pools.
- They will be exposed to different selection pressures, resulting in natural selection* taking place.
- Natural selection:
  • Either mutations will occur independently within a population or directional selection will occur (a single extreme phenotype is favoured due to the selection pressure).
  • Selected organisms will survive and reproduce/pass on their alleles to their offspring. This will lead to a change or an increase in the allele frequency.

This leads to differences in the gene pools of the separated populations, changing the phenotype frequencies. Eventually, individuals from different populations will be unable to interbreed and produce fertile offspring = reproductively isolated. There are now separate species.

*Instead of natural selection, genetic drift can also occur - organisms reproducing and passing on alleles is due to chance rather than environmental factors/selection pressures. Genetic drift usually occurs within smaller populations.

I'm not sure if this is a great explanation, but hope it makes sense and is helpful anyway.
That looks about right. To put it another way:

If an original population gets split up by a geographical barrier (like the clearing of a forest across the middle of a species range, or a mountain range, a river, etc), and prevented from joining back up together again, then each of the new populations will tend to evolve according to whatever the conditions in their new environment favour. So if one population gets stuck on the sunny side of a mountain, and the other part of the population gets stuck on the cold side, the "sunny side" might favour alleles and mutations that help with cooling (like a thinner fur coat), whilst the "cold side" might favour those that help retain warmth (like a thicker fur coat). Over time, the differences in gene frequencies might get so extreme that even if the two populations join up again, they can no longer interbreed. You now have 2 new species - that is speciation.

Another mechanism is due to a "founder effect". This is where the population that separates contains a different mix of alleles just by chance (for example, all the individuals with the Z allele might find themselves on the sunny side of the mountain, when they are cut off). This means the two populations have a different range of alleles for natural selection to work on. Speciation can occur over time when selection operates on these two different mixes.
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