Evolution by Speciation

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Evolution by Speciation

It is thought that natural selection, as well as altering allele frequencies according to the advantage they give, is the force behind the production of all the different species that have ever lived on Earth.

A group of organisms with similar morphological, physiological and behavioural features, which can interbreed to produce fertile offspring, and are reproductively isolated from other species.

Therefore, donkeys, which look and behave like horses, can breed with horses, but their offspring (mules) are infertile. Donkeys and horses belong to different species.

For one species to form 2 species, they must therefore be reproductively isolated. This may happen because of several reasons.

Isolating mechanisms

Geographical: A population becomes physically separated by a barrier that prevents them from mixing. For example; a stretch of water (as has happened in the Galapagos Islands) or a road being cut into a forest.

In the two areas there could be very different selection pressures, resulting in different alleles being advantageous and thus increasing in frequency.

Over time, the morphological, physiological and behavioural differences are so great that they can no longer interbreed.

Habitat: A population becomes separated because two groups may live on the same mountain but at different altitudes, or in the same area but in differing types of soil.
Seasonal: A population becomes separated because two groups breed at different times.
Behavioural: A population becomes separated because two groups behave differently. For example; one group of birds may sing one song, another group sings a different song and neither group recognises the other.

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