This is where a change in the environment creates selection pressures which are ideal for two extreme traits.

For example, grass plants found in Welsh copper mines. Normally, copper in the soil kills grass plants. However, a chance mutation created a plant that could cope with these conditions. This plant prospers in contaminated soil but in normal soil it grows more slowly and so loses out to the other variety of grass. Today, these two varieties now live in close proximity to each other.

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Speciation

A species is a group of populations capable of interbreeding with each other. This makes them reproductively isolated from other groups. When two groups are reproductively isolated it means that sexual reproduction is not possible, whether for physical, developmental, behavioural, ecological or temporal reasons.

Speciation is where one species forms two species. In order for this to happen they need to be reproductively isolated. One way in which this can happen is geographically.

Geographical isolation, or allopatric speciation, is where a physical barrier, for example mountains, desert or water, divides a population. This can occur when there’s a natural disaster or due to a number of individuals in a population migrating.

If the two populations are different, in an abiotic or biotic sense, then they’ll evolve differently because they’ll have different selection pressures to contend with. If the environments are similar speciation can still occur due to random genetic drift. Either way, changes in the allele frequencies occur in both populations causing a difference in the gene pools.

If it happens that these two population meet again they can no longer interbreed because they’ve now evolved into two different species and are now reproductively isolated from each other. They might even both be different from the species from which they both originated.