Isolation and Speciation

Populations are group of interbreeding individuals of the same species occupying the same habitat. In theory, any individual in a populationis capable of breeding with any other. However, breeding sub-units may become separated in some way, that is, become isolated, and evolve along seperate lines. If, reunited after many generations, the sub-units were found to be incapable of breeding successfully with each other, they would have become a seperate species. Separation by geographical features, habitat changes, change in the body form and changes in breeding mechanisms may lead to the formation of new species.

Speciation

Within a population of one species there are groups of interbreeding individuals. Within each population there are breeding sub-units called Demes. Individuals within a deme tend to breed with each other more often than they do with individuals of other demes. New species arise when some barrier to reproduction occurs so that the gene pool is divided and the flow of genes between seperate demes may cease. Such a barrier which effectively prevents gene exchange between demes is called an isolating mechanism. If, the separation is long-term, eventually the two groups will be so different that two new species incapable of interbreeding are formed. The separate species will each have their own gene pool. This process is called Speciation.

Isolation leading to Speciation

For new species to develop from a population, some form of isolating mechanism is required.

There are two main forms of isloating mechanisms:-

• Geographical Isolation
• Reproductive Isolation

Geographical Isolation

This occurs when populations become physically split into separate demes. The physical barrier may be a mountain or a river or any feature which prevents the population of the same species from interbreeding. The evolution of a new species is very probable, given time. This sort of speciation is known as Allopatric Speciation.

Consider an Isolation model:-

• A population of birds with short flight range feed and breed only in the cool of a valley and the lower slopes of two mountains a considerable distance apart. The birds are only able to breed at a certain temperature provided by the cool conditions. The mountain peaks are too cold for the birds to survive.
• The climate then changes and it gets warmer. The birds tend to inhabit the nearest mountain and become confined to the cool mountain peaks. The birds are split into two…