Selection, Genetic Drift and Speciation

Natural selection changes allele frequencies when heritable phenotypes differ in reproductive success. Directional selection favours one end of a distribution, stabilising selection favours intermediate phenotypes, and disruptive selection favours different extremes over intermediates under the stated conditions.

Genetic drift involves chance changes in allele frequency due to finite sampling of reproductive contributions. Its relative effect is especially important in small populations, where the loss or reproduction of a few individuals can greatly change the gene pool. An allele can increase by drift without providing an advantage; a useful trait is not guaranteed to escape chance loss.

A geographical barrier can reduce gene flow between populations. Mutation, different selection pressures and genetic drift can then produce differences in their gene pools. Over many generations, divergence may create reproductive barriers, so members can no longer interbreed successfully to produce fertile offspring in the biological-species model.

Geographical separation is therefore an initiating condition, not automatically the final proof of a new species. If separated populations can still interbreed freely when reunited, the evidence for completed reproductive isolation is weaker. Explain the inherited differences and their reproductive consequence.

Sympatric speciation involves reproductive separation without an initial geographical barrier. Differences in ecology, mating behaviour or breeding time can reduce gene flow within a shared region; chromosome-number changes such as polyploidy can also contribute in plants under suitable circumstances.

Disruptive selection may favour divergent phenotypes, but does not by itself guarantee two species unless interbreeding between the groups becomes sufficiently restricted. Evidence should address mating, offspring fertility or gene flow as well as traits. Phenotypic variation can also reflect environment, so do not assume every visible difference is an inherited reproductive barrier.

Try each question before opening the worked response. Use precise biological terms, connect each cause to its effect, and check units and assumptions when interpreting data.