Species and reproductive isolation

Explanation

The biological species concept groups natural populations whose members can interbreed and produce viable, fertile offspring, while being reproductively isolated from other such groups. It focuses on gene flow rather than appearance alone.

The concept is difficult to apply to fossils, asexual organisms and geographically separated populations whose mating cannot be observed. Hybridisation between otherwise distinct groups also complicates sharp boundaries. A classification should state which evidence and species concept are being used.

In allopatric speciation, a geographical barrier reduces gene flow. Mutation, selection and other population processes can then produce divergence. If reproductive barriers persist when contact resumes, separate species may have formed. Geographical separation alone does not instantly create a new species.

In sympatric speciation, reproductive isolation develops within the same geographical region. Behavioural differences such as mating signals or timing, and physiological differences affecting compatibility, can reduce interbreeding. Host preference or ecological differentiation can contribute; in some plants polyploidy provides a further mechanism, though the central required idea is isolation without a geographical split.

Pre-fertilisation barriers can prevent mating or fertilisation; post-fertilisation barriers can reduce hybrid survival or fertility. Explain the barrier's effect on gene exchange rather than merely naming it.

Adaptive radiation is the rapid evolution of many species from one ancestral species, each adapted to a different niche. It often follows the colonisation of a new area with many unoccupied niches and few competitors, such as an island group. Populations on different islands or using different food sources are isolated, face different selection pressures and diverge. The finches of the Galapagos Islands, with beaks suited to different foods, are a well-known example.