Speciation is the formation of a new biological species resulting from a population splitting into two daughter populations, and each daughter population being reproductively isolated from the other. Once reproductively isolated, each daughter population faces different mutations, selective pressures, and patterns of gene flow and genetic drift, leading to independent changes in allele frequency, and independent evolutionary changes that could lead to the formation of the new species.

A biological species is broadly defined as a group of organisms who are capable of interbreeding and producing fertile offspring.

There are four key mechanisms of speciation:


The populations are split by some geographical barrier, such as a mountain range or a body of water, that prevents reproductive contact between them. As a result, the forces of mutation, selection, gene flow and genetic drift in each population is different and the two populations undergo different evolutionary changes. Eventually the two populations diverge to such an extent that they are no longer reproductively compatible with one another. This means that two new species have been formed.


This involves the isolation of a small population that has broken off from a larger one. In the smaller population, there is a greater incidence of genetic drift and this is
believed to be a key factor in the rapid evolutionary change and eventually speciation of the smaller population. This concept is closely related to the founder effect.


In this form of speciation, the two 'isolated' populations are not completely physically isolated and there may be some contact and interbreeding between the two. However, interbreeding between the two populations leads to offspring with reduced fitness and so selective pressures for certain behaviours or mechanisms ensures that interbreeding is kept largely within each population, thus leading to the same eventual evolutionary divergence between them as seen in all types of speciation.

This is speciation where there is no geographic isolation between populations. Instead the mechanism of isolation may be behavioural (for instance, the mating ritual of one group no longer becomes attractive to the other); mechanical (incompatibility of gametes or genitals, or premature termination of embryos); or perhaps temporal (different flowering times or mating seasons). Thus, although the two 'populations' co-habit, their genetic lineages undergo different patterns of selection, gene flow and genetic drift, and consequently they evolutionarily diverge and often form new biological species.