spéciation

Spéciation, or speciation in English, is the evolutionary process by which new biological species arise. It happens when populations of a single lineage diverge sufficiently to become reproductively isolated, so that gene flow between them declines or stops altogether. Over time, these isolated lineages accumulate genetic differences that reinforce isolation and may lead to distinct ecological roles or geographic ranges.

Different modes of speciation are described in population genetics. Allopatric speciation occurs after geographic separation, either by physical barriers or by colonization of new areas, which reduces gene flow and allows divergence by drift and selection. Parapatric speciation happens when neighboring populations experience limited gene flow and diverge along environmental gradients. Sympatric speciation occurs within a single geographic area, often driven by ecological specialization or chromosomal changes such as polyploidy in plants. Peripatric speciation is a special case of allopatric speciation involving a small, peripheral population that becomes isolated and diverges.

Speciation is driven by genetic mechanisms including natural selection, genetic drift, mutation, and assortative mating, which can reinforce isolation. Reproductive barriers can be prezygotic, such as differences in mating time, habitat, or pheromones, or postzygotic, such as inviable or sterile hybrids. Polyploidy is a particularly common route to speciation in plants, producing immediate reproductive isolation.

Speciation rates vary: some events occur rapidly, others over millions of years. Once established, new species occupy distinct ecological niches and contribute to biodiversity through adaptive radiations and other evolutionary processes. The concept is central to biology and underpins classification and the study of evolutionary history.