speciation

Speciation is the evolutionary process by which populations evolve to become distinct species. It involves the development of reproductive isolation, so that gene flow between diverging populations declines and may become effectively zero. Over time, accumulated genetic differences, ecological divergence, and reproductive barriers lead to separate evolutionary lineages.

There are several modes of speciation. Allopatric speciation occurs when a geographic barrier creates isolated populations that diverge, through drift and local adaptation. Peripatric is a small, isolated peripheral population. Parapatric speciation occurs when neighboring populations experience limited gene flow and divergent selection across a gradient. Sympatric speciation happens without geographic isolation, often via ecological specialization or chromosomal changes such as polyploidy in plants.

Speciation is driven by the evolution of reproductive barriers. Prezygotic barriers prevent mating or fertilization (temporal, habitat, behavioral, mechanical, or gametic isolation). Postzygotic barriers reduce the fitness of hybrids (hybrid inviability, sterility, or breakdown).

Genetic divergence accumulates with restricted gene flow, aided by natural selection, genetic drift, mutation, and potentially reinforcement when hybrids have low fitness. Polyploidy can produce instant speciation, especially in plants.

Evidence includes ring species, adaptive radiations such as Darwin’s finches and cichlid fishes, and observed cases of polyploid speciation. The ecological speciation framework emphasizes how divergent natural selection across environments can drive reproductive isolation.

In genomics, speciation may involve heterogeneous genomic divergence, with islands of differentiation amid shared backgrounds.

Understanding speciation provides insight into biodiversity, evolutionary history, and conservation, including managing hybridization and preserving distinct lineages.