speciatsioon

Speciatsioon, commonly called speciation, is the evolutionary process by which one lineage splits into two or more reproductively isolated lineages, leading to the formation of distinct species. It involves the divergence of populations and the reduction or elimination of gene flow between them.

There are several pathways by which speciation can occur. Allopatrische (allopatric) speciation happens when geographic barriers physically separate populations, allowing independent genetic drift and selection. Sympatrische speciation occurs within the same geographic area, often driven by disruptive selection, habitat specialization, or polyploidy (especially in plants). Parapatrische (parapatric) speciation arises along a contact zone or environmental gradient, with limited gene flow. Peripatrische (peripatric) speciation is a special case where a small peripheral population diverges from the main range.

Reproductive isolation is central to speciation. Prezygotic barriers prevent mating or fertilization (temporal, habitat, behavioral, mechanical, or gametic isolation). Postzygotic barriers reduce the viability or fertility of hybrids (hybrid inviability or sterility), reinforcing isolation after mating occurs.

Speciation rates can be gradual or relatively rapid, influenced by population size, gene flow, selection, mutation, and ecological opportunity. Plants frequently show polyploid speciation, where chromosome duplication creates instant reproductive isolation. Species concepts differ in defining what constitutes a species; the Biological Species Concept emphasizes reproductive isolation, while phylogenetic and other concepts use lineage history and diagnostic traits.

Examples include allopatric radiations in island systems and rapid sympatric radiations in certain cichlid fishes, illustrating the diversity of pathways by which speciatsioon can unfold.