The formation of new species from a common ancestor in the course of evolution. Within a species, gene flow between populations gets less for some reason (eg. geographical isolation), selection pressures differ, genetic drift acts relatively rapidly if a population is small, and populations diverge. Species are created within the ancestral range when a population exploits a slightly different niche, the habitat fragments, a hybrid persists or ploidy changes.

This definition is abridged from A – Z of tree terms: A companion to British arboriculture.
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