The Elements of Evolution (48-2) Variation

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Variation blossoms into speciation after interbreeding generally ceases. Populations diverge into distinct species by relative isolation, however achieved, and by adaptation to niche habitats. Isolation may happen geographically or behaviorally. Behaviors such as feeding strategy, mate selection, and even breeding timing form bases for speciation. Over time, associative selection and genetic drift differentiates a new species. In referring to allele frequency, genetic drift is also called allelic drift.

In many instances, what starts as behavioral differences and breeding choices turn into mechanical isolation: the original and descendant species are no longer breeding-compatible. Sometimes hybrids do not develop properly. Crosses of sheep and goats do not survive to reproductive age. In other instances, sterility/infertility results. Mules, the offspring of a male donkey and a mare (female horse), cannot reproduce; and so with hinnies, which are the offspring of a stallion (male horse) and a female donkey.