species

Meaning: In the biological species concept, a species is a group of populations whose members interbreed in nature and produce viable, fertile offspring. Members are reproductively isolated from other species. Isolation can be prezygotic (habitat, temporal, behavioral, mechanical or gametic barriers) or postzygotic (hybrid inviability, hybrid sterility, hybrid breakdown). Speciation occurs when gene flow between populations is reduced enough for them to diverge, for example after a geographic barrier (allopatric) or through divergence within the same range (sympatric).

On the MCAT: A passage describes two populations and asks whether they are separate species. Check whether they mate in nature and whether the offspring are both viable and fertile. A horse and a donkey produce a mule, which is usually sterile, so they count as separate species. You may also be asked to predict how a barrier, a change in mating season or a change in courtship behavior affects gene flow and speciation.

Common confusion: Producing a hybrid does not make two groups one species; the offspring must be viable and fertile. Different dog breeds are one species because they can produce fertile offspring. The biological concept is a working definition with limits: it fits asexual organisms and fossils poorly, and some distinct species occasionally produce fertile hybrids. Do not mix up species with population, which is a local group within a species.