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A species is a group of interbreeding organisms that are capable of, and do under natural conditions, reproduce fertile offspring. The key words/phrases are fertile and "under natural conditions." Members of different species, so defined, may mate and even produce live offspring, but if the offspring are infertile, we still say they belong to different species (mules, which are the result of horse/donkey crosses, and ligers, which are the result of lion/tiger crosses). Members of different species, so defined, may be capable of producing fertile offspring under artificial conditions, but do not do so in the wild (domestic/wild ducks who are kept in captivity). With living populations, researchers can test this. They can see plants and animals reproduce normally and have fertile offspring. It is not always easy to define, however, even in living populations. With fossil populations, it is more difficult and often somewhat arbitrary, since researchers can't test reproductive compatibility directly. If two similar fossils exhibit the same amount of morphological difference as two closely related but distinct living species, then researchers assign the fossils to different species. Assigning fossil specimens to species is not an exact science.
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