Scientists use taxonomy, which is a fairly rigid system. Generally a new species is born when it is sufficiently different from the species that it evolved from that the two can’t breed.
Taxonomy is useful for scientists, but not so much for casual conversation. For example there is no such thing as a fish in evolutionary science. For regular speech an animal is unique if the average person thinks it is. Cows are different from bison because we treat the animals differently, not because of any scientific basis.
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