THERE'S NO SUCH THING:

Determining that, say dogs and cats, are different species isn't difficult. The two species of carnivorans don't just look and act very different, they also can't produce viable hybrid offspring. 

Most biologists use exactly that criteria to draw the line between species, whether or not the two groups can reproduce and yield fertile hybrids. 

The biological species concept, as it's called, seems simple enough. But it's not perfect.

To resolve such questions of speciation, biologists have dug into the genomes of organisms to see how readily two groups interbreed. But there's even ambiguity there. For example, ornithologists agree that the blue winged warbler and the golden winged warbler are distinct species. But their nuclear ge  nomes show remarkable similarities.

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