For Charles Darwin, "species" was an undefinable term, "one arbitrarily given for the sake of convenience to a set of individuals closely resembling each other." That hasn't stopped scientists in the 150 years since then from trying, however. When scientists today sit down to study a new form of life, they apply any number of more than 70 definitions of what constitutes a species--and each helps get at a different aspect of what makes organisms distinct.

In a way, this plethora of definitions helps prove Darwin's point: The idea of a species is ultimately a human construct. [...]

Perhaps the most classic definition is a group of organisms that can breed with each other to produce fertile offspring, an idea originally set forth in 1942 by evolutionary biologist Ernst Mayr. While elegant in its simplicity, this concept has since come under fire by biologists, who argue that it didn't apply to many organisms, such as single-celled ones that reproduce asexually, or those that have been shown to breed with other distinct organisms to create hybrids.