There are a lot of simple things out there that aren't really simple once you start trying to understand and explain them. The concept of "species" is one of those non-simple concepts. I had assumed that I had a good gut understanding of "species" until I read an article called "Speciation," by Andrew P. Hendry, published in the March 12, 2009 edition of Nature (available online only to subscribers). Hendry suggests that the term "species" as a technical classification in the field of biology is "ambiguous and amorphous." He starts by quoting Darwin, from on the origin of species: In short, we shall have to treat species in the same manner as those naturalists treat genera, who admit that genera are merrily artificial combinations made for convenience. This may not be a cheering prospect; but we shall at least be free from the vain search for the undiscovered and undiscoverable essence of the term species. Hendry suggests that modern biological research has proved Darwin. No universal easily applicable concept of "species" exists; instead, more than two dozen approaches exist with regard to "species." The most common version is the "biological species concept" (BSC). This definition holds that species are "groups of actually or potentially interbreeding individuals that are reproductively isolated from other such groups (that is, they exchange few genes). Hendry elaborates: The BSC is sometimes interpreted to imply the extreme situation where two groups are separate species only when successful hybrids cannot ever be produced-and any two such groups certainly are separate species. But many other groups that are widely accepted to represent separate species frequently violate the strict criteria; for example, some estimates hold that 25% of all plant species and 10% of all animal species hybridize successfully with at least one other species. Probably for this reason, the BSC is often relaxed to the point that different groups are considered separate species if they can maintain their genetic integrity and nature. This more useful, albeit more ambiguous, criterion allows for some genetic exchange (gene flow) between species as long as they do not become homogenized. Hendry then goes on to discuss various challenges to BSC.