WIKIMEDIA COMMONSEvolution is not concerned with species, but with individuals. The survival and reproduction of those individuals that are best adapted to their environment determine the characteristics of subsequent populations, but neither the process nor the theory requires that these populations be organized into species. The formation of species, when it occurs, is a phenomenon that needs to be explained.
On closer inspection, the very notion of a “species” is difficult to formally define. This has been a matter of debate since before Darwin, who himself concluded that “we shall have to treat species in the same manner as those naturalists treat genera, who admit that genera are merely artificial combinations made for convenience” (Origin of Species, Chapter XIV). Many subsequent authors have proposed more formal definitions, known as species concepts, each useful for particular applications but not without idiosyncrasies.
For example, in sexually reproducing populations, the biological species concept refers to distinct groups of organisms that can only mate successfully with other members of the same group. This definition at first seems reasonable, but situations have been observed where members of group A can ...
