thescientist | Evolution 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 mate with group B, and B with C, but not A with
C. Formation of biological species is therefore not an evolutionary
necessity: under some conditions it happens, under others it does not.
Two other species concepts that are frequently employed are ecological species and genetic species.
