Species Definitions

The concept of a species is something that almost everyone understands intuitively.  However, within the field of evolutionary biology there are a number of definitions that have fairly subtle differences.  The following are some examples of definitions that have been offered recently.

Species: A reproductively isolated aggregate of interbreeding populations. (Mayr. 1970)

Species: The members, in aggregate, of a group of populations that interbreed or potentially interbreed under natural conditions. (Futuyma, 1978)

Species: In the sense of biological species, the members in aggregate of a group of populations that interbreed or potentially interbreed under natural conditions; a complex concept.  Also, a basic taxonomic category to which individual specimens are assigned, which often but not always corresponds to the biological species. (Futuyma, 1986)

Species: A species is a group of populations whose evolutionary pathway is distinct and independent from that of other groups; a distinct and independent path is achieved by the group's reproductive isolation from other groups.  (Futuyma & Mayer, 1980)

Chronospecies: (or successional species, paleospecies, evolutionary species) A lineage constructed from fossil data that exhibits sufficient evolutionary change that a taxonomist deems it appropriate to divide it into two intergrading species. (Stanley, 1979)

Morphospecies: Two populations differing in one or more characteristics to a statistically different degree to be called a species. (Raup & Stanley, 1978)

Biological Species: Groups of actually or potentially interbreeding populations genetically isolated from other groups by one or more reproductive isolating mechanisms; biospecies. (Lincoln, et al., 1982)

Cohesion Species:  The most inclusive population of individuals having the potential for phenotypic cohesion through intrinsic cohesion mecahnisms.

Genotypic Cluster: Species are recognized by morphological and genetic gaps between populations in a local area rather than by means of the phylogeny, cohesion, or reproductive isolation that are responsible for these gaps (Mallet 1995). In a local area, a single species is recognized if there is but a single cluster in the frequency distribution of multilocus phenotypes and genotypes. Separate species are recognized if there are several clusters separated by multilocus phenotypic or genotypic gaps. These gaps may be entirely vacant, or they may contain low frequencies of intermediate genotypes, or hybrids.