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