Species

234 Species

any rate) but not to the definition based on reproductive isolation. It allows that non-
genetic isolation can form species (for example, intracellular infection by Wollbachia),
and further that good species might later hybridize to form a new species. They accept
Mayr’s definition: “species are groups of interbreeding natural populations that are
reproductively isolated from other such groups,”^24 although given their qualifying of
that concept, it should perhaps read mostly interbreeding and isolated.

Evolutionary Species Concepts

Evolutionary species concepts derive from attempts to deal with the time dimension,
something that the biological species concepts of Mayr and Dobzhansky tend to avoid—
for them species exist at a given time horizon, and over evolutionary time, of course, they
can change and split. Paleontologist George Gaylord Simpson proposed a definition:

An evolutionary species is a lineage (an ancestral–descendant sequence of populations)

evolving separately from others and with its own unitary evolutionary role and tendencies.^25

Earlier, in a classic paper, he had expressed it slightly differently:

... a phyletic lineage (ancestral-descendant sequence of interbreeding populations)

evolving independently of others, with its own separate and unitary evolutionary role

and tendencies, is a basic unit in evolution.^26

According to Cain, Simpson characterized the intergrading forms of an evolu-
tionary species as “transients.”^27 Species are thus many things: they are popula-
tions that form phyletic lineages through interbreeding, and which have independent
evolutionary roles and independent evolutionary tendencies. These properties are
reiterated in a later version of the evospecies concept of E. O. Wiley:

A species is a single lineage of ancestral descendant populations of organisms which

maintains its identity from other such lineages and which has its own evolutionary

tendencies and historical fate.^28

By 2000, this has ceased to be a “definition” and is now a “characterization”:

An evolutionary species is an entity composed of organisms that maintains its identity

from other such entities through time and over space and that has its own independent

evolutionary fate and historical tendencies.^29

(^24) Mayr 1995, 5.
(^25) Simpson 1961, 153.
(^26) Simpson 1951, quoted in Ghiselin 1997, 112f, who suggests the shift from the biospecies empha-
sis of this definition to the less explicitly interbreeding concept a decade later is due to his falling
“increasingly under the spell of the set-theoretical treatment of the Linnaean hierarchy by Gregg ...,
who, although mentioning in passing the possibility that species are something else, insisted they are
classes.” However, a set interpretation does not make it, ipso facto, a class interpretation.
(^27) Cain 1954, 111.
(^28) Wiley 1978, 18.
(^29) Wiley and Mayden 2000, 73.