Evolution__3rd_Edition

378 PART 4 / Evolution and Diversity
Summary
1 In practice, species are defined by easily recognizable
phenetic characters that reliably indicate what
species an individual belongs to.
2 The biological species concept defines a species as a
set of interbreeding forms. Interbreeding between
species is prevented by isolating mechanisms.
3 The ecological species concept defines a species as a
set of organisms adapted to a particular ecological
niche.
4 The phenetic species concept defines a species as a
set of organisms that are sufficiently phenetically similar
to one another.
5 The biological, ecological, and phenetic (and several
other) species concepts are all closely related, and are
concerned to explain or describe much the same fact:
that life seems to come in the form of distinct species.
6 Individuals mainly interbreed with other members
of their own species because of isolating barriers that
prevent interbreeding with other species. Isolating
barriers can be prezygotic or postzygotic.
7 Geographic variation can be adaptive or neutral.
The amount of genetic variation among geographic
races of a species can be described quantitatively and is
low in human beings relative to other species.
Further reading
8 The theory of evolution justifies population thinking
rather than typological thinking about intraspecific
variation: all individuals in a population are
equally good members of a species, rather than some
being better specimens than others.
9 Character displacement occurs when two species
have partly overlapping geographic ranges and the two
species differ more in sympatry than in allopatry.
Character displacement maybe caused by ecological
competition.
10 The biological species concept explains the
integrity of species by interbreeding (which produces
gene flow), the ecological concept by selection. The
two processes are usually correlated, but it is possible
to test between them in special cases. Selection can be
strong enough to overcome gene flow, and selection
can maintain a species’ integrity in the absence of gene
flow.
11 Taxonomic entities such as biological species may
be real or nominal. According to the biological species
concept, species can be real, but lower and higher taxonomic
levels are nominal. According to the ecological
species concept, all taxonomic levels can have a similar
degree of realism.
Mayr (1963) is the classic account of the species in evolutionary biology; see also Mayr
(1976, 2001) and Mayr & Ashlock (1991). Coyne (1994) discusses species concepts,
particularly in relation to Mayr’s ideas. Dobzhansky (1970), Huxley (1942), Cain
(1954), and Simpson (1961b) also contain classic material. Ereshefsky’s (1992) anthology
contains many of the important papers on species concepts.
More recent books include the volume edited by Howard & Berlocher (1998), which
has good chapters on species concepts by Harrison, Templeton, Shaw, and de Queiroz
that discuss the use of molecular markers and coalescence. See Levin (2000) on plants.
Two other recent books are by Hey (2001) and by Ereshefsky (2001), both of whom
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