Evolution__3rd_Edition

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Time
Vertical
species concept
Species may be defined by
interbreeding
CHAPTER 13 / Species Concepts and Intraspecific Variation 351
Horizontal
species concept
Biologists are mainly concerned with defining species in the present, and this requires a
horizontal concept. We need to know which eagles are Haliaeetus leucocephalus now,
and are less interested in eagles a million years in the past or the future. This chapter
concentrates on horizontal concepts.
13.2.1 The biological species concept
Figure 13.3
Horizontal and vertical species concepts. A horizontal concept
aims to define species at a time instant and specifies which
individuals belong to which species at one time. A vertical
concept aims to define species through time and specifies
which individuals belong to which species through all time.
The biological species concept defines species in terms of interbreeding. Mayr (1963), for
instance defined a species as follows: “species are groups of interbreeding natural populations
that are reproductively isolated from other such groups.” The expression
“reproductively isolated” means that members of the species do not interbreed with
members of other species, because they have some attributes that prevent interbreeding.
The species concept that is now called the biological species concept actually predates
Darwin a it was the species concept used by John Ray in the seventeenth century,
for instance a but it was strongly advocated by several influential founders of the
modern synthesis, such as Dobzhansky, Mayr, and Huxley, and it is the most widely
accepted species concept today, at least among zoologists.
The biological species concept is important because it places the taxonomy of
natural species within the conceptual scheme of population genetics. A community of
interbreeding organisms make up, in population genetic terms, a gene pool. In theory,
the gene pool is the unit within which gene frequencies can change. In the biological
species concept, gene pools become more or less identifiable as species. The identity is
imperfect, because species and populations are often subdivided, but that is a detail.
The species, in this concept, is the unit of evolution. Organisms do not evolve but
species do, and higher taxonomic groups such as phyla only evolve in so far as their
constituent species are evolving.
The biological species concept explains why the members of a species resemble one
another, and differ from other species. When two organisms breed within a species,
their genes pass into their combined offspring; as the same process is repeated every
generation, the genes of different organisms are constantly shuffled around the species
gene pool. Different family lineages (of parent, offspring, grandchildren, and so on)
soon become blurred by the transfer of genes between them. The shared gene pool gives