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Evolution__3rd_Edition

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..<br />

Time<br />

Vertical<br />

species concept<br />

Species may be defined by<br />

interbreeding<br />

CHAPTER 13 / Species Concepts and Intraspecific Variation 351<br />

Horizontal<br />

species concept<br />

Biologists are mainly concerned with defining species in the present, and this requires a<br />

horizontal concept. We need to know which eagles are Haliaeetus leucocephalus now,<br />

and are less interested in eagles a million years in the past or the future. This chapter<br />

concentrates on horizontal concepts.<br />

13.2.1 The biological species concept<br />

Figure 13.3<br />

Horizontal and vertical species concepts. A horizontal concept<br />

aims to define species at a time instant and specifies which<br />

individuals belong to which species at one time. A vertical<br />

concept aims to define species through time and specifies<br />

which individuals belong to which species through all time.<br />

The biological species concept defines species in terms of interbreeding. Mayr (1963), for<br />

instance defined a species as follows: “species are groups of interbreeding natural populations<br />

that are reproductively isolated from other such groups.” The expression<br />

“reproductively isolated” means that members of the species do not interbreed with<br />

members of other species, because they have some attributes that prevent interbreeding.<br />

The species concept that is now called the biological species concept actually predates<br />

Darwin a it was the species concept used by John Ray in the seventeenth century,<br />

for instance a but it was strongly advocated by several influential founders of the<br />

modern synthesis, such as Dobzhansky, Mayr, and Huxley, and it is the most widely<br />

accepted species concept today, at least among zoologists.<br />

The biological species concept is important because it places the taxonomy of<br />

natural species within the conceptual scheme of population genetics. A community of<br />

interbreeding organisms make up, in population genetic terms, a gene pool. In theory,<br />

the gene pool is the unit within which gene frequencies can change. In the biological<br />

species concept, gene pools become more or less identifiable as species. The identity is<br />

imperfect, because species and populations are often subdivided, but that is a detail.<br />

The species, in this concept, is the unit of evolution. Organisms do not evolve but<br />

species do, and higher taxonomic groups such as phyla only evolve in so far as their<br />

constituent species are evolving.<br />

The biological species concept explains why the members of a species resemble one<br />

another, and differ from other species. When two organisms breed within a species,<br />

their genes pass into their combined offspring; as the same process is repeated every<br />

generation, the genes of different organisms are constantly shuffled around the species<br />

gene pool. Different family lineages (of parent, offspring, grandchildren, and so on)<br />

soon become blurred by the transfer of genes between them. The shared gene pool gives

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