BIOLOGY Volume 3 - A Guide to General Biology - 2004

27. MECHANISMS OF SPECIATION

27.6. The Species Concept

A species represents the lowest taxonomic category that can be defined with any degree of precision. Table 27.2 outlines some of the existing Structure/97.html">Definitions of a species.

Individuals belonging to the same species rarely exist in nature as a single, expansive population. Typically, a species is represented by a series of smaller, interbreeding populations called demes, each possessing its own Gene pool. These populations may occupy contiguous or widely separated geographical areas. The spatial Separation of populations means that a given species may encounter diverse environmental conditions and varying intensities of Selection pressure. Mutation and selection in isolated populations can generate varying degrees of phenotypic Variability within a species, as described below.

Class="center">Table 27.2. Alternative definitions of a species

27.6.1. Geographical races

Populations occupying a vast geographical range or scattered over long periods across widely separated habitats may exhibit significant phenotypic differences. These differences are generally underpinned by adaptations to climatic factors. For example, the gypsy moth (Hymantria dispar) is distributed throughout Japan and eastern Asia. Across this expanse, climatic conditions vary widely, ranging from subarctic to subtropical. Ten geographical races of the moth are recognized, differing in the timing of their hatching from eggs. Northern races hatch later than southern ones. It is believed that the phenotypic traits of these races result from METABOLISM/18.html">The Influence of climatic factors on gene frequencies within their gene pools. The GENETIC BASIS OF this variability is evidenced by the fact that when representatives of these races are placed under identical conditions, they continue to hatch at different times.

27.6.2. Ecological races (ecotypes)

Populations adapted to environmentally distinct habitats may occupy adjacent geographical areas; for instance, in the coastal regions of California, the plant Giliaachilleaefolia is represented by two races. One race, the "sun" ecotype, grows on open, grassy southern slopes, while the other, the "shade" ecotype, is found in the shaded oak woodlands and redwood groves. These races differ in petal size, a genetically determined trait.

27.6.3. Clines

A cline refers to the gradual variation in phenotypic traits of a given species across its geographical range. A single species may have multiple clines oriented in various directions (Fig. 27.14).

Species exhibiting pronounced phenotypic variability within a single population, in accordance with the degree of their geographical variation, are termed polytypic species. A classic example of such a species is found in gulls of the genus Larus (sec. 27.8.4).

All the cases of polytypic variation described above reflect varying degrees of genetic divergence, which may affect the reproductive potential of members of the respective populations should they come into contact.

Fig. 27.14. Phenotypic variation in body size and coat coloration in wood mice (Apodemus) in Scotland.