Specialization within species

The natural world is not composed of a continuum of types of organism each grading into the next: we recognize boundaries between one type of organism and another. Nevertheless, within what we recognize as species (defined below), there is often considerable variation, and some of this is heritable. It is on such intraspecific variation, after all, that plant and animal breeders (and natural selection) work.

Since the environments experienced by a species in different parts of its range are themselves different (to at least some extent), we might expect natural selection to have favored different variants of the species at different sites. The word 'ecotype' was first coined for plant populations (Turesson, 1922a, 1922b) to describe genetically determined differences between populations within a species that reflect local matches between the organisms and their environments. But evolution forces the characteristics of populations to diverge from each other only if: (i) there is sufficient heritable variation on which selection can act; and (ii) the forces favoring divergence are strong enough to counteract the mixing and hybridization of individuals from different sites. Two populations will not diverge completely if their members (or, in the case of plants, their pollen) are continually migrating between them and mixing their genes.

Local, specialized populations become differentiated most conspicuously amongst organisms that are immobile for most of their lives. Motile organisms have a large measure of control over the environment in which they live; they can recoil or retreat from a lethal or unfavorable environment and actively seek another. Sessile, immobile organisms have no such freedom. They must live, or die, in the conditions where they settle. Populations of sessile organisms are therefore exposed to forces of natural selection in a peculiarly intense form.

This contrast is highlighted on the seashore, where the intertidal environment continually oscillates between the terrestrial and the aquatic. The fixed algae, sponges, mussels and barnacles all meet and tolerate life at the two extremes. But the mobile shrimps, crabs and fish track their aquatic habitat as it moves; whilst the shore-feeding birds track their terrestrial habitat. The mobility of such organisms enables them to match their environments to themselves. The immobile organism must match itself to its environment.

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