'Nothing in biology makes sense, except in the light of evolution'. We try in this chapter to illustrate the processes by which the properties of different sorts of species make their life possible in particular environments.

We explain what is meant by evolutionary adaptation and by the theory of evolution by natural selection, an ecological theory first elaborated by Charles Darwin in 1859. Through natural selection, organisms come to match their environments by being 'the fittest available' or 'the fittest yet': they are not 'the best imaginable'.

Adaptive variation within species can occur at a range of levels: all represent a balance between local adaptation and hybridization. Ecotypes are genetically determined variants between populations within a species that reflect local matches between the organisms and their environments. Genetic polymorphism is the occurrence together in the same habitat of two or more distinct forms. Dramatic examples of local specialization have been driven by manmade ecological forces, especially those of environmental pollution.

We describe the process of speciation by which two or more new species are formed from one original species and explain what we mean by a 'species', especially a biospecies. Islands provide arguably the most favorable environment for populations to diverge into distinct species.

Species live where they do for reasons that are often accidents of history. We illustrate this by examining island patterns, the movements of land masses over geological time, climatic changes especially during the Pleistocene ice ages (and we compare this with predicted changes consequent on current global warming) and the concepts of convergent and parallel evolution.

The various terrestrial biomes of the earth are reviewed and their aquatic equivalents touched on briefly. Raunkiaer's concept of life form spectra, in particular, emphasizes that ecological communities may be fundamentally very similar even when taxonomically quite distinct.

All communities comprise a diversity of species: a diversity of matches to the local environment. Environmental heterogeneity, interactions between predators and prey, parasites and hosts and mutualists, and the coexistence of similar species all contribute to this.

Chapter 2

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