Blackwell Publishing

The theory of natural selection (part 2) - What are the sources of polymorphism?


Multiple niche polymorphism

A niche is a distinct region in the species' natural environment or something analogous to a distinct region. Up to two conditions are needed for a multiple niche polymorphism.

• The first (and crucial) condition is that different genotypes should have different fitnesses in different niches. In nature, the members of a species occupy a variety of niches, and these vary in their physical conditions (whether they are dark, light, dry, damp, etc.) and biological conditions (what predators and parasites are locally active, what food is available). The simplest case would have two niches, A and B, with genotypes AA and Aa better adapted to A and aa is better adapted to B. The fitnesses are:

Fitness in niche A11-s1-s
Fitness in niche B1-t1-t1

• A stable polymorphism can more easily evolve if there is habitat selection: each genotype 'chooses' the correct habitat to live in. Individuals with genotype AA or Aa would go and live in niche type A and those with aa pick niches of type B.

Multiple niche polymorphism is a form of frequency-dependent polymorphism. When the genotype AA is rare, it experiences relatively little competition in the niches for which it is well adapted, and it increases in frequency. As AA becomes commoner, it increasingly occupies all the niches of type A and has to make a living in B type niches too. Its advantage decreases as its frequency increases. The fitness of each genotype is negatively frequency-dependent.

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