Mechanisms traditionally considered constraints on local selection can enhance the genetic diversity of a population. Above: Genetic diversity comes originally from mutations in the population or by the immigration of mutations from other populations. Selection acts to remove deleterious phenotypes. The power of selection to reduce genetic diversity is reduced if the heritability of phenotypes is low or if genetic drift is strong.
For many ecologists, the terms in the diagram above seem unusual or unfamiliar. Ecology and evolutionary biology really employ separate languages. As an example, the diagram uses the term "genetic diversity," which is a useful metaphor in ecology but is imprecise in the evolutionary paradigm. More precise terms would be genetic variance or additive genetic variance. This article attempts to bridge some of the distance between ecolgical and evolutionary perspectives on niche evolution.
Whereas it is common in matters of ecological theory to assume that species are genetically and phenotypically homogenous, evolutionary biologists assume that variation arises at every step along the way from reproduction through development. Mutation and immigration force constant change on the genetic pool of the population. Minute variation in the environment forces deviations in phenotype even among identical offspring. When all is said and done, selection then acts on any variation in phenotype, whether it is a product of genetic diversity or merely a random deviation caused by chance events in the course of an individual's life.
What might interest the ecologist about the evolutionary perspective on populations is that it can clarify the process by which niches are formed. Consider a species confined to a particular habitat type in a mosaic landscape. Other habitats are unsuitable. How can this species ever increase its niche breadth? That is, how will it adapt to new habitat types?
If genotype and phenotype are strongly coupled and selection can act effectively, there is a chance that the species will never expand its niche. This is called stasis or niche conservatism. However, there are a few tricks for baffling selection that might actually improve a population's chance of adapting to new habitats. Drift is a familiar one. If a population is small enough, chance events may be sufficient to allow the survival of mutants that might have low local fitness but bear traits suitable to other habitats.
Less commonly, evolutionary biologists consider changes in the strength of coupling between genotype and phenotype. More random noise in the expression of phenotype will reduce the coupling, meaning that selection on phenotype will have less to do with an individual's genes. The same effect can be achieved through canalization, a process by which a single phenotype is produced regardless of the accumulation of genetic variation. Heritability is reduced in both cases. Though not intuitive, reduced heritability can allow genetic variance to accumulate and may increase the probability that a species will adapt to a previously hostile environment.
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