While the mechanism by which selection acts is constant, the genetic and phenotypic consequences to the population can vary under different conditions.  For a population witha given genetic composition and organization, the particular form and outcome of a selection regime depends upon the shape of the fitness curve derived by local environmental conditions. 

In some cases, selection can lead to elaboration of a trait, while in others the result may be stability.  Similarly, selection can either lead to an increase or decrease in the variance of a trait, or the loss or maintenance of polymorphism in the population.  We can classify three major types of selection: directional, stabilizing, and disruptive.  At the same time, the strength of selection can be either frequency-independent or frequency-dependent.
 
 

Directional selection results when one end of the fitness curve is highest, indicating that one extreme of the phenotypic distribution has the highest fitness.  Selection will work to increase the proportion of these high fitness forms in the population, shifting the phenotypic distribution in that direction.  Because the genetic system typically continues to generate new, more extreme phenotypes in each generation, the variance in the phenotypic distribution remains relatively constant. 

As the population shifts towards the high fitness end of the distribution, w bar increases.  The rate at which w bar changes under directional selection is roughly equal to the additive genetic variance, and proportional to the frequency of heterozygotes in the population; this is Fisher's fundamental Theorem of natural selection.  Thus, the frequency of different alleles in the population influences the rate at which the population responds to selection (adapts), and is maximal when there are many equally-frequent alleles.

The degree of dominance of alleles to one another also influences the rate of adaptation (increase in w bar) over time.  Initially, the increase in the frequency of a favrable allele is highest when that allele is dominant; however, it is extremely difficult to eliminate a low fitness, recessive allele.
 

Stabilizing selection results when itermediate phenotypes have the highest fitness, that is, when the fitness curve is higher in the middle than at either end.  In increasing the frequency of the high fitness genotypes, stabilizing selection results in reduced phenotypic variance, but does not shift the mean phenotype in the population (or does so to a limited degree).  Stabilizing selection is probably the most common form of selection.  Because common phenotypes are selected for, stabilizing selection generates evolutionary inertia that resists directional change, and results in stable equilibrium allele frequencies.

If selection is acting ona  single-locus trait, then satbilizing sleection results from heterozygous advantage, also known as heterosis or overdominance for fitness.  While difficult to demonstrate for individual loci (though sickle-cell is an obvious example), there is evidence in many species for an advantage in being heterozygous; often, the degree of heterozygosity of individuals is positivel crrelated with growth rate, etc.  The overall effect of heterozygous advantage is to maintain a stable genetic polymorphism in the population.

While stabilizing selection in a single-locus case results from heterozygous advantage, quantitative polygenic traits are subject to stabilizing sleection without the necessary link to heterozygosity.  Here, intermediate phenotypes have the highest fitness, but can differ in their degree of heterozygosity.

Disruptive or diversifying selection occurs when both extremes of the phenotypic distributions have higher fitnesses than do intermediate phenotypes (i.e., the fitness curve has two peaks).  Disruptive selection increases phenotypic variance but does not shift the mean phenotype in the population.  While apparently less common than the other types of selection, disruptive selection is theoretically important as it can increase genetic and phenotypic diversity and potentially promote speciation, by splitting a single population into two parts.  In a single locus situation, disruptive selection generates unstable equilibrium allele frequencies.

Disruptive selection is part of a broader concepts of selection in variable environments and multiple niche polymorphism.  Here, selection may favor different genotypes in different microenvironments.  Here, polymorphism is likely to be maintained if the environmental variation is coarse-grained and hard selection is at work.  If habitat selection or other form of spatial segregation is coupled with assortative mating, then the potential for speciation exists.  Examples of disruptive selection are relatively rare; more often, the appearance of multiple distinct genotypes/phenotypes within a population can be explained by migration.  

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