Genetic Constraints
Genetic constraints provide another force that inhibits the effectiveness of natural selection. Examples include pleiotropy, epistasis, and polygenic traits.
With pleiotropy a hypothetical gene would be selected by natural selection or other related mechanisms. The trait would be selected for its effects on reproductive success. This is well established as how traits evolve. However, some genes control multiple traits. Selection that influences epistasis is a case where the regulation or expression of one gene, depends on one or several others. This is true for a good number of genes though to differing extents. The reason why this leads to muddied responses is that selection for a trait that is epistatically based can mean that an allele for a gene that is epistatic when selected would happen to affect others. This leads to the coregulation of others for a reason other than there is an adaptive quality to each of those traits. Like with pleiotropy, traits could reach fixation in a population as a by-product of selection for another.
In the context of development the difference between pleiotropy and epistasis is not so clear but at the genetic level the distinction is more clear. With these traits as being by-products of others it can ultimately be said that these traits evolved by not that they necessarily represent adaptations.
Polygenic traits are traits that are controlled by a number of different genes. Counter to what is commonly taught in high school and introductory biology classes with beginner's Mendelian Genetics, traits are rarely controlled by one single discrete, either/or allele. Things like human height vary with a great range because this trait is controlled by several different genes.
To drastically change some quantitative trait controlled by many genes, this could require mutations in more than one gene or changes in regulation of more than one gene. This means that mutations effect these systems more on a one-at-a-time basis. Because of this
Adaptationists are accused by their critics of using ad-hoc "just-so stories" to make their theories unfalsifiable. The critics, in turn, have often been accused of attacking straw men, rather than the actual views of supposed adaptationists.
Read more about this topic: Adaptationism
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