Selection is a directional process that leads to an increase or a decrease in the frequency of genes or genotypes. Selection is the process that increases the frequencies of plant resistance alleles in natural ecosystems through coevolution,  and it is the process that increases the frequencies of virulence alleles in agricultural ecosystems during boom and bust cycles.

How and when do typical genes contribute to fitness? A major puzzle: Most genes appear to be unnecessary!

Half or more can be “knocked out” (fully disabled) in yeast, worms, flies and even mice, without any obvious phenotypic effects (in the lab, anyway).

But these genes are maintained in evolution, so they must be useful. How? Two hypotheses:

• (1) Most are “special-purpose” genes needed only under certain circumstances (stresses that occur in nature but not in the lab).
• (2) Most are “fine-tuning” genes that increase the efficiency or accuracy of some physiological or developmental process in most environments.

Selection occurs in response to a specific environmental factor. It is a central topic of population and evolutionary biology. The consequence of natural selection on the genetic structure and evolution of organisms is complicated. Natural selection can decrease the genetic variation in populations of organisms by selecting for or against a specific gene or gene combination (leading to directional selection). It can increase the genetic variation in populations by selecting for or against several genes or gene combinations (leading to disruptive selection or balancing selection). Natural selection might lead to speciation through the accumulation of adaptive genetic differences among reproductively isolated populations. Selection can also prevent speciation by homogenizing the population genetic structure across all locations.

Selection in plant pathology is mainly considered in the framework of gene-for-gene coevolution. Plant pathologists often think in terms of Van der Plank and his concept of “stabilizing selection” that would operate against pathogen strains with unnecessary virulence. As we will see shortly, Van der Plank used the wrong term, as he was actually referring to directional selection against unneeded virulence alleles.

http://www.apsnet.org/edcenter/advanced/topics/PopGenetics/Pages/NaturalSelection.aspx

Click to access selection.pdf

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