Population Genetics. What is Disruptive Selection?

Disruptive selection, also called diversifying selection, describes changes in population genetics in which extreme values for a trait are favored over intermediate values. In this case, the variance of the trait increases and the population is divided into two distinct groups . Example. Suppose there is a population of rabbits. The color of the rabbits is governed by two incompletely dominant traits: black fur, represented by “B“, and white fur, represented by “b“. A rabbit in this population with a genotype of “BB“ would have a phenotype of black fur, a genotype of “Bb“ would have grey fur (a display of both black and white), and a genotype of “bb“ would have white fur. If this population of rabbits occurred in an environment that had areas of black rocks as well as areas of white rocks, the rabbits with black fur would be able to hide from predators amongst the black rocks, and the rabbits with white fur likewise amongst the white rocks. The rabbits with grey fur, however, would stand out in all areas of the habitat, and would thereby suffer greater predation. As a consequence of this type of selective pressure, our hypothetical rabbit population would be disruptively selected for extreme values of the fur color trait: white or black, but not grey. Disruptive selection and sympatric speciation It is believed that disruptive selection is one of the main forces that drive sympatric speciation in natural populations. The pathways which lead from disruptive selection to sympatric speciation are not often prone to deviation; it is a domino effect which depends on the consistency of each factor. These pathways consist of disruptive selection being based on intraspecific competition, which often leads to reproductive isolation, and finally sympatric speciation. It is important to keep in mind that disruptive selection does not always have to be based on intraspecific competition, so in some cases that step of the pathway can be eliminated. What can happen instead is that disruptive selection supports polymorphisms, which can lead to reproductive isolation, and lastly speciation. When disruptive selection is based on intraspecific competition, it in turn promotes ecological niche diversification and polymorphisms. If two morphs, or forms, of a phenotype occupy different niches, it would promote less competition for resource use. Disruptive selection is seen in high density populations rather than in low density populations because intraspecific competition is more common when accompanied by a higher density population. This is due to the fact that higher density populations often have more competition for resources which drives polymorphisms or changes in niches in order to create less competition. If one morph has no need for resources used by the other morph, then it is likely that neither would feel the need to compete or interact, therefore supporting these two morphs to continue occurring in the population. This theory does not necessarily have a lot of supporting evidence in natural populations, but it has been seen many times in experimental situations using existing populations. These experiments further support that, under the right situations (as described above), this theory could prove to be true in nature. #gene #ympatricSpeciation #intraspecificCompetition #selectivePressure #population #geneExpression #Allele #genomes #GeneticsLecture #diversifyingSelection #dnaMolecule #Cancer #changesInPopulationGenetics #trait #locus #genome #GeneticExamQuestionsSolutions #chromosome #Variance #GeneticsExamQuestionsSolutions #genotype #polymorphisms #Iherb #DNA #GeneticTesting #molecularBiology #eukaryotic #genes #phenotype #Genetics101 #DisruptiveSelection #genetics #alleles