Compare and contrast directional selection and disrup tive s

Compare and contrast directional selection and disrup-

tive selection and provide an example of each.

Solution

Directional selection is a mode of natural selection in which an extreme phenotype is favored over other phenotypes, causing the allele frequency to shift over time in the direction of that phenotype. Under directional selection, the advantageous allele increases as a consequence of differences in survival and reproduction among different phenotypes.

An example of directional selection is fossil records that show that the size of the black bears in Europe decreased during interglacial periods of the ice ages, but increased during each glacial period. Another example is the beak size in a population of finches. Throughout the wet years, small seeds were more common and there was such a large supply of the small seeds that the finches rarely ate large seeds. During the dry years, none of the seeds were in great abundance, but the birds usually ate more large seeds. The change in diet of the finches affected the depth of the birds’ beaks in the future generations.Their beaks range from large and tough to small and smooth

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.

In 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. This is an example of underdominance (heterozygote disadvantage) leading to disruptive selection.

Both of these show the distribution of phenotypes. In directional selection, the distribution of phenotypes forms a \"bell curve.\" Selection against one of the extreme phenotypes causes the distribution to \"move\" in one direction or the other. An example might be plants whose flower color is determined by incomplete dominance: white, pink, or red. Pink flowers may be the average phenotype, but if we start to remove red flowers from the population, the \"mean\" phenotype will be shifted toward white flowers.
In disruptive selection, the average phenotype is selected against. This produces a \"two-humped\" bell-type curve, and the greater distribution is split between the two phenotype extremes. If we have the same type of incomplete dominance as mentioned in the previous paragraph, assume that the pink flowers are selected against. This means that the two \"humps\" shown in the distribution will be centered around the red and white phenotypes.