Sunday, March 6, 2011

Semester 2: Blog # 4

  1. Stabilizing selection favors the normal, the common, average traits in a population. Stabilizing Selection occurs when selection favors the intermediate trait value over the extreme values. Populations under this type of selection typically experience a decrease in the amount of additive genetic variation for the trait under selection. An example would be human birth, babies cnanot be born with really low weight or really heavy weight because it will lead to infant mortality. So thats why our bodies perfer medium weight.
  2. Directional selection is when natural selection favors a single phenotype. It occurs when there is a shift in population towards an extreme version of a beneficial trait. An example for this selection would be A useful example can be found in the breeding of the greyhound dog. Early breeders were interested in dog with the greatest speed. They carefully selected from a group of hounds those who ran the fastest. From their offspring, the greyhound breeders again selected those dogs who ran the fastest. By continuing this selection for those dogs who ran faster than most of the hound dog population, they gradually produced a dog who could run up to 40mph.   
  3. 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. An example for this would be the population of rabbits. The color of the rabbits is governed by two incompletely dominant traits: black fur and white fur. If this population of rabbits were put into an area that had very dark black rocks as well as very white colored stone, the rabbits with black fur would be able to hide from predators amongst the black rocks and the white furred rabbits would be able to hide in the white rocks, but the gray furred rabbits would stand out in both of the habitats and would suffer greater predation. As a consequence of the selective pressures of their environment, our hypothetical rabbit population would be disruptively selected for extreme values of the fur color trait: white or black, but not gray.

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