Bio 182 exam 2

genetic drift

A change in the allele frequency of a population as a result of chance events rather than natural selection.

Problem : What type of distribution would you expect for a trait on which there is no selection pressure? On which there is stabilizing selection pressure? Disruptive selection pressure? Directional selection pressure?

The distribution of a trait on which there is no selection pressure should form a smooth bell-shaped curve. If there is stabilizing selection pressure, the curve should be narrower, but with the same mean. If there is disruptive selection pressure, the curve should be bimodal (have two peaks). If there is directional selection pressure, the mean of the curve should shift up or down the scale, depending on the direction of selection.

Directional selection graph

The effect of directional selection on trait distribution

Disruptive Selection

The effect of directional selection on trait distribution

Stabilizing Selection Graph

The effect of stabilizing selection on trait distribution

Problem : A population of arboreal rodents lives in one species of pine tree. These pine trees have very evenly spaced branches of uniform thickness. Rodents that are too small cannot reach between the branches to move around in the tree. Rodents that are too big and heavy break through the branches and fall out of the trees. The rodent population contains adults of very uniform size. What type of selection has created this uniform size distribution in these rodents?

The uniform size distribution was created by stabilizing selection.

Problem : A population of finches lives on an island with an abundant food supply. These finches have highly variable beak lengths, allowing them to feed on different types of food. Then several other species of birds move into the area, making food supplies scarce. The only types of food left for the finches are small seeds and nectar from long tubular flowers. Finches with small beaks can easily grasp and crack the seeds. Finches with long beaks can reach inside the long flowers to reach the nectar. However, some finches have beaks that are too large to deal with the small seeds, but too small to reach inside the flowers. After several generations, there are two populations of finches: those with short beaks and those with long beaks. What kind of selection has acted on these finches?

These finches have experienced disruptive selection.

Problem : A group of early giraffes lives in a lush forest with many trees and shrubs of different heights to feed on. These giraffes have necks of various lengths, with most giraffes having a medium length neck. Then a blight passed through the area, killing off all of the low growing plants, leaving only tall trees whose leaves are far off the ground. Giraffes with shorter necks now have a hard time getting food, while those with longer necks can feed on the high-growing leaves. After several generations, the giraffes in this area all have longer necks than those before the blight. What type of selection has occurred in this giraffe population?

These giraffes have experienced directional selection.

Stabilizing Selection

When selective pressures select against the two extremes of a trait, the population experiences stabilizing selection.

bottleneck effect

a reduction in the genetic diversity of a population caused by a reduction in its size

genetic drift can lead to

loss of genetic variation within populations and can eliminate beneficial alleles in small populations

founder effect (genetic drift)

occurs when a few individuals become isolated from a larger population

Directional selection

one extreme of the trait distribution experiences selection against it. The result is that the population's trait distribution shifts toward the other extreme. In the case of such selection, the mean of the population graph shifts.

Disruptive Selection

selection pressures act against individuals in the middle of the trait distribution. The result is a bimodal, or two-peaked, curve in which the two extremes of the curve create their own smaller curves.