AP Biology—Unit 8
Modes of Natural Selection
Name:
1. Define Natural Selection.
2. Why is natural selection important?
Modes of Natural Selection
3.
Number of individuals in population
1.
2.
3.
Time
Are the above 3 diagrams showing 3 modes of Natural selection. What do the arrows
represent?
A) Directional selection- moves the frequency distribution of alleles in a steady consistent
direction. The phenotypic character of the population shifts as a whole either in response to
directional changer in the environment or as an adaptive response to a new environment.
B) Disruptive selection- Increases the frequency of two or more alleles that give rise to extreme
forms of the trait, such the intermediate forms are selected against.
C) Stabilizing selection- decreases the frequency of alleles that give rise to extreme forms of a
trait, such that intermediate forms already well adapted to prevailing conditions favored.
1. Match each graph to each mode described above.
Graph
Mode
1.
2.
3.
2. Which Mode tends to counter the effects of mutation, genetic drift and flow?
3. Which mode result in dimorphism (two forms of a trait)
4. Below are descriptions of examples of natural selection in nature. Decide which mode
of natural selection it illustrates and explain your choice.
A) Horseshoe crabs have a distinct flattened shape for pushing through mud and
sand. The fossil record indicates that the form of the crabs has remained
essentially unchanged for more than 250 million years.
B) DDT was used as an insecticide on mosquitoes, especially in those areas where
malaria was a problem. With in a few years the DDT has shown not to be
effective in controlling the mosquito population in the areas most heavily sprayed.
C) On average, human infants have a birth weight of about 7 lbs. Those infants
with a birth weight that differs significantly relative to this value have increasing
incidences of infant mortality.
D) In Africa there are 3 forms of a type of female swallowtail butterfly. On I a
yellow color (like the male) that lives in an area that has few other species of
butterfly. The second is an orange color that looks like the inedible (to predators)
female of another type of butterfly. The third has large spots that mimic the
protective coloration of yet another different species of butterfly. The male in all
cases remain yellow. Hint: you may wish to have different modes for different
scenarios in this example.
6. Another type of selection is sexual selection. Many species show sexual dimorphism- different
external appearance and behavior between males and females. Give an example:
7. If the males has a bright color, than in some respects it would seem the trait would be selected
against (more predation). How can such a trait be, in reality, selected for and remain in the
population?