lecture 10

Question 1

HWE principle

Answer 1

identifies the four mechanisms of evolution: genetic drift, selection, migration, and non-random mating

Question 2

migration

Answer 2

• gene flow: movement of alleles between populations.
• the general effect of migration is to homogenize allele frequencies across populations.
• gene flow prevents populations from diverging, unless it is balanced by other evolutionary forces

Question 3

continent island model (migration)

Answer 3

one way migration to a separated population

Question 4

island model (migration)

Answer 4

random migration between separate populations

Question 5

stepping-stone model (migration)

Answer 5

migrants only come from neighboring populations

Question 6

isolation by distance model (migration)

Answer 6

gene flow from local neighborhoods in a continuously distributed population

Question 7

genetic drift

Answer 7

• is random sampling error

- selection is differential reproductive success that happens for a reason.

Question 8

genetic drift and population size

Answer 8

• sampling error is reduced with increased sample sized

- theoretically, if we sampled indefinitely, then the frequency of A1 would be exactly 0.6 (the initial frequency)

Question 9

founder effect

Answer 9

• small founding populations usually have alleles frequencies that are different from their source population, simply by chance.
• the founding of a new population by a small group of individuals typically involves instantaneous evolution of differences between the new population and the source.
• consequences: fewer alleles at higher frequencies

Question 10

genetic drift and evolution

Answer 10

1. because genetic drift is random, every population will follow a unique evolutionary path
2. genetic drift has a more rapid and dramatic effect on allele frequencies in small populations, slow effect on large pops
3. given enough time, genetic drift can produce substantial changes in allele frequencies even large populations; alleles will drift to fixation or loss and the frequency of heterozygotes will decline (rapid in small, slow in large)

Question 11

Probability that a specific allele will be the one to drift to fixation…

Answer 11

… is equal to that allele’s initial frequency.
- If we start with a finite population in which A1 is at a frequency of 0.73, then there is a 73% chance that the A1 will drift to fixation.

Question 12

inbreeding (nonrandom mating)

Answer 12

• mating between relatives
• increases the frequency of homozygotes and reduces the frequency of heterozygotes in each generation
• allele frequencies do not change
• genotype frequencies do change

Question 13

inbreeding depression (nonrandom mating)

Answer 13

• a decline in average fitness that takes place when homozygosity increases and heterozygosity decreases in a population

Question 14

inbreeding depression results from two processes:

Answer 14

• Increased frequency of homozygous recessive loss-of-function mutations: In heterozygotes, these alleles have little or no effect; but inbreeding increases the frequency of homozygous recessive individuals and thus the frequency of individuals expressing the mutation.
• Loss of heterozygosity: Many genes—especially those involved in fighting disease—are under intense selection for heterozygote advantage. If an individual is homozygous at these genes, then fitness declines.