Pages 465-470; 472-474

Question 1

genetic drift

Answer 1

causes allele frequencies to change randomly. Drift may cause alleles that decrease fitness to increase in frequency.

Question 2

gene flow

Answer 2

occurs when individuals leave one population, join another, and breed.

Question 3

modern synthesis

Answer 3

when biologists began to apply Mendelian genetics to Darwinian evolution.

Question 4

hardy-weinberg principle

Answer 4

serves as a mathematical null hypothesis for the study of evolutionary processes. Wanted to know what happened in an entire population when all of the individuals bred.

allele frequencies: p + q = 1
genotype frequencies: p^2 + 2pq + q^2

Question 5

gene pool

Answer 5

all of the alleles from all the gametes produced in each generation.

Question 6

È The Hardy-Weinberg principle is based on five assumptions:

Answer 6

(1) Random mating – enforced by picking gametes from the gene pool at random.
(2) No natural selection – all members of the parental generation survived and contributed equal numbers of gametes to the gene pool, no matter what their genotype.
(3) No genetic drift – assumed that alleles were picked in their exact frequencies p and q, and not at some different values caused by chance – that is the model behaved as though the population was infinitely large.
(4) No gene flow – no new alleles were added by immigration or lost through emigration.
(5) No mutation – did not consider new alleles the might be introduced into the gene pool.

Question 7

The Hardy-Weinberg principle functions as the null hypothesis.

Answer 7

If biologists observe genotype frequencies that do not conform to the Hardy-Weinberg prediction, it means something interesting is going on: either nonrandom mating is occurring (which changes genotype frequencies but not allele frequencies), or allele frequencies are changing for some reason.

Question 8

genetic variation

Answer 8

the number and relative frequency of alleles that are present in a particular population.

Question 9

Directional selection

Answer 9

changes the average value of a trait. Changes in one direction. Tends to reduce the genetic diversity of populations. Over time, favored alleles will eventually approach a frequency of 1.0 (fixed) while disadvantageous alleles will approach a frequency of 0.0 (lost).

Question 10

stabilizing selection

Answer 10

reduces variation in a trait. There is no change in the average value of a trait over time and genetic variation in the population is reduced.

Question 11

disruptive selection

Answer 11

increases variation in a trait. Eliminates phenotypes near the average value and favors extreme phenotypes. The overall amount of genetic variation in the population is maintained. Sometimes plays a part in speciation, or the formation of new species.

Question 12

balancing selection

Answer 12

maintains variation in a trait. No single allele has a distinct advantage.

• Occurs when heterozygous individuals have higher fitness than homozygous individuals, a pattern called heterozygote advantage.
• The environment varies over time or in different geographic areas occupied by a population – meaning that certain alleles are favored by selection at different times or in different places. Overall genetic variation in the population is maintained or increased as a result.
• Certain alleles are favored when they’re rare, but not when they are common, a pattern know as frequency-dependent selection.

Question 13

purifying selection

Answer 13

when disadvantageous alleles decline in frequency.