Chapter 23 - The Evolution of Populations

Question 1

The three main mechanisms that can cause changes in allele frequencies:

Answer 1

natural selection, genetic drift, and gene flow

Question 2

Microevolution -

Answer 2

evolution on its smallest scale, shown in changes in allele frequencies in a population over generations

Question 3

Genetic drift -

Answer 3

chance events that alter allele frequencies

Question 4

Gene flow -

Answer 4

the transfer of alleles between populations

Question 5

Natural selection/adaptation -

Answer 5

consistently improves the degree to which organisms are well suited for life in their environment

Question 6

Genetic variation -

Answer 6

differences among individuals in the composition of their genes or other DNA sequences

Question 7

Gene variability -

Answer 7

genetic variation at the whole-gene level can be quantified as the average percentage of loci that are heterozygous

Question 8

Nucleotide variability -

Answer 8

genetic variation can be measured at the molecular level of DNA

Question 9

Phenotype -

Answer 9

the product of an inherited genotype and many environmental influences

Question 10

Genetic variation originates from

Answer 10

mutation, gene duplication, or other processes that produce new alleles and new genes

Question 11

Mutation -

Answer 11

a change in the nucleotide sequence of an organism’s DNA

Question 12

“heterozygote protection” -

Answer 12

recessive genes that can maintain a pool of alleles that may be harmful under present conditions, but that could be beneficial if the environment changes

Question 13

Neutral variation -

Answer 13

differences in DNA sequence that do not confer a selective advantage or disadvantage

Question 14

Sexual reproduction then shuffles existing alleles and deals them at random to produce individual genotypes.
Three mechanisms contribute to this shuffling:

Answer 14

crossing over, independent assortment of chromosomes, and fertilization

Question 15

A population -

Answer 15

a group of individuals of the same species that live in the same area and interbreed, producing fertile offspring

Question 16

The characterized genetic makeup of a population -

Answer 16

gene pool, consisting of all copies of every type of allele at every locus in all members of the population

Question 17

“fixed in the gene pool”

Answer 17

If only one allele exists for a particular locus in a population, that allele is said to be _________ and all individuals are homozygous for that allele.

Question 18

Hardy-Weinberg equilibrium -

Answer 18

a population that is not evolving (allele and genotype frequencies will remain constant from generation to generation) is said to be in

Question 19

Condition 1 for Hardy-Weinberg equilibrium -

Answer 19

No mutations

Consequence if Condition Does Not Hold: The gene pool is modified if mutations occur or if entire genes are deleted or duplicated.

Question 19

Condition 2 for Hardy-Weinberg equilibrium -

Answer 19

Random mating

Consequence if Condition Does Not Hold: If individuals mate within a subset of the population, such as near neighbors or close relatives (inbreeding), random mixing of gametes does not occur and genotype frequencies change.

Question 20

Condition 3 for Hardy-Weinberg equilibrium -

Answer 20

No natural selection

Consequence if Condition Does Not Hold: Allele frequencies change when individuals with different genotypes show consistent differences in their survival or reproductive success.

Question 21

Condition 4 for Hardy-Weinberg equilibrium -

Answer 21

Extremely large population size

Consequence if Condition Does Not Hold: In small populations, allele frequencies fluctuate by chance over time (genetic drift).

Question 22

Condition 5 for Hardy-Weinberg equilibrium -

Answer 22

No gene flow

Consequence if Condition Does Not Hold: By moving alleles into or out of populations, gene flow can alter allele frequencies.

Question 23

Hardy-Weinberg equation: q^2

Answer 23

frequency of homozygotes

Question 24

Hardy-Weinberg equation: q

Answer 24

frequency of the recessive allele

Question 24

New mutations (_____________) can alter allele frequencies, but because mutations are rare, the change from one generation to the next is likely to be very small.

Answer 24

violation of condition 1 (No mutations)

Question 25

Nonrandom mating (____________) can affect the frequencies of homozygous and heterozygous genotypes but by itself has no effect on allele frequencies in the gene pool.

Answer 25

violation of condition 2 (Random mating)

Question 26

The concept of natural selection is based on differential success in survival and reproduction:

Answer 26

Individuals in a population have different heritable traits so those with traits better suited to their environment tend to produce more offspring

Question 27

Natural selection in genetic terms:

Answer 27

selection results in alleles being passed to the next generation in proportions that differ from the present generation

Question 28

Adaptive evolution -

Answer 28

in which traits that enhance survival or reproduction tend to increase in frequency over time, typically caused by some alleles favored over others

Question 29

Genetic drift

Answer 29

chance events causing allele frequencies to fluctuate unpredictably from one generation to the next, especially in small populations

Question 30

The founder effect

Answer 30

When a few individuals become isolated from a larger population, this smaller group may establish a new population whose gene pool differs from the source population

Question 31

The bottleneck effect

Answer 31

where a population passes through a “bottleneck” that greatly reduces its size

Question 32

Genetic drift is significant in small populations:

Answer 32

Chance events can cause an allele to be disproportionately over- or underrepresented in the next generation. Although chance events occur in populations of all sizes, they tend to alter allele frequencies substantially only in small populations.

Question 33

Genetic drift can cause allele frequencies to change at random:

Answer 33

Because of genetic drift, an allele may increase in frequency one year, then decrease the next; the change from year to year is not predictable. Thus, unlike natural selection, which in a given environment consistently favors some alleles over others, genetic drift causes allele frequencies to change at random over time.

Question 34

Genetic drift can lead to a loss of genetic variation within populations:

Answer 34

By causing allele frequencies to fluctuate randomly over time, genetic drift can eliminate alleles from a population. Because evolution depends on genetic variation, such losses can influence how effectively a population can adapt to a change in the environment.

Question 35

Genetic drift can cause harmful alleles to become fixed:

Answer 35

Alleles that are neither harmful nor beneficial can be lost or become fixed (reach a frequency of 100%) by chance through genetic drift. In very small populations, genetic drift can also cause alleles that are slightly harmful to become fixed. When this occurs, the population’s survival can be threatened (as in greater prairie chickens).

Question 36

Gene flow -

Answer 36

the transfer of alleles into or out of a population due to the movement of fertile individuals or their gametes. Can also affect how well populations are adapted to local environmental conditions

Question 37

Relative fitness -

Answer 37

the contribution an individual makes to the gene pool of the next generation relative to the contributions of other individuals

Question 38

Directional selection -

Answer 38

occurs when conditions favor individuals exhibiting one extreme of a phenotypic range, thereby shifting a population’s frequency curve for the phenotypic character in one direction or the other

Question 39

Disruptive selection -

Answer 39

occurs when conditions favor individuals at both extremes of a phenotypic range over individuals with intermediate phenotypes

Question 40

Stabilizing selection -

Answer 40

acts against both extreme phenotypes and favors intermediate variants

Question 41

Sexual selection -

Answer 41

a process in which individuals with certain inherited characteristics are more likely than other individuals of the same sex to obtain mates

Question 42

Sexual dimorphism -

Answer 42

a difference in secondary sexual characteristics between males and females of the same species

Question 43

Intrasexual selection -

Answer 43

meaning selection within the same sex, individuals of one sex compete directly for mates of the opposite sex. In many species, intrasexual selection occurs among males.

Question 44

Intersexual selection (mate choice) -

Answer 44

where individuals of one sex (usually females) are choosy in selecting their mates from the other sex

Question 45

Balancing selection -

Answer 45

this type of selection includes frequency-dependent selection and heterozygote advantage

Question 46

Frequency-dependent selection -

Answer 46

the fitness of a phenotype depends on how common it is in the population

Question 47

Heterozygote advantage

Answer 47

If individuals who are heterozygous at a particular locus have greater fitness than both kinds of homozygotes