Chapter 6The Ways of Change: Drift and Selection

Question 1

what is the term for the study of the distribution of alleles in populations and causes of allele frequency changes?

Answer 1

Population genetics

Question 2

__________: location of a specific gene or sequence of DNA on a chromosome

__________: individual carries two copies of the same allele

__________: individuals carries different alleles

Answer 2

Genetic locus
Homozygous
Heterozygous

Question 3

Diploid individuals carry two alleles at every locus
__________: alleles are the same
__________: alleles are different
Evolution: __________________

Answer 3

Homozygous
Heterozygous
Evolution: change in allele frequencies from one generation to the next

Question 4

Hardy-Weinberg equilibrium

Population allele frequencies do not change if:
5 things on population, genotypes, mutation, mating, and migration

Answer 4

Population is infinitely large
Genotypes do not differ in fitness
There is no mutation
Mating is random
There is no migration

Question 5

Allele frequencies predict genotype frequencies

write down the Hardy-Weinberg equation

Answer 5

p2 + 2pq + q2 = 1

Question 6

what does the Hardy-Weinberg theorem prove?

Mechanisms of evolution are forces that do what?

Answer 6

proves that that allele frequencies do not change in the absence of drift, selection, mutation, and migration

Mechanisms of evolution are forces that change allele frequencies

Question 7

Populations Evolve Through a Variety of Mechanisms

analyze and review figure on powerpoint slide pg. 12

Question 8

Key Concept

Hardy-Weinberg serves as the fundamental?

Answer 8

null model in population genetics

Question 9

what does Bottlenecks reduce?

Answer 9

genetic variation

Question 10

Key Concepts

____ ____ is the random, non-representative sampling of alleles from a population during breeding

________ are lost more ____ in ____ populations

Answer 10

Genetic drift is the random, non-representative sampling of alleles from a population during breeding

Alleles are lost more rapidly in small populations

Question 11

Key Concept

Even brief bottlenecks can lead to a drastic reduction in genetic diversity that can persist for generations

Question 12

The Concept of Fitness

Fitness: ________________
Components of fitness:
*
*
*
____ fitness: fitness of a genotype standardized by comparison to other genotypes

Answer 12

Fitness: the reproductive success of an individual with a particular phenotype
Components of fitness:
Survival to reproductive age
Mating success
Fecundity
Relative fitness: fitness of a genotype standardized by comparison to other genotypes

Question 13

Contribution of Alleles to Fitness

_____ ____ _: difference between average fitness of individuals with allele vs. those without

Answer 13

average excess fitness

Question 14

Natural selection more powerful in large populations

Drift is ____ in large populations
____ advantages in fitness can lead to ____ changes over the long term

Answer 14

Drift is weaker in large populations
Small advantages in fitness can lead to large changes over the long term

Question 15

Pleiotropy May Constrain Evolution

Pleiotropy: ________________
Can be antagonistic
Net effect on fitness determines outcome of selection

Answer 15

mutation in a single gene affects many phenotypic traits

Question 16

Natural Selection in Action

Alleles that lower fitness experience ______ ______
Alleles that increase fitness experience ______ ______

Answer 16

negative selection
positive selection

Question 17

Dominance: Allele versus Allele
Relationships Among Alleles at a Locus

All alleles can be either ________ or ________ in terms of their effects on fitness

________: allele yields twice the phenotypic effect when two copies present
It can be good, or twice as good!
It can be bad, or twice as bad!

________: dominant allele masks presence of recessive in heterozygote
Recessive alleles can “________”

Answer 17

• negative or positive
• Additive
• Dominance
• “hide”

Question 18

. Alleles with additive effects on phenotypes are always exposed to selection, so they will increase steadily from the moment they arise due to mutation until they are fixed in the population.

Question 19

Recessive alleles are not exposed to selection initially, because they are likely to occur only in heterozygous genotypes. They may linger for thousands of generations until drift either removes them or increases their frequency. Eventually, if drift increases their frequency sufficiently, homozygous recessive individuals will begin to appear in the population. As soon as this happens, selection will begin to increase the frequency of the allele and swiftly carry it to fixation.

Question 20

Dominant alleles are exposed to selection immediately and their frequency will increase rapidly. However, as dominant alleles become increasingly common, the alternative alleles (by definition, recessive) become increasingly rare. As we’ve just seen, rare recessive alleles are invisible to the action of selection because they are carried in a heterozygous state. Thus, selection alone is unlikely to fix a completely dominant allele.

Question 21

Selection occurs when genotypes

Answer 21

differ in fitness

Question 22

Outcome of selection depends on

Answer 22

frequency of allele and effects on fitness

Question 23

Population size influences power of drift and selection
- Drift more powerful in _______
- Selection more powerful in ________

Answer 23

small population

large population

Question 24

Alleles may have pleiotropic effects
- When fitness effects oppose each other ________ determines direction of selection
Laboratory evolution studies reveal how alleles rise and spread through populations
Rare alleles almost always carried in a heterozygous state
- Recessive alleles invisible to selection
- Selection cannot drive dominant to fixation

Answer 24

• ## environment

Question 25

_______ are the source of new genetic variation in populations
- Can be many in a large population

Answer 25

mutations

Question 26

Balancing selection maintains multiple alleles in populations
Negative frequency-dependent
Heterozygote advantage

Question 27

Inbreeding increases percentage of loci that are homozygous for alleles identical by descent

Genetic bottlenecks often go hand in hand with inbreeding and selection
- Recessive alleles exposed to selection

Question 28

The extent of population subdivision depends on features of the landscape as well as how readily individuals move between locations.

A: When landscapes are homogeneous and/or individuals move readily, populations have little or no subdivision.
B: At the other extreme, subpopulations may be completely isolated from each other, with almost no movement between them.
C: Many populations fall somewhere in between, such that movement between locations is less frequent than movement within but still occurs regularly enough to shuttle alleles from place to place.

Question 29

______ ________ increases faster when subpopulations are small.

Answer 29

Genetic distance

Question 30

Populations with extreme subdivision often show heterogeneity in allele frequencies from place to place called _____ reflecting divergence due to drift

Answer 30

genetic structure

Question 31

_____ _____ enhances the effects of genetic drift
Divergence in allele frequencies

Answer 31

Population subdivision

Question 32

Gene flow counteractions subdivision by

Answer 32

homogenizing allele frequencies

Question 33

Δp = p x (aA1/ϖ)

list the formula pieces for what they represent

Answer 33

Δp = the change in allele frequency due to selection
p = the frequency of the A1 allele
ϖ = the average fitness of the population
aA1 =the average excess of fitness for the A1 allele

Question 34

Equilibrium frequency reached through tug-of-war between negative selection and new mutation
Explains persistence of rare deleterious mutations in populations

Answer 34

Selection occurs when genotypes differ in fitness
Outcome of selection depends on frequency of allele and effects on fitness
Population size influences power of drift and selection
Drift more powerful in small population
Selection more powerful in large population

Question 35

Thanks to genetic drift, isolated populations grow more distant from each other over time, as alleles are randomly fixed or lost from place to place

Answer 35

Rate changes dependent on size of population

Question 36

Populations with extreme subdivision often show heterogeneity in allele frequencies from place to place, called genetic structure, reflecting divergence due to drift. Selection also can drive subpopulations apart. However, since most genetic markers are selectively neutral, spatial patterns in their relative frequencies reflect drift rather than selection.

Answer 36

Over the same landscape, species may differ in the extent of population subdivision due to their habitat preferences and behavior. Lynx, for example, move widely; as a result, their populations show almost no genetic divergence from place to place. Populations of Bighorn Sheep, in contrast, show much more extensive genetic divergence across the same distances.