Chapter 26 - Evolutionary Processes

Question 1

Assumptions of the Hardy-Weinberg Principle with respect to a particular gene:

Answer 1

1. Random mating - Individuals cannot choose a mate.
2. No natural selection - All members of the parental generation survived and contributed equally to the gene pool, no matter their genotype.
3. No genetic drift - Alleles were picked in their exact frequencies p and q and not some different values caused by chance.
4. No gene flow - No new alleles were added by immigration or lost through emigration.
5. No mutation - No new alleles are introduced into the gene pool through mutation.

Question 2

How many and what are the evolutionary processes that can shift allele frequencies in populations over time, causing evolution?

Answer 2

Four processes;

1. Natural Selection
2. Genetic Drift
3. Gene Flow
4. Mutation

Question 3

How does Natural Selection cause evolution?

Answer 3

Natural Selection increases the frequency of certain alleles (the ones leading to repro success). This leads to adaptation (it’s the only one of the four processes that leads to adaptation.)

Question 4

How does genetic drift cause evolution?

Answer 4

Genetic drift causes allele frequencies to change randomly. Drift can even cause fitness-decreasing alleles to increase in frequency.

Question 5

How does gene flow cause evolution?

Answer 5

Gene flow is when individuals leave one population, join another, and breed. This causes evolution.

Question 6

How does mutation cause evolution?

Answer 6

Mutation modifies allele frequencies by continually introducing new alleles. New alleles can be beneficial or deleterious.

Question 7

What are the key ideas of Chapter 26?

Answer 7

Key ideas:

1) Natural selection is not the only agent responsible for evolution.
2) Each of the four evolutionary processes has different consequences for genetic variation and fitness.

Question 8

What was Hardy-Weinberg’s guiding question?

Answer 8

What happens in an entire population when all the individuals – and thus all possible genotypes – bred? They imagined that all of the allales from all the gametes produced in each generation go into a single group called the gene pool and then combine at random to form offspring. This roughly happens in clams and sea stars.

Question 9

Describe at a high level Hardy-Weinberg’s methodology

Answer 9

Calculate what happens when you pluck at random two gametes from the gene pool, mate them to form offspring, and repeat many times.

Question 10

Two fundamental claims of HW principle.

Answer 10

1. Given allele frequencies p and q, then generation after generation will have p+q=1 and p^2 + 2pq + q^2 = 1
2. When alleles are transmitted via meiosis and random combination of gametes, their frequencies do not change over time. Evolution only occurs if other factors occur.

Question 11

How do we use HW as a null hypothesis?

Answer 11

Given a set of allele frequencies, HW tells us how we could expect the genotype frequencies to look given random mating, no natural selection, no gene flow, no genetic drift, and no mutation. If we see a difference, we know one of those things is present.

Either nonrandom mating is happening (changing genotype frequencies but not allele frequencies) or allele frequencies are changing for some reason.

Question 12

Random mating: What’s the basic idea

Answer 12

Random mating happens with respect to a particular gene…i.e. individuals are not choosing their mate based on some gene.

Question 13

Describe inbreeding’s effect on the genotype frequency and allele frequency.

Answer 13

1. Inbreeding increases homozygosity.

2. Inbreeding alone does not cause evolution, because allele frequencies do not change in the population as a whole.

Question 14

Does non-random mating change genotype frequency, allele frequency, or both?

Answer 14

Non-random mating changes genotype frequency only, meaning it is not an evolutionary process.

Question 15

How does inbreeding affect evolutionary change?

Answer 15

Inbreeding doesn’t cause evolution – because it does not change allele frequency – but it can speed up evolutionary change by increasing the rate at which natural selection eliminates recessive deleterious alleles.

Question 16

Inbreeding depression

Answer 16

The decline in average fitness that happens when homozygosity increases and heterozygosity decreases.

Question 17

Causes of inbreeding depression

Answer 17

1. Many recessive alleles represent loss-of-function mutations (double-recessive individuals are often quickly eliminated by selection)
2. Many genes – especially those involved in fighting disease – are under intense selection for heterozygote diversity.

Question 18

When does evolution by natural selection occur?

Answer 18

Evolution by natural selection occurs when heritable variation leads to differential success in survival and reproduction. If certain alleles are associated with favored phenotypes, they increase in frequency while other alleles decrease in frequency.

Question 19

Genetic variation

Answer 19

Genetic variation is the number and relative frequency of alleles in a particular population. Lack of genetic variation in a population is usually bad

Question 20

What are the four modes of natural selection?

Answer 20

1. Directional Selection
2. Stabilizing Selection
3. Disruptive Selection
4. Balancing Selection

Question 21

What happens when directional selection occurs? What happens to average value and genetic variation? What is an example of disruptive selection?

Answer 21

The average phenotype of a population changes in one direction. (distribution shifts right or left). Directional selection usually reduces the genetic diversity of a population.

Think: Swallows in a famine. Small ones die.

Question 22

What is a fixed allele?

Answer 22

A fixed allele is an allele that reaches a frequency of 1.0.

Question 23

What is a lost allele?

Answer 23

A lost allele is an allele that reaches a frequency of 0.0.

Question 24

What is purifying selection?

Answer 24

Purifying selection is when disadvantageous alleles decline in frequency.

Question 25

What happens when stabilizing selection occurs? What happens to average value and genetic variation? What is an example of disruptive selection?

Answer 25

When stabilizing selection occurs, both extremes in a population are reduced.

Two conseqeuences:

1) No change in average value over time
2) Genetic variation in population is reduced

Think: Small babies and large babies are more likely to die.

Question 26

What happens when disruptive selection occurs? What happens to genetic variation? What is an example of disruptive selection?

Why is disruptive selection important?

Answer 26

Disruptive selection is the opposite of stabilizing selection. It eliminates phenotypes near the average and favors phenotypes at the extremes.

Overall genetic variation is maintained.

Think: the bill sizes of black-bellied seed-crackers. There are only small seeds and large seeds; no needs that are perfect for middle-beak-sized birds.

Disruptive selection is important because it sometimes plays a part in speciation.

Question 27

What happens with balancing selection occurs?

Answer 27

Balancing selection occurs when no single allele has a distinct advantage; there is balance among several alleles in terms of fitness and frequency.

Question 28

What are the conditions for balancing selection?

Answer 28

1. Heterozygote advantage. Genetic variation is maintained.
2. The environment varies over time. Genetic variation is maintained or icnreased.
3. Certain alleles are favored when rare but not when they are common – //frequency-dependent selection//

Question 29

What is frequency-dependent selection? Give an example

Answer 29

Frequency-dependent selection is when an allele is favored when it is rare but not when it is common.

Example: Rare alleles for guppy coloration are favored because predators learn to recognize common colors, so uncommon ones become favored. Then frequencies shift. So overall genetic variation is maintained or increased.

Question 30

What is the fundamental attribute of natural selection?

Answer 30

It increases fitness and leads to adaptation.

Question 31

Define intersexual selection

Answer 31

When an individual of one gender selects a mate from the other gender

Question 32

Define intrasexual selection

Answer 32

When individuals within the same gender compete with one another to obtain mates

Question 33

State the observed pattern and hypothesized process of the Bateman-Trivers theory

Answer 33

Pattern: Sexual selection acts on males much more strongly than on females; attractive features are more highly elaborated on males

Process: Eggs cost a lot of energy to make, whereas sperm don’t cost much.

Question 34

Define the fundamental asymmetry of sex. What are its consequences?

Answer 34

Females invest much more in their offspring than do males.

Consequences:
1. Females produce relatively few young, so female fitness is a result not of ability to find a mate but rather is a result of ability to get resources to make eggs and have healthy young.

2. Males can father limitless offspring, so a male’s fitness is a result not of resources but of the number of females he can mate with.

Question 35

What are three predictions of the Bateman-Trivers theory of sexual selection?

Answer 35

1. Females are choosy; males are not choosy.
2. Given equal numbers of M and F in a population, males will compete with each other for mates.
3. If male fitness is limited by access to mates, then alleles increasing male attractiveness or success in male-male competition should increase rapidly, violating HW assumptions.

Question 36

What is sexual dimorphism?

Answer 36

Any trait that differs between males and females. Antlers, horns, peacock ornamentation, size, body hair, etc.

Question 37

Define genetic drift

Answer 37

Genetic drift is any change in allele frequency in a population that is due to chance. Causes allele frequencies to drift up and down randomly over time.

Question 38

Which populations are especially affected by genetic drift?

Answer 38

Small populations

Question 39

Name the three key points about genetic drift:

Answer 39

1. Genetic drift is random with respect to fitness.
2. Genetic drift is most pronounced in small populations.
3. Over time, genetic drift can lead to the random loss or fixation of alleles.

Question 40

Define a founder effect. Which evolutionary process does it contribute to?

Answer 40

A founder event is a change in allele frequencies that occurs when a new population is established. It contributes to genetic drift.

Question 41

Define a genetic bottleneck. Which evolutionary process does it contribute to?

Answer 41

A genetic bottleneck is a sudden reduction in the number of alleles in a population. It contributes to genetic drift.

Question 42

Define Gene Flow. What is the outcome of gene flow as an evolutionary process?

Answer 42

When an individual leaves one population, joins another, and breeds. (More precisely, allele flow.)

Gene flow usually equalizes allele frequencies between the source population and the recipient population. The populations become more alike. In other words: Gene Flow homogenizes allele frequency

Question 43

How does gene flow relate to fitness?

Answer 43

Gene flow is random with respect to fitness. The arrival or departure of alleles can increase or decrease average fitness, depending on the situation.

Question 44

How does mutation relate to genetic diversity? How do the other three evolutionary processes?

Answer 44

Mutation introduces new genetic diversity by introducing new alleles. The other three (natural selection, genetic drift, and gene flow) all tend to decrease genetic diversity. Gene flow can increase diversity in the recipient population.

Question 45

List the three ways mutations can occur.

Answer 45

1. Point mutations
2. Chromosome-level mutations
3. Lateral gene transfer (horizontal gene transfer)

Question 46

Define point mutation

Answer 46

A change in a nucleotide that may result in a polypeptide with a new amino acid sequence.

Question 47

Define Chromosome-level mutation

Answer 47

Often gene duplication can occur

Question 48

Define lateral gene transfer (horizontal gene transfer)

Answer 48

Transfer of genes from one species to another rather than from parent to offspring. Think: Pea aphids that got the ability to produce carotenoid pigments from a fungal symbiont.

Question 49

Define mutation’s relation to fitness

Answer 49

Mutations happen randomly with respect to fitness. Most mutations are deleterious; sometimes they are beneficial.

Question 50

Describe mutation’s degree of effect on evolution

Answer 50

As an evolutionary process, mutation is very slow compared to selection, genetric drift, and gene flow.

It can have a very large effect when combined with genetic drift, gene flow, or selection.

Question 51

What are the key take-home messages about mutations?

Answer 51

1. Mutation is the ultimate source of genetic variation.
2. If mutation did not occur, evolution would eventually stop.
3. Mutation alone is usually inconsequential for changing allele frequencies at a particular gene.

Question 52

What are Darwin’s four postulates?

Answer 52

1. Individuals in a population vary in their traits
2. These differences may be passed on to offspring
3. Variation results in differential reproductive fitness
4. Natural selection occurs when individuals with certain traits produce more offspring than do individuals without those traits

Question 53

Who are the two key figures in the modern synthesis? Their sciences?

Answer 53

Mendelian genetics + Darwinian evolution

Question 54

What is assortative mating?

Answer 54

When individuals mate with other individuals that have the same genotype or phenotype.

Question 55

Give an example of a point mutation

Answer 55

The gene for hemoglobin re: sickle-cell anemia