Ch.23 - Evolution of Populations

Question 1

What is a population?

Answer 1

A population is a group of individuals of a single species living in the same general area.

• Interbreed
• Produce offspring

Question 2

____________ is a change in allele frequencies in a population over generations

Answer 2

Microevolution is a change in allele frequencies in a population over generations

Question 3

To det if a population is evolving, we need to monitor ___________.

Answer 3

To determine if a population is evolving, we need to monitor allele frequency

• Use Hardy-Weinberg models

Question 4

Differentiate genotype and phenotype

Answer 4

Genotype: an individual’s genetic makeup

Phenotype: the product of the genotype and the environment

1. Physical phenotype: flower color
2. Molecular phenotype: blood group (A, B, O, AB)

Question 5

_________ are categorical traits, i.e. either/or.

__________ are traits that vary along a continuum.

Answer 5

Discrete characters are categorical traits, i.e. either/or.

• E.g. purple or white flower petal.
• Often det by single gene locus w diff alleles → distinct phenotypes.

Quantitative characters are traits that vary along a continuum.

• Typ results fr influence of 2+ genes.

Question 6

T/F: Natural selection can only act on genetic variation.

Answer 6

TRUE

Natural selection can only act on genetic variation.

I.e. only variation in genes (heritibal traits) can be passed fr parent to offspring.

Question 7

What is a mutation? Mutation in wh cells are lost? Passed on?

Answer 7

Mutation: change in nt seq of an org’s DNA

• Point mutation: change of a single base in one gene.

In animals, mutations occur in somatic and germ cells, but only variation in germ cells can be passed on to offspring; somatic mutations are lost when an individual dies.

Question 8

Most mutations are considered at least slightly harmful. Why?

Answer 8

Orgs rep thousands of generations of past selection.

Unlikely that a new mutation will improve a phenotype.

Question 9

What does the Hardy-Weinberg principle state?

Answer 9

If only Mendelian segregation and recombination are present:

• Freqs of alleles and genotypes will remain constant in ea gen
• Population said to be in Hardy-Weinberg equilibrium (HWE)

Question 10

What conditions disrupt HWE?

Answer 10

If any of the following occur → no HWE → change in allele freq over generations, i.e. pop is evolving:

1. Mutation
2. Non-random mating
3. Natural selection
4. Genetic drift
5. Gene flow

Question 11

What do mutations, non-random mating, natural selection, genetic drift, and/or gene flow indicate wrt HWE?

Answer 11

These conditions result in changes in allele freq over generations → pop is evolving → no HWE.

Question 12

How can we tell whether a pop is evolving?

Answer 12

1. Det genetic makeup if there was no change in allele freqs.
2. Compare to data collected fr pop
3. Decide whether distribution of allele freqs differ

Question 13

How do you calc the freq of an allele in a population?

Answer 13

Calc Allele Freq:

• For diploid orgs, total # of alleles at a locus:

Total # of individuals x 2

• Total # of dominant alleles at a locus:

2 alleles for ea homo dominant individual, plus 1 allele for ea hetero individual.

• Same logic applies for recessive alleles

E.g. gene w two alleles: A and a. Possible genotypes for pop of 160 individuals:

• AA = 22
• Aa = 68
• aa = 70
• Caution: allele freq is allele count divided by total # of alleles.

Question 14

In HWE, what symbols rep the freq of the dominant and recessive alleles?

Answer 14

p = freq of dominant allele

q = freq of recessive allele

E.g. if freq of dominant allele ‘R’ is 0.8 (p = 0.8), then freq of homo dominant genotype is p² = 0.8² = 0.64, i.e. 64%.

• q = 1 - p = 1 - 0.8 = 0.2 → q² = 0.04, i.e. 4% are homo recessive.
• Rest must be hetero: 1 - 0.64 - 0.04 = 0.32, or 32%
  
  • Alt: p*q = 0.8*0.2 = 0.16 → mult by 2 bc pq or qp → 0.32 or 32%.

Question 15

In HWE, what symbols rep allele freqs vs genotype freqs?

Answer 15

Allele frequencies

• C^R or p
• C^W or q
• p + q = 1

Genotype frequencies

• C^RC^R → p²
• C^WC^W → q²
• C^RC^W and C^WC^R → 2pq

Question 16

The five conditions for nonevolving populations are rarely met in nature.

Answer 16

TRUE

The five conditions for nonevolving populations are rarely met in nature

• W/i a pop, some loci are likely in HWE while others are evolving.

–No mutations

–Random mating

–No natural selection

–Extremely large population size

–No gene flow

Question 17

Which conditions against HWE have minor impact on allele freqs? Which directly alter allele freqs?

Answer 17

Minor impact: mutation and non-random mating.

Directly alter allele freq: genetic drift, gene flow, natural selection.

Question 18

What is genetic drift?

Answer 18

Describes how allele freqs fluctuate randomly and unpredictably b/w gens.

Tends to ↓ GV thru loss of alleles → harmful alleles may become fixed.

As pop size ↓ → impact of ‘chance’ ↑

• Founder effect
• Bottleneck effect

E.g. species colonize an island → resulting pop typ has LESS GV than mainland pop.

Question 19

What is the founder effect?

Answer 19

Founder effect: A new pop might be started fr only a few individuals → gene pool may differ fr source pop.

Question 20

What is the bottleneck effect?

Answer 20

Dramatic ↓ in pop → certain alleles now underrep’d.

• Even if pop size recovers, GV is lower.

Question 21

What is gene flow?

Answer 21

Gene flow is migration: xfr of alleles into/out of a pop, due to movement of fertile individuals (or their gametes).

Tends to ↓ GV b/w pops.

• May result in formation of new pop fr two neighboring groups.
• Can ↓ fitness if neighboring pops live in diff environs.
• Occurs at higher rate than mutation.

Question 22

T/F: gene flow occurs at lower rate than mutation.

Answer 22

FALSE

Gene flow occurs at higher rate than mutation

Question 23

What was Darwin’s view on natural selection?

Answer 23

Alleles are passed to next gen in diff proportions than in current generation.

I.e. differential reproductive success.

Inference: unequal fitness → accumulation of favorable traits in pop over gens → adaptive evolution → better match b/w org and environ.

Question 24

________ fitness is the total # of offspring contributed to next gen, while ______ fitness is the contribution an individual makes to the gene pool relative to the contribution of others.

Answer 24

Darwinian fitness is the total # of offspring contributed to next gen, while relative fitness is the contribution an individual makes to the gene pool relative to the contribution of others.

Remember: entire organism (NOT just one locus) is subjected to NS → phenotypes (NOT genotypes) are selected.

Question 25

Three modes of NS:

________ selection favors individuals at one end of the phenotypic range.

________ selection favors individuals at both extremes of the phenotypic range.

________ selection favors intermediate variants and acts against extreme phenotypes.

Answer 25

Three modes of NS:

Directional selection favors individuals at one end of the phenotypic range.

• E.g. coral reef pop facing rising sea water temps → high T resilient coral selected for.

Disruptive selection favors individuals at both extremes of the phenotypic range.

Stabilizing selection favors intermediate variants and acts against extreme phenotypes.

Question 26

T/F: selection can only act on existing variation.

Answer 26

TRUE

Selection can only act on existing variation.

I.e. can’t create new mutations.

Question 27

Diploidy and balancing selection help preserve GV. Explain.

Answer 27

Diploidy - GV can be ‘hidden’ in recessive alleles.

Balancing selection - NS maintains 2+ alleles

• Two mechanisms: heterozygote advantage and freq-dep selection.

Question 28

What is meant by a heterozygote advantage?

Answer 28

When indiviudals who are heterozygotes at a partic locus have a greater fitness than do both kinds of homozygotes.

Caution: defined in terms of genotype, NOT phenotype.

Question 29

What is frequency-dependent selection?

Answer 29

Frequency-dependent selection: fitness of a phenotype ↓ if it becomes too common in pop.

• Selection can favor whichever phenotype is less common in a population

Question 30

_________ is a form of natural selection in which individuals with certain inherited characteristics are more likely than others to obtain mates.

Answer 30

Sexual selection is a form of natural selection in which individuals with certain inherited characteristics are more likely than others to obtain mates.

Often leads to sexual dimorphism: marked diffs b/w sexes in secondary sexual characteristics; e.g. size, color, ornamentation, behavior, etc.

Question 31

_______ selection is sexual selection w/i the same sex (often among males).

_______ selection is when individuals of one sex, typ females, are choosy in selecting their mates.

Answer 31

INTRAsexual selection is sexual selection w/i the same sex (often among males).

• Compete directly for mates of opp sex

INTERsexual selection is when individuals of one sex, typ females, are choosy in selecting their mates.

• Selects for showiness of males physicality or behavior, which may pose a risk to survival

Question 32

What is the good genes model of sexual selection?

Answer 32

An external characteristic of males (song, coloration, morphology) indicates that he has good genes.

• Females that mate with him would produce high quality offspring.

Question 33

What is balancing selection?

Answer 33

Balancing selection: natural selection maintains two or more alleles

Two mechanisms:

• Heterozygote advantage
• Frequency-dependent selection

Question 34

Genetic variation refers to genetic differences
among individuals within a population.

What are the primary sources of GV in sexually reproducing orgs?

Answer 34

Crossing over, indep assortment of chromos, and fertilization.

Question 35

A population is a localized group of orgs belonging to one species, united by its ________, the aggregate of all alleles in the population.

Answer 35

A population is a localized group of orgs belonging to one species, united by its gene pool, the aggregate of all alleles in the population.

Question 36

For a population in Hardy-Weinberg equilibrium, the allele and genotype frequencies will remain constant if the population is _____ (large or small), mating is _______ (non-/random), mutation is ______ (signif/negl), there ____ (is/isn’t) gene flow, and there ____ (is/isn’t) natural selection.

Answer 36

For a population in Hardy-Weinberg equilibrium, the allele and genotype frequencies will remain constant if the population is large, mating is random, mutation is negligible, there is no gene flow, and there is no natural selection.

• Note: pop size and genetic drift refer to the same characteristic. As pop size ↑ → drift ↓.

Question 37

What is the Hardy Weinberg Equilibrium equation?

Answer 37

p² + 2pq +q² = 1.

Question 38

Which type of selection tends to increase genetic variation?

Answer 38

Disruptive selection eliminates phenotypes near the average and favors the extreme phenotypes → ↑ GV in a population.

Question 39

In bell-shaped selection curves, what does ea axis represent?

Answer 39

X-axis: value of a partic characteristic, e.g. size, color.

Y-axis: # of individuals

Question 40

T/F: Heterozygote advantage refers to the tendency for heterozygous individuals to have better fitness than homozygous individuals. This higher fitness results in less genetic variation in the population.

Answer 40

FALSE

Heterozygote advantage ↑ GV in the population.

Question 41

Which of the following are basic components of the Hardy–Weinberg model?

• Freqs of two alleles in a gene pool before and after many random matings
• Allele freqs, # of individuals in pop
• Allele freqs, phenotype freqs
• Allele freqs in a subset of the pop

Answer 41

Which of the following are basic components of the Hardy–Weinberg model?

• Freqs of two alleles in a gene pool before and after many random matings
• Allele freqs, # of individuals in pop
• Allele freqs, phenotype freqs
• Allele freqs in a subset of the pop

Question 42

What is the frequency of the A1 allele in a population composed of 20 A1A1 individuals, 80 A1A2 individuals, and 100 A2A2 individuals?

Answer 42

Freq of A1 allele is 0.3.
p = (# of a1 alleles) / (total of all alleles)

p = [(2(20) + 80] / [(2 x 20) + (2 x 80) + (2 x100)]

p = 0.3