Lecture 13: Population Genetics

Question 1

What is population genetics?

Answer 1

The study of the genetic variation in populations, and the evolutionary factors that explain this variation.

Question 2

What does population genetics cover?

Answer 2

• Evolutionary processes and evolutionary history at genetic level.
• Disease dynamics.
• Antibiotic resistance.
• Conservation biology.
• Pandemics.
• Breeding of domestic plants and animals.

Question 3

What are constituents of evolutionary processes?

Answer 3

• Mutation and recombination.
• Natural selection.
• Gene flow.
• Genetic drift.
• Non-random mating.

Question 4

What is evolution?

Answer 4

A change in allele and/or genotype frequency over time.

Question 5

Describe the Hardy-Weinberg equilibrium.

Answer 5

• Describes situations in which allele and genotype frequencies do not change.

+ States that allele and genotype frequencies in a population will remain constant from generation to generation in the absence of other evolutionary influences.

• Independently derived by Hardy and Weinberg in 1908.
• Based on Mendelian genetics.

Question 6

What is Hardy-Weinberg equilibrium ratio and formula?

Answer 6

• p2 : 2pq : q2 of AA : Aa : aa
• p + q = 1
• p2 + 2pq + q2 = 1

Question 7

What does the Hardy-Weinberg relation predict? What are its assumptions?

Answer 7

• Genotype frequencies from allele frequencies, and vice versa.
• Assumptions:

~ no selection

~ no migration

~ no mutation

~ large population size (no genetic drift)

~ random mating

Question 8

What are the characteristics of mutations in evolutionary processes?

Answer 8

• Source of new alleles and genetic variation.
• Don’t lead to significant change of allele or genotype frequencies on their own.
• Provide ‘raw material’ for other evolutionary forces to act on.

Question 9

What is natural selection?

Answer 9

The process whereby organisms better adapted to their environment tend to survive and produce more offspring.

~ The idea of natural selection was formulated by Darwin and Wallace (1858).

Question 10

How does natural selection influence evolutionary processes? What are some key factors for natural selection?

Answer 10

• Variation in average reproductive success among phenotypes.
• Important basis is overproduction in natural populations.
• Leads to evolution if phenotypes with different reproductive success have a genetic basis.

Question 11

How does natural selection happen?

Answer 11

• Individuals compete for resources.
• Phenotypic variation within population.
• Some phenotypes might have a better chance of reproducing than others.

Question 12

What is fitness?

Answer 12

Measure of the relative reproductive success of a genotype.

Question 13

What is adaptation?

Answer 13

A consequence of selection.

A feature that has a beneficial function and has become prevalent or is maintained in a population because of natural selection for that function.

Question 14

What is positive selection?

Answer 14

Natural selection that increases the frequency of a favourable allele.

Question 15

What is negative selection?

Answer 15

Natural selection that decreases the frequency of a deleterious (causing harm or damage) allele.

Question 16

What are some characteristics that many human genetic diseases have?

Answer 16

• Deleterious allele rare and recessive.
• Low frequency of homozygotes.
• Deleterious allele remains in population in heterozygous individuals over long period of time.

Question 17

What is balancing selection?

Answer 17

Natural selection that acts to maintain two or more alleles in a population.

Question 18

What is an example of balancing selection? Explain it.

Answer 18

Sickle cell anemia heterozygote advantage.

AA: vulnerable to malaria
SS: affected by sickle cell anemia
SA: protected against malaria, not affected by sickle cell anemia

—> S and A alleles are maintained in population at intermediate frequencies in regions with malaria.

Question 19

What are the types of effects of natural selection on a particular trait?

Answer 19

• Stabilising – maintains status quo and acts against extremes.
• Directional – leads to a change in a trait over time.
• Disruptive – operates in favour of extremes and against intermediate forms.

Question 20

What is sexual selection?

Answer 20

Promotes traits that increase an individual’s access to reproductive opportunities.

Question 21

What is the difference between intersexual and intrasexual selection?

Answer 21

• Intersexual — members of one sex compete for the attention of the other sex (interaction between males and females).
• Intrasexual — members of one sex compete for access to the other sex (interaction between individuals of the same sex).

Question 22

What is gene flow?

Answer 22

The movement of alleles from one population to another (caused by migration).

Question 23

What are the functional characteristics of gene flow?

Answer 23

• Increases genetic diversity in the receiving population.
• Makes populations more similar to each other.
• Can be maladaptive.

Question 24

What is genetic drift?

Answer 24

• Random change in allele frequencies from generation to generation.

Question 25

What is the genetic drift’s effect on small populations compared to large ones?

Answer 25

• Genetic drift has a stronger effect on small populations.

Question 26

What are two ways genetic variation could be lost?

Answer 26

• Population bottleneck — catastrophic reduction in population.
• Founder effect — a few individuals of a population start a new population.

Question 27

What is non-random mating?

Answer 27

• A mating system in which at least some individuals are more or less likely to mate with individuals of a particular genotype than with individuals of other genotypes.
• It leads to a change of genotype frequencies, but not to a change of allele frequencies.

Question 28

What are two types of non-random mating? What do they lead to?

Answer 28

• Assortative – individuals with same genotype more likely to mate than individuals with different genotypes.

~ Lead to excess of homozygotes.

• Disassortative – individuals are more likely to make with individuals of a different genotype.

~ Lead to excess of heterozygotes.

Question 29

Does natural selection always lead to evolution?

Answer 29

No.