Lecture Notes: Population Genetics

Question 1

population genetics overview

Answer 1

studies the variation in gene/allele frequencies through time

1940’s Integration of evolutionary theory with genetics; neodarwinism (modern synthesis)

Question 2

neodarwinism

Answer 2

population are the units of evolutions, not individuals
natural selection is the most important mechanism, but there are others
gradualism

Question 3

population

Answer 3

localized group of interbreeding individuals of the same species

Question 4

Gene Pool

Answer 4

total set of genes in a population

fixed alleles: those for which all of the individuals in a population are homozygous

Question 5

Hardy Weinberg Model

Answer 5

• gene pool in equilibrium
• meiosis and random fertilization maintain the same allele/genotype frequency as in P1, then the population is equilibrium
• if values start to deviate, then the population is evolving

Question 6

Hardy Weinberg Assumptions

Answer 6

1. Large population size
2. no random genetic drift
3. no gene flow
4. no mutation
5. panmixia ( complete random mating)
6. No natural selections

Question 7

microevolution

Answer 7

change in genotype/allele frequencies in a population from generation to generation

Question 8

Mechanisms of evolultion

Answer 8

1. random genetic drift
2. gene flow
3. mutation
4. non-random mating (inbreeding and assortative mating)
5. Natural Selection

Question 9

Random Genetic Drift

Answer 9

• changes in the gene pool of a small population due to chance
• sampling error: disproportionate results in small samples
• new generations get alleles at random so:
  a. the smaller population size, the greater the chance that the allele frequencies in the new generation will not represent P1

Question 10

Conclusion of random genetic drift

Answer 10

the changes in frequencies from one generation to the next is unpredictable (random sampling of alleles)

on average small populations will have some alleles fixed faster than larger populations

leads to two phenomena: bottleneck and flounder

Question 11

bottleneck effect

Answer 11

• a drastic environmental change can results in small sample of population surviving
• the smaller the sample, the less the chance that its gene pool is representative of the original population
• leads to the decreased variation

Question 12

founder effect

Answer 12

1. few colonizing individuals that survive
2. sample unlikely to be representative of population of origin
3. specially important in islands
4. wallace line

Question 13

gene flow

Answer 13

• genetic exchange by migration of fertile individuals or exchange of gametes between populations
• directions and flow affects the change in the gene pool
• if rates of flow are equal, gene flow has a homogenizing effect among divergent population

Question 14

Mutations

Answer 14

• basic origin of variation
• if it occurs in gametes, then it is transmitted to the next generation, and affects the gene pool
• rates vary among groups and gene loci
• small effects in large populations
• rate of spread of mutations can increase by the disproportionate reproduction of some populations

Question 15

non-random mating

Answer 15

can occur by:

a. inbreeding: mating among close relatives or self-fertilization
b. assortative mating: selection of mates based on specific phenotypes
- leads to loss of variation
- specially important in sessile organisms

Question 16

Natural Selection

Answer 16

• Differential reproductive success based on the variation among individuals within a population
• leads to adaptation: accumulation and maintenance of more favorable genotypes for a specific enviornment

Question 17

Mode of natural selection

Answer 17

• stabilizing
• directional
• diversifying (disruptive)

Question 18

Stabalizing Selection

Answer 18

• selection against extreme phenotypes
• selection for average phenotypes
• results in reduced variation

Question 19

Directional Selection

Answer 19

• Selection against one extreme
• opposite extreme phenotype is favored
• results in shift in average phenotype

ex: increase horns in cattle to protect the calfs

Question 20

Disruptive Selection

Answer 20

• selection against average phenotypes
• extreme phenotypes are favored
• results in bimodal distribution

Question 21

Variation

Answer 21

• continuous: height
• discrete : ABO blood groups
• polymorphism: when 2 or more “morphs” of a discrete character are present in a population

Question 22

Clines

Answer 22

• Gradual changes in a character along a geographical axis
• interbreeding populations line up along the axis
• environmental variables change gradually along the axis
• some times the population at the extremes are reproductively isolated from each other, although there is a continuous gene flow along the axis

Question 23

origin of variation

Answer 23

mutation
recombination
important aspect to remember:
a. generation time
b. neutralist vs selectionist
c. macromutation- "hopeful monster" vs homeotic genes

Question 24

Maintenance of variation

Answer 24

diploidy

balanced polymorphism

Question 25

diploidy

Answer 25

hiding the variation in heterozygote | a. the rarer the recessive allele is the greater the proportion in heterozygotes

Question 26

balanced polymorphism

Answer 26

NS maintains it in a population

Question 27

Examples of balanced polymorphism

Answer 27

a. heterozygote advantage b. Hybrid Vigor c. patchy environment + Natural Selection d. frequency-dependent selection

Question 28

heterozygote advantage

Answer 28

heterozygotes with greater survivorship and reproductive success than homozygotes ex: sickle cell anemia in areas with malaria

Question 29

Hybrid Vigor

Answer 29

crossbreeding between inbred varieties | leads to increased heterozygosity at many loci and decrease of deleterious recessives

Question 30

patchy environment + natural selection

Answer 30

heterogeneous environment within the boundaries of a population leads to maintenance of different phenotypes

Question 31

frequency-dependent selection

Answer 31

reproductive success of phenotype decreases if the density of the phenotype increases