Week 6: Factors Influencing Genetic Diversity

Question 1

What two factors have universal relevance to genetic diversity?

Answer 1

• genetic drift
• natural selection

Question 2

What two factors have species-specific relevance to genetic diversity?

Answer 2

• reproduction
• ecology/life history traits

Question 3

What is genetic drift?

Answer 3

changes in allele frequencies over generations as a result of genetic drift

Question 4

What are some results of genetic drift in a population ? (3)

Answer 4

• reduced genetic diversity
• non-adaptive evolutionary change
• alleles are driven to fixation

Question 5

Why is genetic drift stronger in small populations?

Answer 5

Demographic stochasticity - random fluctuations in survival probabilities

Question 6

What is the census population size? (Nc)

Answer 6

the true population size (Nc)

Question 7

What is the Effective Population Size? (Ne)

Answer 7

a measure of the number of individuals in the population that will contribute genetically to the next generation

Question 8

What does effective population size (Ne) reflect?

Answer 8

the rate at which genetic variation is lost as a result of genetic drift (in an ideal population)

Question 9

What is an ideal population?

Answer 9

one that meets the assumption of HWE

Question 10

What is effective breeding population? (Nb)

Answer 10

the number of adults breeding in one reproductive season

Question 11

How can you quantify the rate of genetic loss due to drift?

Answer 11

Track the ratio of Ne/Nc

Question 12

Between Nc and Ne, which is smaller?

Answer 12

Ne

Question 13

What 4 factors influence Ne?

Answer 13

1. variation in reproductive success
2. uneven sex ratios
3. population bottlenecks
4. life history

Question 14

What is variation in reproductive success?

Answer 14

some individuals will have more offspring than others

Question 15

What life history traits influence reproductive success?

Answer 15

• competition for mates
• r-selected survival stochasticity (ex. coral gametes)

Question 16

Why do uneven sex ratios reduce diversity?

Answer 16

• fewer reproduction events, or one male mating with multiple females
• less variation in next generation!

Question 17

Why do population bottlenecks affect diversity?

Answer 17

• when population recovers, less founding diversity means less diversity overall

Question 18

What is an application of Ne?

Answer 18

• Seychelles warbler
• low genetic diversity, but historical specimens show higher diversity
• therefore, bottleneck event at some point in history

Question 19

How does life history affect Ne?

Answer 19

• longer lifespans = longer generation time, less generational turnover
• long, stable generations will drift less than sporadic populations

Question 20

How is effective population size measured?

Answer 20

• changes in allele frequencies due to genetic drift (if there is drift, there is non-random mating etc, so smaller Ne!)
  (assumes changes are not due to selection, mutation, migration etc)

Question 21

How does drift affect Ne?

Answer 21

lowers it!

Question 22

What can natural selection do to genetic diversity?

Answer 22

increase OR decrease it!

Question 23

How do population bottlenecks affect genetic diversity?

Answer 23

• decrease Ne
  –> increase genetic drift
  –>decrease genetic diversity

Question 24

What two factors determine how much genetic diversity is lost in a bottleneck?

Answer 24

1. number of individuals lost (affects how much diversity is lost)
2. Time to recover (longer time means greater loss due to genetic drift)

Question 25

How could you estimate genetic drift from microsatellite data?

Answer 25

random alleles are lost, leaving random gaps in the distribution of allele frequencies

Question 26

What are founder effects?

Answer 26

a type of bottleneck via the introduction of a small population to a new area (subset of diversity)

Question 27

How are Isle Royale Gray Wolves an example of genetic drift?

Answer 27

- small founding population - bottleneck caused by parvovirus - (also high inbreeding, was genetically rescued)

Question 28

What types of natural selection decrease genetic diversity?

Answer 28

- directional, stabilizing, and disruptive selection favour a small subset of diversity - positive frequency-dependent selection eliminates rare phenotypes (ex. snail shells, warning colouration)

Question 29

What types of natural selection increase genetic diversity?

Answer 29

- balancing selection (ex. heterozygote advantage/overdominance) - negative frequency-dependent selection (maintains diversity by favouring rare alleles) (type of balancing selection)

Question 30

What is the paradox of sexual reproduction?

Answer 30

- sex is costly - males are inefficient

Question 31

Why is sexual reproduction so successful?

Answer 31

Genetic recombination - meiosis - recombination - independent assortment

Question 32

In cloning species, how might we measure genetic diversity?

Answer 32

clonal richness - proportion of indidivuals with unique genotypes

Question 33

When might a species switch from asexual to facultative sexual reproduction?

Answer 33

When conditions are bad and genetic diversity becomes more important

Question 34

How do you calculate clonal richness?

Answer 34

R = (G-1)/(n-1) - G = number of genotypes - n = number of individuals - R=1 (each ramet is distinct) - R=0 (all individuals are same genet)

Question 35

What life history/ecology/landscape traits can influence genetic diversity? (landscape genetics)

Answer 35

- life span and age to maturity - migration patterns - habitat type - habitat connectivity

Question 36

How did landscape genetics influence spadefoot toads?

Answer 36

- some individuals expanded their range into novel desert habitat - low Ne/diversity in new habitat = bottleneck - genetic rescue by hybridizing with an adapted species