Hardy-Weinberg Equilibrium: Genetic Drift and Neutral Theory

Question 1

what is drift

Answer 1

• chance events
• Random events that cause allele frequencies to change

Question 2

Does drift cause changes in allele frequencies?

Answer 2

yes

Question 3

Does drift adapt animals to environment?

Answer 3

no because its completely random

Question 4

does drift violates the HW assumption 4 that population sizes are infinitely large

Answer 4

yes it does bc it changes allele frequencies

Question 5

Does drift increase or decrease variation?

Answer 5

• Decrease bc some alleles go into fixation by chance
• Randomly leads to some alleles disappearing

Question 6

what are 3 patterns about genetic drift

Answer 6

1. unique evolutionary path
2. effect in small populations
3. changes in large populations

Question 7

3 patterns about genetic drift - unique evolutionary path

Answer 7

Because fluctuations in allele frequencies are caused by random sampling error, every population follows a unique evolutionary path

Question 8

3 patterns about genetic drift - effect in small populations

Answer 8

Genetic drift has more rapid and dramatic effect on allele frequencies in small populations that in large populations

Question 9

3 patterns about genetic drift - changes in large populations

Answer 9

Given enough time, genetic drift can produce substantial changes in allele frequencies even in populations that are fairly large

Question 10

if genetic drift is the only evolutionary mechanism at work, what is the outcome of a locus with 2 alleles?

Answer 10

Eventually one allele will drift to a frequency of 1 (fixation) and all others will be lost

Question 11

What is the probability that a given allele (A vs a) will be the one that is fixed?

Answer 11

It is equal to its initial frequency

Question 12

If frequency of A1 in a population is 0.1 (10%), then its probability of fixation is _____

Answer 12

• 0.1 (10%)
• 90% chance of becoming lost

Question 13

Wandering of allele frequencies produces what two important effects

Answer 13

1. Random Fixation of Alleles
2. Loss of Heterozygosity

Question 14

two important effects - loss of heterozygosity

Answer 14

As allele frequencies in a finite population drift toward fixation or loss, the frequencies of heterozygotes decreases

Question 15

loss of heterozygosity - why does this happen

Answer 15

• Heterozygotes are at their highest frequency when A1 = 0.5 and A2 = 0.5
• As one allele or the other drifts to fixation, you get farther from this value

Question 16

example of random fixation and loss of heterozygosity in natural populations

Answer 16

Templetonn et al. Ozark
collared lizards

Question 17

random fixation and loss of heterozygosity - Templetonn et al. prediction

Answer 17

• glade population of lizards is small in size and had genetic isolation
• bc of this, they predicted that the lizards would bear a strong imprint of genetic drift

Question 18

Templetonn et al. experiment predictions - loci fixation within populations

Answer 18

within populations, most loci should be fixed for a single allele, and genetic variation should be very low

Question 19

Templetonn et al. experiment predictions - allele fixation

Answer 19

Which allele became fixed in any particular population should be a matter of chance, however, so there should be considerable genetic diversity among populations.

Question 20

Templetonn et al. experiment - result

Answer 20

• most glade populations were fixed for a single multilocus genotype, and different genotypes were fixed in different glades
• seven distinct multilocus genotypes

Question 21

Important points about drift

Answer 21

1. causes allele frequencies to change at random
2. does not adapt animals to environment
3. impacts small populations more than large ones
4. causes loss of genetic variation within populations
5. can cause harmful alleles to become fixed BY CHANCE

Question 22

important points about drift - causes loss of genetic variation within populations

Answer 22

alleles become fixed AND heterozygosity is lost

Question 23

important points about drift - can cause harmful alleles to become fixed BY CHANCE

Answer 23

Because it is a completely random process

Question 24

what causes small genetic population sizes?

Answer 24

• founder
• bottleneck

Question 25

what causes small genetic population sizes? - founder

Answer 25

• Few individuals found the population and did not bring many genes with them
• Not a lot of genetic diversity if the population size grows to become large

Question 26

what causes small population sizes? - bottleneck

Answer 26

• Massive population gets squeezed so the population is lowered
• Even if population, recovers, genetically it cannot recover bc it got so small

Question 27

what was neutral theory used to explain

Answer 27

• the discovery that amino acids sequences change constantly like a clock
• there is one fixation every 2 years

Question 28

neutral theory - beneficial mutations

Answer 28

Beneficial mutations are rare, start at low frequency, and are often lost—uncommon—can be fixed via positive selection

Question 29

neutral theory - deleterious mutations

Answer 29

Deleterious mutations are removed by purifying selection (so don’t contribute to evolution)

Question 30

neutral theory - neutral mutations

Answer 30

Neutral mutations rise and fall b/c drift

Question 31

what are neutral theory ideas

Answer 31

• most of the mutations going to fixation must be neutral
• drift dominates sequence evolution
• rate of molecular evolution is equal to the mutation rate
• asks if these neutral mutations cause adaptations

Question 32

what are neutral theory predictions

Answer 32

• non-coding regions of the genome (pseudogenes) should be neutral - no effect on fitness
• synonymous substitutions should be more frequent than nonsynonymous

Question 33

neutral theory predictions - how to test that pseudogenes should be neutral

Answer 33

they should have the fastest mutation rates

Question 34

neutral theory predictions - why would synonymous substitutions > nonsynonymous

Answer 34

• Synonymous/silent changes are not exposed to selection so they operate under drift.
• Nonsynonymous should be mostly eliminated via negative or purifying selection

Question 35

how can you test for positive selection

Answer 35

• when polymorphism is present
• you can compare ratio of nonsynonymous vs synonymous substations
• allows us to measure the signature of positive selection

Question 36

what is polymorphism

Answer 36

different individuals in population have different versions/alleles

Question 37

test for positive selection - nonsynonymous-synonymous ratio < 1

Answer 37

• deleterious
• When sequences evolve by drift and negative selection, synonymous substitutions outnumber replacement substitutions.

Question 38

test for positive selection - nonsynonymous-synonymous ratio = 1

Answer 38

neutral

Question 39

test for positive selection - nonsynonymous-synonymous ratio > 1

Answer 39

• positive
• When sequences evolve by drift and positive selection, replacement substitutions outnumber synonymous substitutions.