Lecture 11

Question 1

Size of an idealized population that would have the different or same effect of random sampling on allele frequencies as that of the actual population

Answer 1

same
= Ne is the actual size that corresponds to effects of genetic drift

Question 2

Ne is smaller than N (and sometimes much smaller):

Answer 2

• Not everyone in population contributes to reproduction
• Previous bottlenecks & population size fluctuations have significant effect on variation

Question 3

How do we have to estimate Ne because it is difficult to measure

Answer 3

so we try to estimate it indirectly

Question 4

Effect of genetic drift on genetic variation:
1. Smaller population size =

Answer 4

more genetic drift = lower heterozygosity / higher homozygosity
genetic variation seems to be lost

Question 5

Effect of genetic drift on genetic variation:
2. Smaller population size increases…

Answer 5

the chance that a completely neutral new mutation will become fixed!

Question 6

When certain population sizes were put into popG what happened?

Answer 6

where they have a certain number of populations, just by chance at least one of those populations, the big A allele was completely replaced by little a allele even though there was no natural selection
- things can happen just by chance

Question 7

What would happen if you increased the population size in pop g?

Answer 7

just by chance big A will become lost

Question 8

Effect of genetic drift on genetic variation:
3. Smaller population size increases

Answer 8

the chance that a new deleterious mutation will become fixed!

Question 9

Which process (Genetic drift vs natural selection) is likely to have a bigger effect on allele frequencies?

Answer 9

This depends on how strong each process is

Question 10

how can we describe the strength of natural selection

Answer 10

We can describe strength of natural selection using ‘s’
It’s all about fitness and differential fitness
- very big s is 1
- very small s is 1% or 0.1%

Question 11

how can we describe the strength of genetic drift ?

Answer 11

We can describe the strength of genetic drift using ‘1/Ne ’
It’s all about population size and essentially effective population size
Large population sizes are not gonna be subject to genetic drift
Whereas an organism that has a very small effective population size the reciprocal of that is going to be a much bigger number

Question 12

If s&raquo_space; 1/Ne which will have the prevailing effect on allele frequencies

Answer 12

natural selection will have the prevailing effect on allele frequencies (genetic drift is likely to have little effect).

Question 13

If s &laquo_space;1/Ne which will have the prevailing effect on allele frequencies

Answer 13

genetic drift is likely to have the prevailing effect

Question 14

Effect of genetic drift on genetic variation:
4. New slightly advantageous mutations

Answer 14

are much more likely to become fixed in large populations (no genetic drift)

Question 15

why are new slightly advantageous mutations are much more likely to become fixed in large populations (no genetic drift)

Answer 15

becuase S is small so you’re wanting to counterbalance the s with the forces of genetic drift.
so organisms that have much smaller effective population sizes are gonna win out over that small selection

Question 16

What happens in popG when there is no genetic drift and no populations:
there are 3 fitnesses that are really close to each other
We are introducing a new mutation
- little a
- its advantageous, but just barely (at 1 and bi A is at 0.99)

Answer 16

It’s expected to replace the big A allele
started almost 100% with big A allele and then this new thing comes in and its got barely higher fitness and it spreads (but takes a long time, common after 10000 and fixed by 50000)
again this is only with bigger populations (no genetic drift)

Question 17

Inbred wolf populations on Isle Royale collapses

Answer 17

• population founded in 1950, became longest predator-prey study ever
• predators were wolves and prey was moose
• population had fluctuated at around 25 wolves (~50 max) but became overrun with defects, dropped to just 2 wolves
• genetic rescue by mainland wolves was proposed

Question 18

How did you think of the Isle Royale experiment

Answer 18

an experiment as a small population where you have a decrease in genetic variation
more homo and were starting to mate with eachother
- any deleterious mutations that are becoming going from being excessive

so you have the double whammy of lower genetic
variation due to genetic drift and also due to inbreeding
effects because you have relatives

Question 19

what was genetic rescue proposed in the experiment?

Answer 19

To cause gene flow and movement genes, new alleles, from the mainland populations to to both increase the number of individuals on the island and to rescue the genetic background
of the wolves that were still alive

Question 20

results of the genetic rescue

Answer 20

there was genetic rescue
- few individuals were removed and able to have offspring
- there was a big boost in individuals

Question 21

Gene flow

Answer 21

movement of individuals (or their gametes) between populations

Question 22

Gene flow can change…

Answer 22

genetic structure of populations by moving genes between populations that differ in genetic composition (same species but genetic structure is different)

Question 23

Gene flow can counteract…

Answer 23

divergence among populations (leads to greater homogeneity of populations)

Question 24

How do we measure gene flow (and genetic structure)?

Answer 24

F statistics

Question 25

Who developed F statistics

Answer 25

developed by Sewall Wright

Question 26

What are F statistics

Answer 26

measure reduction in heterozygosity at one hierarchical level relative to another

Question 27

what is Fst

Answer 27

a popular & useful measure of population differentiation (how alleles in different populations look different in a species)
* ranges from 0 to 1 (<0.05 is considered very little structure: lots of gene flow distribution of A and a are about the same, and >0.25 a lot: not lots of gene flow distribution of A and a are very different)

Question 28

Fst =0 when

Answer 28

populations have the
same alleles in the same frequencies
look the same might not even be different pop

Question 29

Fst= 1 when

Answer 29

populations are fixed for different alleles
the “f”
fixation= reached 100%
no gene flow, lost genetic variation

Question 30

What happens in popG when there is no genetic drift and no populations:
there are 3 fitnesses that are really close to each other
We are introducing a new mutation
- little a
- its advantageous, but just barely (at 1 and bi A is at 0.99)

Answer 30

It’s expected to replace the big A allele
started almost 100% with big A allele and then this new thing comes in and its got barely higher fitness and it spreads (but takes a long time, common after 10000 and fixed by 50000)
again this is only with bigger populations (no genetic drift)