Lecture 11 Flashcards
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
same
= Ne is the actual size that corresponds to effects of genetic drift
Ne is smaller than N (and sometimes much smaller):
- Not everyone in population contributes to reproduction
- Previous bottlenecks & population size fluctuations have significant effect on variation
How do we have to estimate Ne because it is difficult to measure
so we try to estimate it indirectly
Effect of genetic drift on genetic variation:
1. Smaller population size =
more genetic drift = lower heterozygosity / higher homozygosity
genetic variation seems to be lost
Effect of genetic drift on genetic variation:
2. Smaller population size increases…
the chance that a completely neutral new mutation will become fixed!
When certain population sizes were put into popG what happened?
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
What would happen if you increased the population size in pop g?
just by chance big A will become lost
Effect of genetic drift on genetic variation:
3. Smaller population size increases
the chance that a new deleterious mutation will become fixed!
Which process (Genetic drift vs natural selection) is likely to have a bigger effect on allele frequencies?
This depends on how strong each process is
how can we describe the strength of natural selection
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%
how can we describe the strength of genetic drift ?
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
If s»_space; 1/Ne which will have the prevailing effect on allele frequencies
natural selection will have the prevailing effect on allele frequencies (genetic drift is likely to have little effect).
If s «_space;1/Ne which will have the prevailing effect on allele frequencies
genetic drift is likely to have the prevailing effect
Effect of genetic drift on genetic variation:
4. New slightly advantageous mutations
are much more likely to become fixed in large populations (no genetic drift)
why are new slightly advantageous mutations are much more likely to become fixed in large populations (no genetic drift)
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
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)
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)
Inbred wolf populations on Isle Royale collapses
- 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
How did you think of the Isle Royale experiment
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
what was genetic rescue proposed in the experiment?
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
results of the genetic rescue
there was genetic rescue
- few individuals were removed and able to have offspring
- there was a big boost in individuals
Gene flow
movement of individuals (or their gametes) between populations
Gene flow can change…
genetic structure of populations by moving genes between populations that differ in genetic composition (same species but genetic structure is different)
Gene flow can counteract…
divergence among populations (leads to greater homogeneity of populations)
How do we measure gene flow (and genetic structure)?
F statistics
Who developed F statistics
developed by Sewall Wright
What are F statistics
measure reduction in heterozygosity at one hierarchical level relative to another
what is Fst
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)
Fst =0 when
populations have the
same alleles in the same frequencies
look the same might not even be different pop
Fst= 1 when
populations are fixed for different alleles
the “f”
fixation= reached 100%
no gene flow, lost genetic variation
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)
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)
