PopG Practical Flashcards
What is the relationship between the average time to fixation and population size?
The average time to fixation has a linear relationship with population size
Why are new favourable alleles often lost in combination?
Random genetic drift
Are favourable alleles lost more often in larger populations? Explain?
No, favourable alleles are lost just as often
- Drift is reduced in in large populations, but the starting allele frequency is also lower
- These effects cancel each other out and mean that favourable alleles are just as likely to be fixed/lost irrespective of the population size
How does the response to selection for a recessive allele differ from selection for a dominant allele i) when the allele is rare; ii) when the allele is common?
For a dominant allele, selection is effective when the allele is rare, but relatively ineffective when the allele is common.
When the A allele is rare it is most often present in heterozygotes. So, every allele copy is “visible” to selection, and the frequency of A will therefore increase rapidly.
When the allele becomes common, the other allele (the a allele) must be rare, meaning the other allele is most often present in heterozygotes.
These Aa heterozygotes cannot be distinguished from the AA homozygotes, and so selection is only acting to punish the aa homozygotes. There are relatively few of these, and so selection becomes less effective.
Does selection in favour of heterozygotes prevent genetic drift?
No. Genetic drift occurs in all populations as a consequence of sampling from a finite collection of individuals
In selection in favour of a heterozygotes (homozygotes of unequal fitness), why do allele frequencies stay at intermediate values?
When A allele is rare it is most often present in heterozygotes, and is therefore driven up in frequency.
As the A allele becomes common it starts to be present more often in homozygotes, and therefore experience negative selection.
Equivalently, we can say that the a allele is now rare, and therefore experiences positive selection – an increase in a is the same as a decrease in A.
These forces balance out a stable intermediate frequency.
How does gene flow allow recovery of an allele frequency from fixation?
An allele might become fixed in one population, but still present at high frequency in another. In this case a migrant from the other population can re-introduce this allele into the population. However, eventually, given enough time, all populations will become fixed for the same allele in the presence of gene flow. In this case even the introduction of a migrant cannot save the population – only mutation can.
