Question

You find a trait in corn that is under positive selection (gives a fitness advantage), however when looking at your corn population over time you see the trait is not moving towards fixation. What are 2 reasons that can cause this? Explain what they are.

Answer 1

Ans:

For a trait to undergo positive selection, it must have two characteristics. The trait must be beneficial and heritable.

If the trait of the corn population is not moving towards fixation, the possible reasons can be described as below :

1) Weakly deleterious mutations can fix in smaller populations through chance, and the probability of fixation will depend on rates of drift (~ 1/Ne) and selection (~s), where Ne is the effective population size. The ratio Nes determines whether selection or drift dominates, and as long as this ratio is not too negative, there will be an appreciable chance that a mildly deleterious allele will fix.

2) Probability of fixation is also influenced by population size changes. Selection coefficients are more effective in the case of growing population causing the beneficial alleles to become fixed and recessive alleles to be lost. Whereas for size shrinking population, the selection coefficients are not as effective. Thus, there is a higher probability of beneficial alleles being lost and deleterious alleles being fixed.