Persistence of Variation - Mod. 8
The issue with this question, or rather perspective on the topic, is that it implies selection is the only force acting on a population. While selection can limit the variation in a population, especially a small one, other forces will also act on that same population to counteract the effects of selection and revamp variation. So to redefine variation in evolutionary terms, genetic variation is essentially the diversity of allele frequency at a certain gene locus. On a molecular level, this results from single nucleotide polymorphisms (SNPs) in the DNA to produce different variations of the same gene. The most major player in increasing genetic variation is the force of mutation. No matter how great the strength of selection is, mutations will always be present in a population, adding variance with their effects, and creating a new gene pool for selection to act on again and again. In the the Module 7 R exercise, we saw the effects of the molecular clock, ticking away and producing viral mutations at a constant pace. This is evidence for mutation's pressure against the force of natural selection.
Secondly, genetic drift can play a role in variation as it randomly knocks out pieces of a gene pool, leaving random individuals as mating pairs behind. However, after the drift event, selection is again able to act on the gene pool and will likely cause a decrease variation. Migration is also a player in the push against selection's homogenizing effect as gene flow between populations can introduce new alleles or cause certain alleles to become more or less frequent through mating. Furthermore, the alleles present may also play roles in dominance or recessivness, also adding to the population's variation. Additionally, as we spoke about a couple of weeks ago, mate choice is important as inbreeding can increase the similar homogenizing effects of selection while the lack of inbreeding in a population leads to greater genetic diversity.
Given that there are a multitude of forces acting on a singular trait at any given time, it is evident that selection does not always eliminate genetic variation because it is hardly ever acting alone. Therefore, genetic variation is almost always maintained in natural populations.
I thought it was great that you addressed the assumption of this question and redefined variation! I think we have seen many examples in class thus far to prove that selection is never the only driving force of evolution.
ReplyDeleteI think it is an interesting concept that you bring up on how when there are multiple forces acting on something, one force can't overpower the rest. With that, are you saying that all the forces you listed act with the same power? Additionally, a couple of weeks ago in class, I remember our professor asking us which evolutionary force is the most important, (or has the most influence). I believe he said that he thought selection was the most important. What would you say?
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