Module 8: Genetic variation

Selection is often assumed to eliminate genetic variation. So, why is genetic variation then not eroded over time? Genetic variation has not eroded over time because there are many different factors that affect it other than selection. We have learned about a lot of different ways that genetic variation is maintained in class from the past few modules. For example, quantitative traits which are controlled by multiple traits and the environment. Not all quantitative traits are normally distributed because one trait may be dominant and the other may not be. Another factor that affects genetic variation is drift. If there is strong genetic drift, this will lead to small populations, and this swamps out selection. Genetic drift is very different than selection because it does not lead to adaptation unlike selection which increases the average fitness across populations. There is also a major difference in the selection on dominant vs. Recessive traits. Dominant alleles reap advantages right away because they first reach fixation. The recessive allele does not reach fixation because the recessive allele can hide in the heterozygote state. By hiding in the heterozygote state,  the recessive allele becomes invisible to selection.