Genetic Variation

I think that one major reason genetic variation is still present is because oftentimes the strength of selection is not strong enough to completely eliminate a certain trait. While some are more advantageous to have, other traits are still able to survive and reproduce enough to maintain that trait being present in the population. There are different levels of selection, and depending on how strong it is will determine how much variation there is in a population for that trait.

Mutations will also always arise in populations and there is often a new advantageous trait to have. Generally if these mutations are advantageous, they may be able to fixate in the population. If traits are constantly changing, then there will always be variation in a population. It would be very difficult for all members of a species to obtain a trait if the alleles for this trait change often. As seen in some R exercises, phenotypes that are not advantageous may still be difficult to eliminate from a population depending on how strong the selection is.

Another reason there may be genetic variation is due to concepts such as heterozygote advantage. There are different traits that have different advantages, but it is possible to live without these. Additionally, these advantages keep heterozygotes in the population, which contribute to variation. There are 3 possible genotypes that could be created when two heterozygotes mate, and this would keep genetic variation in the population.