Genetic+Variation

**Genetic Variation** **Diploidy** Most eukaryotic organisms are diploid, which means that a lot of the variation in them is not seen because of recessive alleles. Usually, natural selection would "select" out an unfavorable trait, but that doesn't happen in diploidy. What happens is that heterozygous individuals keep on passing down the "bad" (I use that term loosely) trait. Then, as long as the children are heterozygous, the trait isn't selected out (1).

**Balancing Selection** Another way nature preserves genetic variation is **balancing selection**, which is when natural selection keeps a steady frequency of at least two phenotypes in a population. This state of equilibrium is called **balanced polymorphism.** This certain type of selection "includes heterozygote advantage and frequency-dependent selection (2)."

**Heterozygote Advantage** Heterozygous organisms, if more fit at a gene locus than homozygotes, will be naturally selected to keep and maintain at least two alleles at that locus (3).

**Frequency-Dependent Selection** Sometimes, you can have too much of a good thing. This is true in **frequency-dependent selection** where if one favorable trait becomes too widespread, it's fitness level goes down (4).

**Neutral Variation** Finally, some genetic variation really has no effect at all, and therefore natural selection does not affect these alleles. Many alleles found in untranslated parts of the genome have no selective advantage. This process is called **neutral variation.** Also, there are **pseudogenes**, which over time have become inactivated by one reason or another. These genes make "genetic noise" and do not affect the gene.

(1) AP Edition Biology, Seventh Edition, Campbell and Reece (2) AP Edition Biology, Seventh Edition, Campbell and Reece (3) AP Edition Biology, Seventh Edition, Campbell and Reece (4) AP Edition Biology, Seventh Edition, Campbell and Reece (5) AP Edition Biology, Seventh Edition, Campbell and Reece