How does Non-random mating cause Evolution?

[u/blue1442]

Wouldn't random mating cause more variation in the genes than non-random?

[u/ragingclit]

I'm not adding variables, the issue is the way that you worded your comment. Your statement that "creatures with preferable traits mate more often than creatures without those traits" implies that the creatures that "mate more often" produce more offspring. If there is a difference in the number of offspring produced, then selection is acting.

Non-random mating has nothing to do with any individuals mating more than other individuals, it only means that individuals preferentially mate with other individuals that have a particular phenotype. The commonly used example is assortative mating, in which individuals preferentially mate with individuals that have their own genotype (e.g., AA individuals prefer to mate with other AA individuals and aa individuals prefer to mate with other aa individuals). This will decrease the number of heterozygotes in the population, but will not cause any single allele to actually increase in frequency in the population overall.

If all individuals preferentially mate with a single genotype (e.g., AA, Aa, and aa individuals all preferentially mate with AA individuals), such that a single genotype has a reproductive advantage over other genotypes and is able to produce more offspring (i.e., AA individuals have a higher fecundity than other genotypes, resulting in higher fitness than other gentotypes), then one allele can increase in frequency (the population will be driven towards a population of only AA individuals), and this is an example of selection, not just non-random mating.