Super Fantastically Mediocre Evolution Blog : Inbreeding

Banjos playing in the distance

Inbreeding can have some desirable benefits. Inbreeding within small groups has the potential to fix desirable genotypes within that group. It can also help maintain a level of genetic "purity" in that bloodline that can pass on advantageous phenotypes or evolutionary traits. In an example of a hermaphrodite organism, it can fertilize itself (there's a joke here, but I'm not gonna say it) and has the advantage of passing on two copies of its genes into each of its offspring. Another impact it could have is mutations, which could be beneficial to the species. It's these kind of benefits that can lead to assortative mating among species. The big difference is that inbreeding will affect all loci, while assortative mating affects only those that play a role in similar phenotype characteristics.

Downsides to inbreeding are many. Inbreeding impacts the allele combinations that are on the same locus of diploid individuals, and more inbred organisms are more likely to have two copies of the same allele at a locus vs if they were randomly mating. This creates a problem of them being more likely to be homozygous with an increased expression of the effects of the recessive alleles. Since many of those alleles are deleterious, the organisms' fitness level will be reduced. Inbred organisms can also develop mutations that aren't beneficial to them, creating health problems and impacting their fitness.

It's also been found that inbreeding in smaller, isolated populations can actually increase their risk of becoming extinct, something they found out with the Glanville fritillary butterfly (oof). Another problem is the lack of genetic variation. Evolutionary advantages that they could receive through outbreeding are lost to them, whether it's by choice or force (i.e., isolated/cut off from other potential mates). There's also the problem of inbreeding depression, which gives a lower survival rate or fertility of progeny of inbred matings. This problem occurs when there's a high frequency of homozygosity in a population that comes from inbreeding and drastically cuts down on heterozygous organisms.

These reasons are what can lead to the disassortative matings. Disassortative matings can increase/improve heterozygosity, genetic variation, achieve evolutionary advantages, and have better overall fitness as a species.

I could honestly probably list even more problems associated with inbreeding (and maybe some other benefits), but I'm not trying to bore you to death.

(Memes ahoy, m'fellows)