One Fish, Two Fish, Yellow, Green, Blue Fish

When it comes to the concepts of evolution, you could throw big words and long-winded explanations at me all day - and I still wouldn't get beyond the metaphorical 2 + 2 = 4. Sure, I can read words and use brain power to turn those words into understanding. If I want a headache. Or I could just talk about fish. 

...I'm gonna talk about fish. 

The proposed question is this: what's with non-random mating? It's not really an evolutionary force, because it can't change allele frequencies. But it is important to evolution, because it provides a situation on which other evolutionary forces can act. And non-random mating does this by messing with genotype frequencies. 

There I go, chucking around big words. Let's get into the fish. 

Pretend we have two alleles in a population of fish. Two copies of one (say, BB) makes blue fish, and having two copies of the other (bb) makes yellow fish. Having one of each (Bb) makes a green fish (This is called incomplete dominance, yada yada jargon jargon, back to the fish). 

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                                                                   BB           Bb             bb

Look! Fish!

Now let's say this population starts some serious non-random mating. Interbreeding, specifically. Also known as "getting it on" with your close relatives. 

Maybe you've heard of some human populations doing this before. The European royal family, perhaps? Or the Egyptian line of Pharaohs? Both illustrate the potential negative impacts of long-term interbreeding: a rise in recessive genetic disease. But why? Well. Let's go back to the fish. 

When a population interbreeds, we know one thing for certain: the genotype frequencies are going to change. We expect to see a serious drop in green fish (heterozygotes) and a rise in blue and yellow fish (homozygotes). This is because yellow fish can only give yellow alleles and blue fish can only give blue alleles - so the more they mate amongst their close relatives (who are likely to share the same only-blue or only-yellow allele situation), the more those alleles are going to propagate. 

It's a long stretch to connect fish to royal bloodlines, but consider this: what if the yellow color is really disadvantageous? Maybe it makes fish easy to spot.  Inbreeding will result in a lot more easy-to-eat yellow individuals, where there might've been predominately blue and green individuals in a randomly mating population. In such as case as this, the fish might evolve disassortative mating, where individuals try to mate with others who don't look like themselves to decrease the number of homozygous recessive yellow fish. Disassortative mating is advantageous because it increases genetic diversity and lowers the chance of inheriting recessive genetic diseases or recessive, disadvantageous alleles.

In a similar manner to the yellow fish, a recessive disease known as hemophilia plagues the European royal line. The one-time mutation in Queen Victoria probably wouldn't have been a big deal - but the royals just had to keep marrying their cousins (who were a lot more likely to also carry the trait in its harmless heterozygous form), which upped the chances of their "pure-blooded royal" offspring having a serious genetic disease. 

So. Frequently, inbreeding = wimpy children. Populations evolve to avoid it. 

But like everything in science, there are exceptions. 

Some populations have evolved assortative mating, or the selection of a mate who looks phenotypically similar to yourself. Why risk the chance of shagging your cousin? Well, because the benefits outweigh the risk, of course. 

If the coloration of our yellow and blue fish indicate their health (while green is drab and does not), then non-random mating will reinforce those bright colors. Bright fish will mate with bright fish so that their children also inherit that communicator of "I'm bright healthy, breed with me!". 

Or if our fish are highly territorial, breeding with others who look like them will result in even more territorial offspring - which enhances the chance that those offspring will survive and have their own offspring, thus propagating the genetic line. 

And if you have two really good alleles and a really disadvantageous heterozygote, interbreeding can reinforce the good alleles while getting rid of the heterozygotes. 

Before you go judging these assortative maters, just remember: you're one of them! Humans fall right into this category. I'd hypothesize it is because of our communal nature. We would much rather mate with someone from our own culture (who we can understand easily) than someone who looks and acts differently. It's a survival tactic, designed to promote the genetic line we're a part of. But don't quote me on that. I'm speculating for fun, not fact. 

So are our inbreeding fish in a good situation or bad situation?

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                                                                   BB           Bb             bb

Well, it depends on who you ask.