Syngnathidae's Secret to Sexy Success

 The title says it all, folks! Want to get all the guys? Want to look sexy as hell? We've got just the lady to advise you!

Meet Microphis deocata - the pipefish who has it all! She's got slimness, she's got color, she's got....something that looks like the DNA diagram in my Bio 198 textbook...

And yeah, that's right! I said "she"! Like a lovely woman blessed with bone structure, this stunning Syngnathidae (the family of all pipefish) has been evolutionarily gifted with beautiful stripes that her mate just can't match. 

No, literally. It's sexual dimorphism. The males of Microphis deocata don't have stripes. They look less like DNA diagrams and more like your everyday living pipe-cleaner. 

This is pretty weird in the animal world. While use humans are used to both sexes getting dolled up to hunt for potential mates, that just doesn't hold for most animals. Without opposable thumbs and makeup, animals must instead rely on their genetically-given gifts to attract the opposite sex. But usually it's males who do the attracting, and females doing the choosing. These funky flat fish are just pain strange! 

What's with their strange sexual selection? Well, sexual selection itself ain't straight. In fact, sexual selection is rampant in the animal kingdom. Just one trip to Aggieville on a Friday night and I guarantee you'll see it in action yourself! 

Jokes aside, sexual selection is found pretty much anywhere there is a disparity between amount of effort exerted by each half of a mated pair. This disparity can be small, like how a female frogs have fewer eggs over their lifetime than a male frog has sperm. Or the disparity can be MASSIVE, such as a female deer gestating and then guarding a fawn long after the male has gallivanted away. Generally speaking, the more selective the judging individual, the greater the gap in expended energy is going to be. 

Which brings us to the proposed question of pipefish. What's the deal with the flashy female? 

Well I won't leave you in suspense. Remember that "disparity" talk above? Well turns out not every species regulates the offspring protection to the female. And Microphis deocata just so happens to be in the same family as another creature well-known for it's fatherly prowess: 

The Seahorse!

Turns out male incubation of female eggs is a pretty common trait among all members of Syngnathidae - a family which includes seahorses, seadragons, and pipefish. In fact, it's one of the family's defining characteristics - aside from funky faces and a resemblance to horses, dragons, or pipes of course. Male incubation of eggs shifts the responsibility of offspring protection from the female to the male - effectively swapping which sex puts more energy into reproduction. To keep things fair, the female pipefish became the showy sex - letting the males sit back and choose at their leisure! 

You know, if you find DNA diagrams sexy. 

Why Isn't Everything The Same?

Someone smart once asked me, "If natural selection is often assumed to eliminate genetic variation, why is genetic variation not eroded over time?" 

It was Michi. Michi asked me. He asked all of us. 

Yay, blog post time! 

But really, it's a good question. I hadn't really thought of it before, but it makes a lot of sense that diversity-eliminating selection would homogenize living organisms if not given constant new sources of diversity. My first thought was "Oh! Mutation probably keeps selection from overtaking diversity."

Then I remembered our calculations about the frequency of mutation rates. And then I promptly panicked, wondering if we were just a few generations away from a world-shattering loss of diversity. 

But fear not, citizen. For mutation is not the only source for maintaining genetic variety! Really, when you think about it, it's not even the main source. Selection maintains variety all on its own. 

Selection is often assumed to eliminate diversity, in cases like directional and stabilizing selection. But we know those aren't the only kinds of selection. Remember those fish that eat off different sides? Negative frequency-dependent selection isn't an eliminator of diversity, but a promoter. Not to mention heterozygote advantage, where two different alleles must come together to make the most fit organism. Or even just plain ol' heterozygotes, hiding a bum allele behind a strong dominant allele. Genetic variation is tricky, y'all. 

Hell, even standard directional or stabilizing selection isn't perfect. It's gonna miss a few individuals here and there, allowing alleles to survive from one generation to the next. And if you have a phenotype that is vulnerable to a virus but who never gets infected with a virus, selection isn't going to act to take those genes out of the population! Selection is only as strong as the environment around it. 

Consider if you have an environment that changes a lot. There could be different selective forces working on a population at once, depending on the state of the environment. Some parts of the equator experience both desert and flood conditions, depending on the time of year. Selection is forced to balance lots of different, probably conflicting alleles to handle both situations - and in the process maintains genetic diversity. 

This happens a lot in sexual selection, too! Red individuals get more mates, but green individuals survive better because of camouflage - that means two different kinds of selection are each gunning for different traits. It may be that no selective force will overpower the other, and instead populations retain their diversity.  

Of course, some variation we see can be attributed to phenotypic plasticity and the like. And I'm sure I haven't thought of all the ways genetic variation avoids natural selection. There's, like, a lot of ways that variation sticks around in a population. Like. A lot. 

It's sneaky that way. 

A, T, G, C, M & M

 How is the theory of neutral evolution different from the theory of evolution by natural selection? How are they the same?

What's up, science people?! Who wants to talk about ~neutral evolution~?! 

...Yeah, me neither. But I don't make the assignments. Let's get into it. 

Neutral evolution is a big fancy way to describe a weirdly specific situation. Basically, genes mutate all the time. A lot of those mutations don't actually affect anything beyond the DNA sequence. These harmless or "silent" mutations occasionally become fixed into the genome - and they do or do not become fixed at a relatively constant rate. 

This sounds a lot like evolution by natural selection. But it can't be natural selection, since the mutations are fixating at random (and not because they're more or less advantageous). It's evolution done by drift. 

That's a lot of science jargon. I'm gonna translate it into M&M jargon instead. 

Let's say someone is putting five M&Ms of the same color in your bowl every minute. Sometimes they're all red, or all green, etc. But you're also eating a random assortment of 5 every minute. By chance, you might end up with way more green M&Ms over time. It doesn't matter to you, because they all taste the same; you just happened to not grab a bunch of green ones. That's neutral genetic drift. 

Now if you do this experiment over and over again eventually you'll wind up with a bowl full of every M&M color. That's fixation of neutral mutations by genetic drift. It doesn't matter to you that the colors have changed, that doesn't affect how you eat them, but they've still changed. And you'll get solid-colored bowls at roughly a constant rate, with some inputs (those 5 identical M&Ms) sticking around and others not sticking around. Since who stays and who goes is random, it happens at a constant pace. 

Do this experiment enough times, and you should see fixation for each color of M&M at roughly equal rates. 

Now pretend that there are only 4 types of M&Ms, and they're called A, T, C, and G. 

Boom. Neutral evolution via genetic drift. 

The theory of evolution by natural selection is almost identical to the M&M scenario described above. But this time, instead of eating a random 5, you target M&Ms of a certain color. Maybe you're trying to eat all the green ones, or you're trying to eat ever color but the green ones. Either way, since the change is no longer random, it's going to reach "fixation" (elimination of one or most colors) much faster. 

In the real world, this difference between randomly eating 5 M&Ms and choosing a color to target is analogous to neutral evolution and evolution by natural selection, but it can also be pictured as silent and non-silent genotypic change. Remember, natural selection can only act on phenotype; it needs to see the color of the M&Ms its eat. But neutral evolution works even on mutations that don't cause a phenotypic change; thus, it doesn't need to see the color of M&Ms. So while both cause change, one cares about the phenotype and works towards a fitter organism, while the other doesn't care about the phenotype and fixes alleles at random.  

Strangely, because drift fixes alleles randomly, it actually does so at a predictable rate. We can't tell which mutations will be fixed, but we know that some changes will stick around purely by chance. You can think of it as you doing the M&M experiment with your friend. 

Let's say you and your friend work together for five bowls. Then they get greedy and start doing their own experiment for five more bowls. If this scenario is replicating neutral evolution, you'll both be fixating into solid-color bowls at about the same rate. You could then look back on the number of bowls and their colors to determine how long ago you guys separated. If you have five sets of differently colored bowls, that means you separated five bowl-generations ago - five ticks of the metaphorical "clock". 

That's how we use neutral evolution to determine temporal distance between lineages. 

Of course, we're talking candy here, so the analogy isn't perfect. Real-world analysis doesn't care about the "color" of the mutation so much that it happens at a relatively constant rate. If you already know two lineages are related, you can tally up the number of changes between them to get a pretty good idea of how long ago they separated and started neutrally evolving on their own. Scientists are also super smart and know how long it takes for one mutation to fixate, allowing them to literally estimate the years between two lineages. Which is a lot more applicable than counting in bowl-time, let me tell you. 

So go share some M&Ms! Talk to your friends about neat science concepts! Pavlov train them to associate your science ramblings with candy! 

Now if you'll excuse me, I'm going to go ask my roommate to proofread this and thank her with candy. 

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). 

        (o_>_)<    (o_>_)<      (o_>_)< 

                                                                   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?

        (o_>_)<    (o_>_)<      (o_>_)< 

                                                                   BB           Bb             bb

Well, it depends on who you ask. 

A Case Study in Sneks

May I present to you,  this meme made reality: 

Michi wants science, so science we shall give him! The question in such: How can we use the One-Island Model of Migration to explain the dataset below? 

Lotsa big words in there, so let's break it down. What's the situation? 

There are a bunch of snakes on a bunch of islands. Now, anyone versed in internet culture knows that snakes generally prefer planes, a la the Samuel-L-Jackson-starring movie Snakes on a Plane, but we will ignore common convention for the purpose of this blog post. And these island snakes, well, they look different from mainland snakes of the same species. Mainland snakes tend to be stripy (Phenotype D), to blend in with undergrowth. But island snakes are frequently less stripy (Phenotypes B and C) or not stripy at all (Phenotype A), because their island homes are rocky and grey. 

Darwin would tell us that the island snakes should all, eventually, become not-stripy-at-all (Phenotype A), because that pattern is best for blending in on the island environment. Natural selection should remove all lightly stripy genes, and fix the non-stripy gene in the island populations. And there certainly shouldn't be any very stripy bois on islands! 

But the data says there are very stripy bois on some islands! Why? A very fancy model called the One-Island Model of Migration is why! Because unlike me, sitting here at my computer and living in my COVID-proof apartment box, animals don't stay still. They move! And when animals move between populations, so do their genes. 

If it was just up to natural selection, island populations would weed out any individuals of phenotype D pretty quickly - and phenotypes B and C would soon follow. But in this real-world scenario, sometimes mainland snakes find their way onto the islands. And those mainland snakes bring with them their genes for phenotype D. Because island snake populations are so small, just a handful of these mainland immigrants can have a relatively large impact on the phenotype frequencies seen on the islands - effectively preventing natural selection from weeding out the stripy phenotypes. It like if you were trying to eat all the green M&Ms in a bowl, but someone kept dumping more green M&Ms in faster than you could eat them! 

Great, now I want M&Ms. 

Looking back at the graphs above, we can see that they're not all equal. Kelley's Island has way more stripy individuals (phenotypes B, C, and D) than Middle & Pelee Islands! What gives? They're both islands, so shouldn't migration affect them the same way? 

But there's the catch! These sneaky water sneks don't migrate to every island equally. The further away an island is, the harder it is to get to - so the fewer migrants it receives. Since Middle & Pelee Island are the most isolated, they receive the fewest migrants, and thus the fewest interjections of disadvantageous, stripe-coding alleles. Without those alleles, the island populations are free to phenotypically diverge much further from the mainland populations. And bada-big, bada-boom - you get different ratios of phenotypes. 

We can even use this distance-equals-number-of-migrants reasoning to explain the differences in the graphs of the two mainlands - Ohio and Ontario. Ontario is dominated by phenotype D, while Ohio has a smattering of the island phenotypes. To figure out why, look at the map. 

The closest island to Ontario is Pelee Island - and it's almost half a lake away! Conversely, it's just a hop skip and jump from Kelley's and Bass Complex Islands to Ohio. That means island migrants are going to have a much easier time getting to Ohio than Ontario! 

That's not to say that Ontario doesn't get any migrants. It probably does. But when you have just one island migrant among an entire population of mainland snakes, trying to find it is looking for the proverbial needle in a haystack (Snake in a lake?). Because mainland populations are much bigger than island populations, island migrants will have a drastically lower impact on allele frequencies than vice versa. 

Moral of the story? Small populations are vulnerable to the allele frequency change that migrants bring, whereas large populations couldn't care less. Islands that are further away receive fewer migrants. And watch out for snakes on planes (and islands)!