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)!