How Fish Adapt to Darkness
There are millions of examples of plants and animals adapting to their environment. Need to blend in? Change your coloring. Need to crack a different kind of nut? Change your beak. But while we have lots of examples of adaptation, we rarely have the ability to look at what genetic changes caused the physical ones. Not every physical trait shares the same kind of genetic basis. An example is anteaters, aardvarks, and armadillos. They all have sticky tongues for catching ants even though they aren’t closely related species. This is called convergent evolution. Scientists at the University of Victoria in Canada have been able to find the exact genetic changes that lead to differences in vision in a super cool little fish called the threespine stickleback, and it turns out that those genetic changes are the exact same ones that happened in the ancestor of spiny-rayed fish like salmon, carp, and bass over 198 Million years ago! It’s super difficult to track genetic changes that lead to physical adaptations since they usually take place over many generations and hundreds of years. Whereas the career of a scientist is maybe about 50 years. If you’re lucky! To study genetic changes easily you need a place where an adaptation is occurring multiple times on a timescale fast enough that you can actually see it happen. And that is exactly what these researchers did! They looked at a small fish called the threespine stickleback that lives in a variety of lakes in an archipelago off the coast of British Columbia, but it turns out that not all of these lakes are created equally. There are two different types of lake. Clearwater and blackwater. Clearwater lakes are just like they sound. The water looks pretty clear and there’s a lot of light so the fish have no trouble seeing, and much like the ocean or other bodies of clearwater, the light in them is mostly blue. Blackwater lakes are different. They’re created when the water is stained with tannins, which are molecules that you might be familiar with if you jumpstart your morning with beverages like tea or coffee. The color and astringent or bitter-ish flavor of coffee is caused by tannins. This tannin stained water actually absorbs most of the blue light that’s shining from the Sun, and any light that would reflect off the sides or the bottom of the lake is also absorbed. This means that the fish are essentially in a lake like a photography darkroom where the light is mostly red and it’s only shining from directly above. Because of the huge differences between clearwater and blackwater, the sticklebacks have adapted their vision to one lake or the other. But how did they do it? To find the genetic source of the differences in fish vision between clearwater and blackwater lakes, the researchers took fish from each of the different kinds of lakes and looked at their DNA to see how it was different. But in addition to looking at fish that were already adapted to each of their environments, they also took some fish that were already living in a blackwater lake and moved them to an unoccupied clearwater lake. They then checked on how they were adapting for the next 19 years! Through these two types of experiments, the researchers found the source for the vision adaptation. It all boils down to seven amino acids in a fish eye protein that recognizes light. Those seven amino acids help tune that protein from being sensitive to blue light to being sensitive to red. Just like you would tune a radio from one station to another. The researchers also checked out the genes of fish that are distantly related to the threespine stickleback and they saw that the same amino acid changes that are happening in the stickleback right now occurred to those fish almost 198 million years ago! It’s pretty cool how this happened. So about 198 million years ago in the ancestor of all spiny-rayed fish, the gene which encodes for the protein that recognizes light accidentally got duplicated! Just like hitting copy and paste. But then, over time, those two copies started to become different. One recognized blue light and the other copy recognized red. Exactly like what’s happening in the sticklebacks now! But wait… If all spiny-rayed fish have these two genes… and the threespined stickleback is a spiny-rayed fish… then it should already have both the blue and red sensitive protein! It should already be able to adapt! Well it turns out the stickleback actually lost one of the copies, just like you might lose your great-grandmother’s pearl necklace in going from generation to generation. So, now they have to reevolve the same amino acid changes to adapt to blackwater versus clearwater every time they move into a new lake. Like Sisyphus rolling his rock uphill! As a major Batman nerd, I’m reminded of when Bane says his classic quote in The Dark Knight Rises: I choose to think that he’s referring to evolution! There is a difference between being put into a dark environment and actually thriving there for multiple generations. Just look at the threespine stickleback! Thanks so much for watching! Subscribe to my channel for new videos every other Friday! And feel free to follow me on any of my other social media platforms, which you can find in the description below. See you next time!