Different stories, same conclusion: how evolution is sometimes like a “choose your own adventure” book

When I was growing up, I really enjoyed “choose your own adventure” books. The adventures in these books would start out with the same problem and allow you to make decisions that would affect the story’s outcome from there. For instance, you might start out at the entrance to a cave and have to decide whether to go in or walk in the opposite direction. Typically, these different decisions would lead to different outcomes, although occasionally two or more stories would converge on the same ending.

A similar “convergence” sometimes happens as organisms evolve in nature, and is known as convergent evolution. One well-known example of this is that of wings in bats, birds, and butterflies. Although these animals are not closely related (and they made different evolutionary “decisions” throughout each of  their evolutionary histories), they all ended up with functionally similar structures that allow them to fly. Of course, flight isn’t the only evolutionary “problem” for which organisms might need a solution. All sorts of challenges exist to which living things must adapt in order to be successful in their environments. One such challenge that Nick Barts and his colleagues have investigated is that of fish living in toxic water.

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Two members of the research team – Michi Tobler (left) and Ryan Greenway (right) dissecting fish for later analysis

Nick and his team went to Florida, Mexico, and the Dominican Republic to find fish living in streams that had high levels of hydrogen sulfide – a naturally occurring chemical that is typically toxic to living organisms. One reason hydrogen sulfide is so toxic is because it binds to proteins in the blood which normally carry oxygen, rendering them incapable of their usual function. If these proteins can’t carry oxygen anymore, the fish may die — that is, unless they have adaptations that allow them to survive in the toxic environment. Specific protein-coding genes give cells the blueprints to make these oxygen-carrying proteins. These genes are the potential sites of mutations that could lead to modified proteins that are better suited to carrying oxygen in toxic environments.

Nick and his team study multiple fish lineages (groups of organisms that descended from a common ancestor) that are successful in toxic streams. But each lineage in toxic water is actually more closely related to  lineages that live in nearby streams without high levels of hydrogen sulfide (check out our previous post about how scientists figured this out). Because the ancestors of each toxic-water fish lineage lived in non-toxic streams, the research team was able to compare proteins of the fish that live in toxic streams with the proteins of fish that live in non-toxic streams to test whether or not each lineage of toxic-water fish used the same evolutionary “decisions” (mutations in the genes for oxygen-carrying proteins) to reach  the same “conclusion” (surviving in toxic water).

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One of the toxic-water fish that Nick and his team study.

The team found something interesting – the proteins that they were investigating were not modified in the same way in all of the toxic-water fish they studied. This means that the different lineages of fish that live in toxic water sometimes used different strategies to live successfully in water with high levels of hydrogen sulfide. This result is like opening up the “choose your own adventure” book, making three or four different decisions as to how to solve the same problem, and ending up with the same conclusion every time.

It is sometimes tempting to look at two similar looking organisms and think, “These must be related – look at those wings. They’re the same for both organisms!” Before jumping to the conclusion that two or more lineages are closely related, it’s important to open up the “choose your own evolution” book and trace back the “decisions” the lineages made to get to that same conclusion. Why do these lineages look/behave/function the same? The answer might just be convergent evolution.

This post is a highlight of a research article Nick Barts and colleagues recently published in Genome.

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This post was written by Lauren Konrade.  Lauren is a biology Ph.D. student who uses sunflowers to study how genome size changes in response to evolutionary forces.

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