We think of evolution as moving in a linear progression from the sea to the land. But some creatures, such as whales and dolphins, clearly adapted to the land, then returned to the sea. Dr. Curt Stager and Martha Foley talk about convergent evolution.

Martha Foley: I have a little question about evolution, which I think of from the pictures in the books as kind of a linear thing. You start in the water, and then you climb on the land and you’ve got, you know, fish and mammals and stuff. Where do the whales fit in, and the dolphins? Because they’re still in the water, and people thought they were fish, right, for a long time?

Curt Stager: Yeah, people argued, I think as recently as a hundred, hundred fifty years ago about if they were fish or not.

MF: So why aren’t they fish?

CS: Well the reason it doesn’t make sense is because the description of evolution that way is not correct. It’s not just a one directional thing and it doesn’t just always go from the sea, to the land with the amphibians, and then the reptiles, and then the mammals and then us, and that’s what the whole story was about.

MF: So you just have throwbacks, or something?

CS: Well, yeah, if you assume there’s a certain direction you may describe them as throwbacks or reverse evolution, but the way an evolutionary biologist would describe it is it’s just that whatever the environment is makes natural selection, and favors whatever happens to be there at the time over something else. If the environment changes or the conditions of the species change, maybe you’ll go off in another direction.

MF: So the dolphin or the whale is a mammal, but the shape of a fish works better in the water than the shape of what you would consider a more ‘normal’ mammal, with legs and feet and things like that.

CS: Yeah, you could say for whatever reasons the ancestors of whales went into the water, they came from a land-dwelling ancestor, and you can tell without even the genetics or something. Look at whales; they’ve got fins it looks like, the flippers. If you analyze that, there are bones in there with a humorous, a radius and an ulna just like a land-dwelling thing and if you look at the fossils, pretty sure the evidence shows that, too, they’ve got a pelvis with no back legs still floating around in there. So for whatever reason they went in the water, you could say it’s the same reason hippos are in the water now, they’re semi-aquatic too, that would be a case where if you think of evolution as going in one direction inevitably towards us, then you’d have to describe them as reverse-evolution, or a side branch.

MF: But it’s just evolution. It’s just change.

CS: That’s what an evolutionary biologist would say, and you could even say that it’s not inevitable to go in one direction, that you could anticipate other situations where something might have gone from the land to the water even earlier, and sure enough there are multiple cases. There’s a really neat one from the age of the dinosaurs. If you went back in time there, while the dinosaurs were tromping around on land, there were big ones in the ocean, too. And there were some that looked like whales or dolphins or fish, you could say, but they were reptiles. They’re like ichthyosaurs, they look like a dolphin with the big nose and the big teeth and stuff. You look at their skeletons and sure enough, just like the whales, they’ve got the bones in their fins or flippers, it’s got the humorous and the radius and the ulna just like the forelimb of a land-dwelling reptile, so they would’ve come from a land-dwelling ancestor and moved into the water. And by the same token, as an evolutionary biologist would say that a species adapts whatever it happens to have in response to the environment, you can tell that these animals came from a reptile on land and not a mammal when you look at their tail. So it looks like flukes that a whale would have, but it’s tilted up and down, and when they waved their tail it wasn’t up and down like a whale or a dolphin does.

MF: It’s back and forth.

CS: Sideways like a crocodile does or something like that, and it’s evidence that a transition away from the land towards the water with reptile ending up adapting to the water like the same way a fish does, but using a reptile body, and then later whales did it with their mammal-type body.

MF: So do your evolutionary biologists have a name that’s better than ‘reverse evolution’ for this?

CS: Yeah, one way you can call it is ‘convergent evolution’ or the effects of natural selection.

MF: Okay. Thanks very much Dr. Curt Stager at Paul Smith’s College, I’m Martha Foley at St. Lawrence University.