Using Robotics to Test Evolutionary Hypotheses for Fish

Hypotheses about the evolution of traits in ancient species are difficult to test, as the relevant animals have often been extinct for thousands or millions of years. In the present study, a population of physical, free-swimming robots modeled after ancient fish evolved vertebrae under selection pressures for predator avoidance and foraging ability, showing how evolutionary robotics can be used to help biologists test hypotheses about extinct animals .

Millions of years ago, during the Cambrian explosion, fish started to evolve tiny proto-vertebrae on the long flexible rods (notochords) that had previously given their bodies structure and lent some stiffness to their tails. As evolutionary traits go, vertebrae were very successful: they have been preserved through these millions of years, through fish, amphibians, reptiles, birds, mammals, and eventually ended up in your backbone.

So why did they evolve in the first place?

One theory is that the sudden increase of genetic diversity during the Cambrian explosion led to an “arms race” between predators and prey, the prize being either dinner (in the case of the predator) or your life (in the case of the prey). Take speed as an example: as prey animals evolve faster escape maneuvers to better evade a particular predator, the predatory species will be under more selection pressure to increase its speed as well. Arms races like this could have led to innovations like vertebrae, which enable fish to displace more water with every tail movement and thus swim faster for only a small increase in energy usage.

We were interested in testing how plausible it is that selection pressure for predator avoidance and foraging ability could drive the evolution of vertebrae, but unfortunately, all of the relevant animals were long extinct. Fortunately, when the desired study animals are unavailable, it’s often possible to create models of animal behavior using other animals that have some characteristics in common with the species of interest, computer simulations of the animals and their environments, or physical simulations of the animals in similar environments in order to test hypotheses like those we were interested in here.

Computer simulations allow researchers to build huge populations of model organisms to which selection can be applied, but are not limited by the laws of physics. The behavior we were interested in here, which involves composite, flexible solids of varying stiffnesses bending in fluids, is difficult to accurately simulate with a computer; however, the number of generations that we wanted to be able to run to perform an evolutionary experiment would make building enough robots difficult. Rather than choose one method and be slave to its flaws, we did both: we created a physical simulation in our lab at Vassar and collaborated with two groups at Lafayette College to develop a computer simulation of the same system. If both of these simulations came up with similar results, there is stronger evidence that the results were not simply due to flaws in simulation.

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