CONVERGENT EVOLUTION

Are creatures that look alike always closely related?

ANSWER: Polar bears share many traits with their brown-bear cousins—both species are recognizable as bears despite obvious differences in color. The fact that polar bears resemble brown bears is persuasive evidence that the two species share a recent common ancestor. But common ancestry is not the only reason that two species might appear similar. Even species that are not closely related may share similar adaptations as a result of independent episodes of natural selection, a phenomenon called convergent evolution.

Cold-dwelling fish provide a good example. In the frigid waters of the Antarctic Ocean, fish have a unique adaptation that keeps them from becoming ice cubes: their blood is pumped full of “antifreeze.” Fish blood contains molecules called glycoproteins that lower the temperature at which body fluids freeze; the glycoproteins surround any tiny ice crystals that may form in the blood and stop them from growing. Arctic fish, at Earth’s other pole, also have “antifreeze proteins,” but the genes that code for them are different.

Arctic and Antarctic fish diverged from their common ancestor long before each species developed antifreeze proteins, which means these adaptations must have evolved more than once. In other words, at least two separate, independent episodes of evolution occurred with the same functional results. Sometimes similar environmental challenges will favor the same adaptations time and time again.