Improbable Destinies: Fate, Chance, and the Future of Evolution

by Jonathan B. Losos

Bergmann’s Rule, named for the nineteenth-century German biologist who described it, refers to the tendency for size to increase with latitude among individuals within a species or among closely related species.

Natural selection has no foresight—it won’t favor a detrimental feature just because it is an early step on a path leading to an ultimately superior condition. Rather, for a feature to evolve by natural selection, every step along the way must be an improvement on what came before it—natural selection will never favor a worse condition, even if it’s only a transient evolutionary phase.

The prevalence of convergence among closely related populations and species is easy to understand. Close relatives tend to be similar genetically, so selection is likely to have the same genetic systems on which to work. Moreover, relatives tend to be similar in many phenotypic attributes. Because of these similarities, closely related species and populations share the same evolutionary predispositions, more likely to evolve in some ways than in others. Some evolutionary biologists refer to these predispositions as constraints or evolutionary biases. These biases could operate in a number of ways. The most obvious is the genetic similarity of close relatives, presenting the same target to natural selection, but more subtle biases could occur as well. An evolved trait in an ancestor could preclude some evolutionary options, forcing evolution to occur in a limited number of other ways among the species’ descendants. Alternatively, an ancestor could evolve a trait that paves the way for the evolution of a second trait. Such potentiation, as it is now referred to by molecular biologists, would have the effect that closely related species would all evolve the second trait, one unlikely to arise in species not descended from that ancestor. For all these reasons, related species are more likely to convergently evolve the same traits when faced with similar selective pressures. That’s not to say that distant relatives can’t converge—it certainly does happen, just less frequently.

“Against this backdrop of parallelism, or repeatability, the longer the LTEE has been running, the more we see that each population really is following its own path,” he said, “and both sets of forces—the random and the predictable, as it were—together give rise to what we call history.”

So, can we predict evolution? In the short-term, yes, to some extent. But the longer the passage of time and the more different the ancestors or conditions, the less likely we are to prognosticate successfully. Dinosauroid? I don’t think so. Perry the Platypusoid? Alas, no. Were we destined to be here? Hardly.