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

by Jonathan B. Losos

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.