That the wings of a bat and the wings of a bird differ profoundly in their anatomical detail, despite serving similar aerodynamic ends, is a textbook fact of comparative biology. Less often emphasised, but more interesting on inspection, is the converse phenomenon: that distantly related organisms can arrive at strikingly similar forms — wings, eyes, streamlined bodies — through entirely separate evolutionary histories. The term for the phenomenon, 'convergent evolution,' is one of the older organising ideas in biology and has, over the last century, accumulated examples whose accumulation has shifted what the idea is supposed to explain.
The simplest version of the idea holds that natural selection, working on roughly comparable problems, tends to produce roughly comparable solutions: a marine vertebrate that needs to swim fast will, in time, acquire a streamlined fusiform shape regardless of whether it began as a fish or as a four-legged land animal returning to the sea. The version is not wrong but is, by itself, less informative than it sounds; it explains why convergence might occur in principle without specifying when, in particular cases, one should expect it and when one should not.
More careful work over the past few decades has tried to push the idea past the principle and toward predictions. Among the things that have emerged: convergence is more common at the gross morphological level than at the molecular level; it is more common in lineages that share substantial developmental machinery than in those that do not; and it is more common when the selective pressure is well defined and stable over long periods than when it shifts. Each of these refinements narrows the conditions under which convergence can be expected, and so makes the appeal to convergence in a given case less of a free pass and more of a substantive prediction.
None of this is to say the original idea has been replaced. It has been disciplined, in ways its older defenders would have found congenial in principle even if exacting in practice. Convergence is still a real and important feature of evolutionary history; what has changed is the standard a biologist is expected to meet before invoking it. To point to two similar-looking organisms and call them a case of convergent evolution is now, where it was once a sufficient explanation, an opening claim that the patterns of development, the timing of the lineages, and the stability of the relevant selection pressures still have to support.
Which one of the following best captures the main idea of the passage?