There are two concepts regarding the process of evolution: contingency and convergence.
This theory holds that species evolve, due to natural selection, randomly according to whichever new adaptation is useful. The present is contingent on random events that occurred in the past (such as an asteroid hitting the earth). There is no directionality toward more advanced organisms; there is no predictability; anything is possible. If one were to re-wind the clock and start Life over at Year Zero, the world evolving from that would look very different from our world today. There would be no mammals, fish, plants, or anything necessarily resembling them. As Stephen Jay Gould states, “We are the accidental result of an unplanned process …. The fragile result of an enormous concatenation of improbabilities, not the predictable product of any definite process.”
This theory agrees that natural selection is the motor that drives evolution, but that it is largely influenced by circumstances that cause separate evolutionary lineages to arrive at the same conclusion. Circumstances may include gravity, water, climate, and the general environment/habitat (of co-evolving plants). For example, both ducks and the platypus evolved duck bills, a very specific adaptation for a similar ecological niche. Both Red-winged Blackbirds and Fan-tailed Widowbirds evolved to have males with black bodies and red shoulders (with white borders) and sing from reeds in marshes, while the females are streaked brown. They have converged to become similar species, although they are quite unrelated. In this way, many species independently evolve in predictable ways that are replicated in time and space around the world. They converge over time and look like each other. Moreover, directionality is also evident. Species evolve from simple to complex, from blob to human. (Some have taken the theological step that convergence and directionality imply universal rules which suggest the work of a Divine Hand. That discussion is beyond the scope of this blog– for now at least.) If one were to re-wind the clock and start over in Year Zero, we’d still end up with a world pretty similar to the one we have now, with a similar mix of species fulfilling similar functions. As Simon Conway Morris states, “Although there may be a billion potential pathways for evolution to follow from the Cambrian explosion, in fact the real range of possibilities and hence expected end results appear to be much more restricted.” That is, there are only so many options and species will fit into those options. Anything is not possible.
If only contingency was the case, birds would evolve in random ways, developing ever more unpredictable and bizarre shapes, sizes, and colors. Bird identification would be easy, as they’d all be so different (except right when closely related species are separating from each other). If convergence was the rule, we’d expect the same patterns to recur time and time again. Bird plumages would be limited to certain patterns and colors, and probably correlated with certain behaviors, habitat preferences, and ecological functions. There would be birds from completely different families, very distantly related to each other and on different continents, which might evolve looking very similar to each other. As my son puts it, the gods of evolution would look like plagiarists. I believe this is clearly the case, for reasons that are not always easy to understand. For example, the six Passerines above, all inhabitants of woodsy areas but only very distantly related, have solid blue-gray upperparts, reddish underparts, and white eyebrows. It’s not clear what purpose this serves, but clearly it’s a pattern they have all converged on.