Convergence in Biology

… it is vanishingly improbable that exactly the same evolutionary pathway should ever be travelled twice. And it would seem similarly improbable, for the same statistical reasons, that two lines of evolution should converge on exactly the same endpoint from different starting points.

It is all the more striking a testimony to the power of natural selection, therefore, that numerous examples can be found in real nature, in which independent lines of evolution appear to have converged, from very different starting points, on what looks very like the same endpoint. When we look in detail we find—it would be worrying if we didn't—that the convergence is not total. The different lines of evolution betray their independent origins in numerous points of detail. For instance, octopus eyes are very like ours, but the wires leading from their photocells don't point forward towards the light, as ours do. Octopus eyes are, in this respect, more ‘sensibly’ designed. They have arrived at a similar endpoint, from a very different starting point. And the fact is betrayed in details such as this.

Such superficially convergent resemblances are often extremely striking, and I shall devote the rest of the chapter to some of them. They provide most impressive demonstrations of the power of natural selection to put together good designs. Yet the fact that the superficially similar designs also differ, testifies to their independent evolutionary origins and histories. The basic rationale is that, if a design is good enough to evolve once, the same design principle is good enough to evolve twice, from different starting points, in different parts of the animal kingdom.

After that, there are a bunch of examples, including echolocation—which developed independently among bats, birds, and whales (including dolphins)—and, a favorite of mine, periodicity in cicadas (which is also mentioned in Promotional Rates). It's possible, but not certain, that 13- and 17-year periods are good because the numbers are prime, but in any case …

… there must be something special about those numbers, because three different species of cicada have independently converged upon them.

Convergent evolution doesn't just operate on a single level, either.

Examples of convergence on a large scale occur when two or more continents are isolated from one another for a long time, and a parallel range of ‘trades’ is adopted by unrelated animals on each of the continents. By ‘trades’I mean ways of making a living, such as burrowing for worms, digging for ants, chasing large herbivores, eating leaves up trees. A good example is the convergent evolution of a whole range of mammal trades in the separate continents of South America, Australia, and the Old World.

Again, there are a bunch of examples. The most compelling one, for me, is one that I somehow completely failed to notice when I first read the book.

In Australia the thylacine, or marsupial ‘wolf’ (often called the Tasmanian wolf because it survived in Tasmania for a little longer than in mainland Australia), was tragically driven extinct within living memory, …

Curious, I checked online, found The Thylacine Museum, and learned two things. One, thylacines had stripes! Two, there is, or was, a project under way to bring thylacines back into the world via cloning, as with the dinosaurs in Jurassic Park.

I have to say, it never ceases to amaze me that for almost any subject, no matter how obscure, there's someone who cares enough about it to put together a really good web site.