Let There Be Sight!
A Celebration of Convergent Evolution
Compiled by Connie Barlow
April 2003 (revised April 2011)
Click for a downloadable PDF of the annotated list (90 examples) of convergent evolution, which includes all of this introductory narrative.
New ONLINE WEBSITE on evolutionary convergence with SIMON CONWAY MORRIS
WIKIPEDIA has a (less detailed) list of examples of convergent evolution * * *
THE LATE STEPHEN JAY GOULD popularized an understanding of evolution that focused on the role of randomness and chance. "Rewind the tape" (of evolution), he would say, and imagine the whole process unfolding from the start once again: everything would be different.
At one level, this interpretation is indisputably true: DNA sequences surely would be different; indeed, there might well be a genetic template other than DNA. But at another level, the level that matters to me and surely to many others, the central issue is whether there would be "trees" reaching into the sky and plump "fruits" beckoning mobile creatures to swallow them and thus carry their progeny on a journey. What matters is whether there would be swimming and flying and running and slithering expressions of life, and even whether there would be a form who, like us, would come to know and celebrate the 13 billion year story of the universe.
For all except perhaps the last possibility, the best answer is an unqualified Yes! We can have confidence in this conclusion, based on the fact that these forms and lifeways have independently evolved, time and again, during the actual 3.8 billion year epic story of life on Earth. This propensity, this drive for life to evolve in the same ways in unrelated lineages is known as convergent or parallel evolution (also, homoplasy). Birds and bats and insects and pterosaurs have wings not because a common ancestor had wings but because wings independently, convergently, evolved multiple times in very distinctive lineages. (Click here for an illustration of convergent evolution in marsupial v. placental mammals.)
Something indeed is going on in evolution. It is not just "one damn thing after another." The visible results of natural selection and other shaping processes are by no means best characterized as a random pageant of form and function. Whether examples of convergence owe to the inward pull of form (developmental biology) or to the push of Earth itself (ecology; "life rediscovering the same synergies," as Peter Corning speaks of it), we can count on particular forms and lifeways to develop time and again. (The 26 June 2007 issue of The New York Times contained a good summary of how the field of "evo-devo" is beginning to de-mystify the underlying processes that lead to convergent evolution: "From a Few Genes, Life's Myriad Shapes").
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The Inevitability of Emergence
I like to say that "Convergence is a sign of the inevitability of emergence." That is, because complex forms and functions have evolved on multiple occasions, we can surmise that the Universe / Divine Creativity easily produces wonders of the living world that seem miraculous to our sensibilities. We can discern that the "lesser" (simple forms) can in fact yield the "greater" (complex forms) even if scientists have not yet figured out the genetic shifts and other steps by which such evolutionary "leaps" actually take place.
Whether or not something like humans would develop "if the tape were rewound" is not the central concern for those of us who love Life. For me, it is comforting to know that Earth is determined that there be trees, that there be eyes to see trees, that there be songs sung in trees to greet the dawn and other songs sung to greet the twilight.
An awe of the power and performance of convergent evolution profoundly shaped the worldviews of great biologists of the past (notably, Charles Darwin and Julian Huxley) and is beginning to do so again today (John Maynard Smith, Richard Dawkins, Simon Conway Morris, Mark McMenamin). Finally, with Simon Conway Morris's book, Life's Solution (published in autumn 2003), and Richard Dawkins's book, The Ancestor's Tale (2004), convergent evolution is on the rise of biological concern and application once again.
Simon Conway Morris, a paleontologist, contributed the cover story "Convergence" for the 16 November 2002 issue of New Scientist magazine. There he wrote:When you examine the tapestry of evolution you see the same patterns emerging over and over again. Gould's idea of rerunning the tape of life is not hypothetical; it's happening all around us. And the result is well known to biologists evolutionary convergence. When convergence is the rule, you can rerun the tape of life as often as you like and the outcome will be much the same. Convergence means that life is not only predictable at a basic level; it also has a direction.
Richard Dawkins, zoologist, highlights convergent evolution in the final chapter of his book, The Ancestor's Tale:It seems that life, at least as we know it on this planet, is almost indecently eager to evolve eyes. We can confidently predict that a statistical sample of reruns [of evolutionary life on Earth] would culminate in eyes. And not just eyes, but compound eyes like those of an insect, a prawn, or a trilobite, and camera eyes like ours or a squid's, with color vision and mechanisms for fine-tuning the focus and the aperture. Also very probably parabolic reflector eyes like those of a limpet, and pinhole eyes like those of Nautilus, the latter-day ammonite-like mollusc in its floating coiled shell. And if there is life on other planets around the universe, it is a good bet that there will also be eyes, based on the same range of optical principles as we know on this planet. There are only so many ways to make an eye, and life as we know it may well have found them all. (page 588)
Like any zoologist, I can search my mental database of the animal kingdom and come up with an estimated answer to questions of the form: 'How many times has X evolved independently?' It would make a good research project, to do the counts more systematically. Presumably some Xs will come up with a 'many times' answer, as with eyes, or 'several times', as with echolocation. Others 'only once' or even 'never', although I have to say it is surprisingly difficult to find examples of these. And the difference could be interesting. I suspect that we'd find certain potential evolutionary pathways which life is 'eager' to go down. Other pathways have more 'resistance'. (page 590)
FOR MORE QUOTATIONS, ALONG WITH A LONG, ANNOTATED LIST OF EXAMPLES OF CONVERGENT EVOLUTION, click for a downloadable PDF. In addition to the foregoing narrative, you will find some 80 examples of convergent evolution, grouped by
vertebrate forms forms across phyla plant forms biochemicals food and nutrient acquisition emergence onto land return to the water lifeways human culture
Please use this list in whatever ways you wish (and hotlink it to your own websites). And do let me know of other examples so that I can expand and improve what is offered here.
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