Growing the Tangled Bank

Pharyngula / by PZ Myers /

Darwin is best known for natural selection, but he saw the power of chance and development, too.

Alison Schroeer

Charles Darwin fortuitously published his grand book, On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life, in 1859, which means that this year we celebrate the sesquicentennial of the idea and the bicentennial of the man. Despite the general enthusiasm for a party, the celebration puts scientists in a curious position: However influential and brilliant a scientist Darwin was, science does not thrive on idolatry, but on constant criticism and inspection. It tends to put a damper on the party when the guest of honor is also under the microscope.

Fortunately, Darwin’s accomplishments hold up surprisingly well for a scientist who worked 150 years ago, and whose field has since been revolutionized by remarkable progress in genetics, biochemistry, computer science, physiology, and microscopy, among other things. The average undergraduate student in the sciences has access to a wealth of technologies and concepts that no one imagined in Darwin’s day. Yet still, in Darwin’s work we find the seeds of whole disciplines and various broad perspectives in evolutionary history, which is why we honor him this year.

Darwin’s success as a biologist was in no small part due to his own talent and dedication; he was a meticulous scientist and an excellent observer with a strong network of colleagues. But he also had an unfair advantage: He was the first! He and Alfred Russell Wallace came up with a way of looking at the world that opened up new lines of research and productive ways of thinking, and Darwin in particular was prolific in writing down his ideas. Even now, as we mine his books, it’s tempting to imagine him as a prescient sage who laid out the future of the whole of biology. But I think it’s more appropriate to see him as a well-prepared mind receptive to the possibilities of his theory, and to recognize that the idea of evolution itself was so powerful that, in some sense, it created the intellectual capital that has benefited us all for more than a century.

I learned about one excellent example of Darwin’s foresight in a recent talk by John Beatty, of the University of British Columbia. Darwin’s name is usually associated with a few evolutionary processes: Darwinian evolution tends to refer specifically to natural selection, and we usually think of Darwin’s chief insights as natural and sexual selection. But he was well aware of the importance of chance and historical contingency in constraining biological solutions, and he also repeatedly mentions the significance of embryology. These themes are beautifully illustrated in his work on orchids, known by the unwieldy title On the Various Contrivances by Which British and Foreign Orchids Are Fertilized by Insects. As usual, Darwin found deep insights while looking into seemingly narrow questions.

Orchids are beautiful and diverse, and the source of their aesthetic appeal lies in the plants’ elaborate flowers. To the flower, of course, the “goal” of these petals isn’t to become a corsage or an attractive centerpiece, but to draw in pollinators and sprinkle them with pollen, which is then carried to other orchids of the same species. One of the best-known instances of a prediction of purely Darwinian evolution was the analysis of the Madagascar star orchid, a flower with a foot-long tube containing nectar. Darwin predicted that there would be an insect with a proboscis of a matching length, which turned out to be true: Morgan’s sphinx moth. The two were the product of an evolutionary arms race, in which variants of the orchid that had slightly longer nectaries were more successful at drawing the pollinator more deeply into the flower for dusting with pollen, while the insect was evolving a longer proboscis to avoid the inconvenience of the same situation. This is natural selection in the strictest sense, in which organisms are in a generation-by-generation competition to better survive in the environment.

But Darwin also noted less well-known orchid features that could not be explained as mere advantages produced by selection of more optimal variants. Some elements are best explained as pure accidents, as relics of an ancestral state, or as developmental by-products.

One of the defining features of an orchid is that a central upper petal is modified and enlarged. It has a name, the labellum or lip, and most often the flower is actually rotated 180 degrees during development to put the labellum on the side nearest the ground, where it can act as a kind of large landing pad for insects. This twist during development is called resupination, and it leaves a hallmark torsion of the flower ovary. It is an entirely adaptive peculiarity — orchids have three petals; the central one happens to originate in the embryo from a topmost primordium; and natural selection has favored variants that twist it into a position that better provides a walkway for insects — although it is also the product of a contingent property of the primitive arrangement of the petals.

Now we have a twist, both metaphorical and literal. Darwin noted that some species of orchids, specifically Malaxis paludosa, secondarily shifted their labellum to an uppermost position. How would a sensible designer have accomplished this change? By switching off the developmental rotation of the flower, obviously. This is not the case, however: Malaxis puts the labellum on top by continuing the rotation to a full 360 degrees. At the same time, other orchids of the genus Catasetum, which also place the labellum on top, achieve their morphology by shutting down rotation, so their ovaries are not twisted at all.

The differences between these two species cannot be explained by natural selection. The end result in both cases has a functional purpose that is a consequence of selection, but the process by which it is achieved is partly arbitrary, the result of simple chance, and partly a result of a developmental contingency. The same end can be reached by different paths, and which path is followed is the product of undirected chance.

Darwin appreciated the significance of this observation. It sets the process of evolution apart from any driving purpose, despite the fact that as natural selection hones the results of small, random changes, it creates the illusion of purposeful design. It also explains the riotous diversity of life on Earth. Even when organisms share a set of simply definable goals, such as to maximize the frequency of outcrossing by insect pollinators, chance piles on chance piles on chance to grant each lineage a novel path to reach that goal, so that we end up with not one optimal orchid, but 25,000 unique species.

In the early days of evolutionary theory, Darwin saw all this, and for these insights we rightfully praise him in this bicentennial year of his birth. But it is also important to keep in mind that Darwin was not the end of biology. Darwin had no knowledge of genetics, and since his lifetime we’ve developed a detailed understanding of the role of chance in recombination and mutation. He knew nothing of the molecular nature of the hereditary material, and now we use the steady ticking of random changes in DNA to measure the pace of the molecular clock. Now we couple genetics, molecular biology, and biochemistry to study the fundamental mechanisms behind the twists and ramifications of development, striving to discern the precise nature of the small changes that lead to the differences in patterns among flowers.

In 2009 we will cheer at the accomplishments of a great scientist, but most important, we will recognize the results of his incredibly successful theory, one that has blossomed in thousands of fruitful directions.

Originally published April 10, 2009

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