About Rationally Speaking
Rationally Speaking is a blog maintained by Prof. Massimo Pigliucci, a philosopher at the City University of New York. The blog reflects the Enlightenment figure Marquis de Condorcet's idea of what a public intellectual (yes, we know, that's such a bad word) ought to be: someone who devotes himself to "the tracking down of prejudices in the hiding places where priests, the schools, the government, and all long-established institutions had gathered and protected them." You're welcome. Please notice that the contents of this blog can be reprinted under the standard Creative Commons license.
Friday, May 30, 2008
The Platypus, evolution, and why Piattelli-Palmarini is wrong
Far from being a joke, the platypus is a strong piece of evidence for the theory of evolution (not that it really needed additional ones) and, scientifically speaking, a rich source of insight into the evolution of mammals from reptile-birds (birds are considered a group of reptiles, in particular, part of the same line of descent as dinosaurs).
Biologists already knew quite a bit about this strange creature, for instance that it produces milk (like any mammal) though it doesn’t have nipples (unlike any other mammal). This suggested that the ability to produce milk evolved before specialized anatomical structures to deliver it, or -- less likely -- that the platypus lost the nipples sometimes after its divergence from the rest of the mammalian lineage. Molecular biology now confirms that platypuses have genes that produce casein proteins, an essential element of true milk, which means that milk production evolved about 166 million years ago, after the mammal-sauropsid split (living sauropsids include snakes, lizards, crocodiles, turtles and of course birds; extinct ones comprise plesiosaurs and pterosaurs).
The platypus genome project has confirmed what evolutionary biologists had surmised from the morphological and physiological features of the platypus: the animal’s biology is a bridge (a “missing link,” if you will) between reptiles and mammals, with genetic features found in each group (again, including birds, which are really flying reptiles). Moreover, some of the features of the platypus are not the result of common ancestry with the reptiles, but of another well known evolutionary phenomenon: convergence. For instance, male platypuses have a spur that can deliver a venom powerful enough to kill a dog. A chemical analysis of the venom reveals that it is made of a “cocktail” of various peptides (short proteins), similar to the venom of snakes. Except that the platypus did not inherit its venom-making ability from reptiles, it evolved it independently, as the molecular studies clearly show.
At any rate, you would think that the Nature article would be yet another spectacular cause for celebration among biologists, one more triumph of evolutionary theory. Not so fast. My Italian colleague Massimo Piattelli-Palmarini (now at the University of Arizona in Tucson) wrote a disconcerting article about the platypus for a major Italian newspaper, Il Corriere della Sera. In the article, Palmarini says that the new discoveries “testify against the classical Darwinian theory that biological evolution proceeds only by small cumulative changes ... Platypus one, Darwin zero.”
This is a rather grandiose claim, and its basis appears to be that biologists have discovered that some genes in the platypus are part of a family of genes that split up through molecular duplication millions of years ago. Members of each half of this family then evolved independently, one affecting the proper placement of testicles in male platypuses, the other contributing to the formation of the placenta in females.
I don’t know of any serious biologist who would be surprised by the discovery of gene duplications in platypuses (they have been known for decades in many animals, plants and bacteria); moreover, gene duplication events followed by the specialization of one or both of the duplicates for novel functions is precisely what current evolutionary theory predicts should happen. So what’s the big deal?
The problem is that, pace Palmarini, nobody believes in the “classical” Darwinian theory, that is in the theory as originally proposed by Darwin, just in the same way in which no modern physicist “believes” in the Newtonian theory, as originally proposed by Sir Isaac. To point to modern discoveries and proclaim out loud that they contradict “the classical Darwinian theory” is disingenuous, and no different from the old and trite tactics of creationists. Serious scientists like Palmarini should not indulge in this sort of easily misconstrued double talk.
Moreover, the explanation that Palmarini gives for his interpretation is next to ridiculous (here Italian has a better fitting phrase than English: “rasenta il ridicolo,” meaning ‘flies within millimeters from the ridiculous’). Speaking of gene duplications, Palmarini says: “You cannot have two and a half copies, or three copies and a tenth. Gradualism, that is the classical Darwinian idea of small random steps, one after the other, can go to get blessed” (the latter turn of phrase, “vanno a farsi benedire” is ironic Italian for they are so untenable that it would take a miracle to consider them).
Duh, is my technical comment. Of course gene duplications produce an integer number of genes, how could it be otherwise? But that has nothing to do with Darwin’s ideas about gradualism (remember that Chuck didn’t even know about the existence of genes). Moreover, Darwin’s strict gradualism was questioned already by his friend and colleague, Thomas Henry Huxley (known as “Darwin’s bulldog”), and is not accepted in the original form by most biologists today. Finally, and this was rather surprising coming from a scientist (Palmarini is not a biologist, but a physicist turned linguist-psychologist), even “classical” Darwinism was never about random steps. Au contraire, the very concept of natural selection implies anything but randomness. And again, it is creationists who -- willfully or through ignorance -- throw randomness into the mix, without understanding, or refusing to understand, that neither Darwin nor any modern biologist has ever claimed that evolution takes place “at random.”
I do not want to give the reader the impression that evolutionary theory is static and that there aren’t serious discussions about the scope of natural selection and the possibility of additional “non-Darwinian” mechanisms to play important roles in evolution. Indeed, most of my active research program is precisely about this sort of discussion. But to write in a newspaper “Platypus one, Darwin zero” (as much as I appreciate soccer metaphors) on the basis of discoveries that in fact spectacularly confirm many tenets of current evolutionary theory is unconscionable. We have enough trouble defending science from religiously inspired know-nothings and from ignorant and presumptuous postmodernists, do we have to guard our rear from our own colleagues too?