Darwin was … a long time ago
posted by saurabh in Biology |Ugh. New Scientist is drumming up subscriptions by pasting the title “DARWIN WAS WRONG” on their cover. Everyone loves a good controversy, right? Let’s swan-dive right into the muck of the inane culture war going on and get good and dirty.
The basic thrust of the article is: the idea of a “tree of life” is wrong. The standard picture of evolution is of a divergent process - an ancestral species separates, differentiates, forms two daughter species. Over time this produces a binary tree of life, which we can trace backwards to its root via comparative genomics. The problem with this neat picture is lateral gene transfer - exchange of DNA across species (or even higher taxonomic classes) boundaries. If DNA can be exchanged back and forth willy-nilly, it confuses the parenting process. If my genome contains both human and frog DNA sequence, am I human, or am I frog? Or both? The graph becomes complex and hard to unravel, and certainly no longer resembles a tree.
This is most problematic in prokaryotic species, which have small genomes with only a few thousand genes, and many mechanisms for exchanging DNA with random strangers (or even taking it up from the environment). That sort of incestuous interchange makes it very hard to draw a simple tree. The stuff gets most confused near the root of the tree, where a debate has long raged about who came first, eukaryotes, prokaryotes, or archaebacteria. The best reading on the subject is W. Ford Doolittle, who rubbishes the notion of a singly-rooted tree in this blessedly free PNAS review:
Some evolutionists believe (i) that a single rooted and dichotomously branching representation of the relationships between all life forms is appropriate (at all levels above species), because it best represents their history; (ii) that we can with available data and methods reconstruct this tree quite accurately; and (iii) that we have in fact done so, at least for the major groups of organisms. … [O]ther evolutionists, ourselves included, question even this most fundamental belief, that there is a single true tree.
So, what am I saying? Is New Scientist right? Well, yes and no. The confusions indicated above are confined: while lateral gene transfer may be rampant in prokaryotes, it’s only sporadic in metazoans (animals), and a tree-of-life metaphor works pretty well there, especially for the species we’re most concerned with (viz., ourselves and our mammalian relatives). There are some notable exceptions, the most famous probably being the discovery that the entire genome of the fruit fly endosymbiont Wolbachia was at some point incorporated into the genome of some Drosophila ancestor. But we shouldn’t expect to find these events playing a significant role in the evolution of large, complex organisms. So while I find this interesting, and don’t mean to contradict Doolittle, et al., in any way, I think the “revolution” is relatively muted, and certainly doesn’t have any of the broad scope that the introduction of Mendelian genetics, for example, had on biology, as New Scientist seems to be suggesting.
Finally, a scrap for the wolves, lest they begin salivating too much, from Doolittle:
Holding onto this ladder of pattern [the tree of life] is an unnecessary hindrance in the understanding of process (which is prior to pattern) both ontologically and in our more down-to-earth conceptualization of how evolution has occurred. And it should not be an essential element in our struggle against those who doubt the validity of evolutionary theory, who can take comfort from this challenge to the TOL only by a willful misunderstanding of its import. The patterns of similarity and difference seen among living things are historical in origin, the product of evolutionary mechanisms that, although various and complex, are not beyond comprehension and can sometimes be reconstructed.
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