Perhaps Charles Darwin was wrong about how species originated? This shouldn’t be so surprising. If you read The Origin of Species you’ll be struck by how much Darwin got wrong, and, how much he got right. As a fully grown adult with some knowledge of evolution re-reading Darwin in the original confirmed for me what a genius he was. The man was writing over 150 years ago in a pre-genetic era. Not only did he not have molecular phylogenetics, but he didn’t even have a proper theory of inheritance! It’s a miracle Charles Darwin got so much right. It’s no sin that he missed the mark even on the big questions.
With that I have to admit it’s awkward when people ask me about big picture evolutionary questions (as opposed to, for example, the rate of new mutations in humans), as unfortunately I’m not very well versed in macroevolution. Much of what I know about speciation in particular I know from Jerry Coyne and H. Allen Orr’s mid-2000s book Speciation. It’s true that both are population geneticists by training, so that might give me a biased view, so I’ve also read Stephen Jay Gould’s books (e.g, The Structure of Evolutionary Theory). But with Gould it’s hard to tell when he’s transmitting consensus, and when’s presenting his own heterodox views, unless you engage in very close reading. Simon Conway Morris wrote The Crucible of Creation specifically to rebut what he felt were Gould’s misrepresentation of his research in Wonderful Life (by the way, I feel that Gould’s literary style is best suited to essay format; in books he’s interminable).
A new open access paper in MBE, Tree of Life Reveals Clock-Like Speciation and Diversification, concludes that speciation is a clocklike and neutral process. As you can see from the figure above it looks as if ~2 million years is some major peak when it comes to the point at which lineages which are species converge in terms of their last ancestral populations. First, it seems clear that they’re using the biological species concept. Second, their mechanism is totally unoriginal. Rather than positive selection due to exogenous natural pressures (think Darwin’s finches, though he himself gave a lot of thought to sexual selection as the driver of speciation) the authors indicate that neutral mutational differences between diverged populations eventually lead to genetic incompabilities. This was an idea that was part of the neo-Darwinian Synthesis, so we’re talking about modulating weights, not overturning of the established order. This is an active area of research in population genetics today, see Emergent speciation by multiple Dobzhansky-Muller incompatibilities. It’s not crazy on the face of it as a hypothesis.
But it’s a whole different thing to generalize about the tree of life. Quanta Magazine has a collection of responses from researchers in the field, A Surprise for Evolution in Giant Tree of Life: Researchers build the world’s largest evolutionary tree and conclude that species arise because of chance mutations — not natural selection. I think the title is misleading for the general audience, as this isn’t a novel thesis at all, but the piece itself is more evenhanded. As noted within phylogenetics is a highly statistical enterprise. Leibniz’s famous injunction “let us calculate” is a bit more complicated when you have biased data to put into your inference generation machine. Obviously the authors couldn’t sample extinct lineages, as noted by some in the Quanta piece. They try to account for this, but the devil is in the details. Overall a ~2 million year figure invariant across arthropods and vertebrates strikes me as strange, and likely a statistical artifact if I had to put money on it (or perhaps low effective population size lineages exhibit more build up of neutral alleles producing genetic incompabilities, while large Ne groups are more impacted by positive diversifying selection, when it comes to speciation?).
Time will tell. This is not the final answer, and my passing acquaintance with this field suggests that a “first look” often does not hold up because people are missing part of this very big picture. The whole tree of life is a big thing to tackle when it comes to generalization. But, I am optimistic that this generation shall not pass before we have enough sequence across the tips of the tree and computational power to analyze it to come to more robust conclusions.
Citation: S. Blair Hedges, Julie Marin, Michael Suleski, Madeline Paymer, and Sudhir Kumar, Tree of Life Reveals Clock-Like Speciation and Diversification, Mol Biol Evol (2015) 32 (4): 835-845 first published online March 3, 2015 doi:10.1093/molbev/msv037
Addendum: I should note also that natural selection itself is somewhat stochastic over short enough time intervals. Don’t know if that would produce neutrality in speciation over the long term.
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