Blogger Bilbo, an occasional visitor here, has paid a rare compliment to biochemist and creationist-witchfinder Larry Moran for his clear explanation of the difference between neutral theory and genetic drift.
Moran’s intention is to clarify a post by biologist P Z Myers, and it succeeds. Myers’ “strapline” is that near-neutral theory has “won”, over adaptationism:
The neutral theory states that most of the variation found in evolutionary lineages is a product of random genetic drift. Nearly neutral theory is an expansion of that idea that basically says that even slightly advantageous or deleterious mutations will escape selection — they’ll be overwhelmed by effects dependent on population size. This does not in any way imply that selection is unimportant, but only that most molecular differences will not be a product of adaptive, selective changes.
Note the nod to the (unspecified) importance of natural selection. Moran’s piece leaves that out, and says:
What Neutral Theory tells us is that a huge number of mutations are neutral and there are far more neutral mutations fixed by random genetic drift that there are beneficial mutations fixed by natural selection. The conclusion is inescapable. Random genetic drift is, by far, the dominant mechanism of evolution…
Replying to objectors, he goes on:
They think that random genetic drift is only important when the alleles are neutral (or nearly neutral). Then they use this false equivalency as a way of dismissing random genetic drift because it only deals with “background noise” while natural selection is the mechanism for all the interesting parts of evolution. I think we should work toward correcting this idea by separating the mechanisms of evolution (natural selection, random genetic drift, and others) from the quality of alleles being produced by mutation (beneficial, detrimental, neutral)…
…The revolution is over and strict Darwinism lost. We now know that random genetic drift is an important mechanism of evolution and there’s more to evolution than natural selection. Unfortunately, this blatantly obvious fact is not understood by the vast majority of people and teachers.There are even many scientists who don’t understand evolution.
If I had £10 for every time I’ve heard people use that last sentence as a jibe (Jerry Coyne used it of James Shapiro, for example), I would be rich! Phylogeneticist and population geneticist Joe Felsenstein (there are big hitters on this thread, as you see) protests slightly in a longish comment:
I am not sure what has been proven:
…2. That the many adaptations that we see are not the result of natural selection? Gould and Lewontin made the point in their “Spandrels of San Marco” paper that we could not assume that any adaptation we saw was itself directly brought about by natural selection for that purpose. But the high level of adaptation of living systems (a level they have to have, else they could not survive and reproduce) must have been the result of natural selection. Mutational processes alone and/or genetic drift alone just could not have made a bird that flies or a fish that swims, not ever.
…I think that it would be a tragedy if the promotion of a series of newer and newer evolutionary theories causes people to forget the one important point — that if there is no natural selection you will not get (or maintain) adaptations.
Pause to note the basis of his argument here – I’ll return to it later. Various other comments bring us to the happy conclusion that nobody is really disagreeing (perhaps after somebody mentioned that creationists might pick up on the argument!). There’s room for both selectionism and neutralism. Somehow the agreement seems superficial, and Tom Mueller’s remark on the thread bears careful reflection:
I think what needs to be understood here is the concept of “theory” as working “model” that can crank out useful answers in deliberately and precisely defined (perhaps even contrived) situations.
The whole thing needs to be seen in the context of Moran’s overall position, and that is helped by reading (a long read, I’m afraid) a series of posts he hosted in 2010, by Arlin Stoltzfus, on The Mutationist Myth. I won’t try to reiterate all his arguments, which you can read yourself, but his thesis is, in the end, that the Neo-darwinian synthesis got its view of evolution fundamentally wrong in various centrally important ways, partly by disregarding the importance of mutation for evolution (preferring to emphasize gene pools and changing frequencies), but mainly by inheriting Darwin’s view that organisms are closely adapted to their environments.
So he argues that mutationally-based theories such as the near-neutral theory are a replacement for, not a mere adjustment of, the Modern Synthesis. This would seem not to be an isolated view (at least Larry Moran concurs) – and his reasoning certainly makes a good case for it. The fact that neutral theory retains some place for natural selection, then, does nothing to detract from the fact that the long-running neutralist-adaptationist debate, despite evolution’s being an incontrovertible fact as sure as gravity, etc etc, has been a Malthusian struggle for survival between two entirely different concepts of evolution. Read the Stoltzfus series and you’ll agree, I think.
Understanding this (even if, as seems inevitable in such a climate, you don’t understand evolution) can help give a context for a paper like that of Austin Hughes (2008) , looking at an in-the-field (rather than theoretical) example, the evolution of rhodopsins in the eye, demonstrating that it best fits a pattern of non-adaptive evolution. He claims the role of positive selection is often over-emphasised (actually he claims that it is carelessly and wrongly assumed in most published research):
Contrary to a widespread impression, natural selection does not leave any unambiguous “signature” on the genome, certainly not one that is still detectable after tens or hundreds of millions of years. To biologists schooled in Neo-Darwinian thought processes, it is virtually axiomatic that any adaptive change must have been fixed as a result of natural selection. But it is important to remember that reality can be more complicated than simplistic textbook scenarios.
TOF’s recent post is applicable here: as soon as near-neutral fixation and selection are both operating together – plus alternative splicing, overlapping genes, epigenetics, cultural inheritence and so on, all mutually interacting – you have a case of “organised complexity” that makes makes understanding specific outcomes harder than predicting the path of individual ice-blocks in Saturn’s rings. Michael Clegg writes:
It is simply not possible to incorporate the complexities of variable selection driven by temporal and spatial environmental change together with drift, migration, and demographic structure into a comprehensive model that is susceptible to analysis. Yet, evolution is actually determined by the continuous operation of these and other factors. Thus, while the conclusions of population genetic models are mathematically true, they may not always contribute to empirical knowledge.
…It is also important to realize that the limits to knowledge in population genetics, and hence also the limits to knowledge in the allied sciences of molecular evolution and conservation biology, are not obvious. Workers in these fields have only come to a partial appreciation for the limits of knowledge as they have explored the empirical content of population genetic theory over a period of more than sixty years.
It’s because of arguments like these that Eugene Koonin’s 2009 overview (which I cited in a recent comment) echoes Myers’ obligatory hat-tip to adaptive selection, but severely downplays its actual importance:
Natural (positive) selection is an important factor of evolution but is only one of several fundamental forces and is not quantitatively dominant; neutral processes combined with purifying selection dominate evolution.
The majority of the sequences in all genomes evolve under the pressure of purifying selection or, in organisms with the largest genomes, neutrally, with only a small fraction of mutations actually being beneficial and fixed by natural selection as envisioned by Darwin.
You may feel the technicalities of all this go over your head (I urge you to read the Stoltzfus series anyway, though, as the history is interesting). But let me tell you what seems to be the bottom line. Darwin’s use of natural selection was to explain, in lieu of a creator, why he found nature to be so exquisitely crafted, seeing selection as:
…daily and hourly scrutinizing, throughout the world, every variation, even the slightest; rejecting that which is bad, preserving and adding up all that is good; silently and insensibly working, whenever and wherever opportunity offers, at the improvement of each organic being in relation to its organic and inorganic conditions of life.
This observational belief (from one of history’s great field naturalists) seems to have inspired Felsenstein’s comment on Moran’s blog. Felsenstein doesn’t really argue the theory – he just appeals to the eyes of the beholder of actual nature. But it is here, whatever the differences in theory, that the fundamental disagreement lies. Stoltzfus, in his last post of the series, writes of Neodarwinist adaptationism:
And this problematic view of variation is based on reasoning from the premise that organisms are exquisitely and pervasively adapted to their niches, to the conclusion that variation must play just the right role of supplying abundant raw materials to make this possible. I believe that there is something fundamentally wrong with this mode of reasoning. Perhaps it betrays a kind of subconscious Panglossian agenda.
The difference, then, is that near-neutral theorists tend to interpret what they see in nature as showing that organisms are not superbly adapted to their environments, contra Darwin’s “Panglossian” rose-tinted view. Theoretical technicalities aside, it’s a question not merely of how evolution enables survival, but whether it has produced a jerry-built bodge, or “endless forms most beautiful.”
“Understanding evolution” is one thing – but here we’re talking about the things that have struck awe into field biologists from Aristotle onwards – the giraffe’s specialisations for height, the mole’s for digging, the octopus’s camouflage, the compound eye at the very theoretical limit of optical possibility, the eagle’s camera eye spotting rabbits a mile away, the orchids matched to their many individual pollinators – those things that in the past made nature a byword for “perfection” and “wisdom.” Naturalists find that level of adaptation in any organism they study. They even see it in the lion that kills its step-children, if one finds the selfish-gene a beautiful concept. They see it in the ecosystems that organisms comprise. They see it in the totality of the interdependent biosphere.
But the neutral theorists, looking at the same things (assuming they get out into the natural world, which I suppose isn’t necessarily so) see mostly imperfections – and their theory, apparently the more correct, is bound only to deliver imperfection. They see what their theory predicts. But so do the adaptationists, so who’s right? Neutral theory’s father, Moto Kimura, hedged his bets:
Kimura argued that molecular evolution is dominated by selectively neutral evolution but at the phenotypic level, changes in characters were probably [my emphasis] dominated by natural selection rather than genetic drift. (Wikipedia)
Positive selection, then, is “rare” and a “departure from the norm” (Austin Hughes, op cit), but nevertheless manages to account for the myriads of wonderful features we actually see all around us, and yet these are in some sense independent of the genetic level of organisation responsible for them. Why would that work? “The F1 car works fine, but the components were taken from a skip and assembled by monkeys – but hey, an engineer dropped by every few days too.”
“Adaptation” is a word we all learned in school biology – but in Felsenstein’s comment it’s actually loaded in favour of his preferred explanation. “To adapt” means to change something to fit a role, which is what natural selection does by dint of killing everything that doesn’t fit – assuming positive selection is working effectively, contra neutral theory.
If forms and behaviours in fact arise by neutral mechanisms and happen to work astonishingly well, they are not “adapted” but preposterously lucky: remember, selection is not operating on them, or it would still be strict Darwinism at root. Similarly, I suppose, if these forms and behaviours were created by God, they would not be “adapted” any more than Beethoven “adapted” Eroica as a symphony: they would be designed for purpose.
But admitting “adaptation” as a descriptive term, let me summarise some propositions that seem to arise :
(1) Neutral theory appears to fit the scientific data better than adaptation theory.
(2) Neutral theory appears to account for most changes in evolution: adaptation is the exception, not the rule.
(3) Neutral theory itself has no capability whatsoever of producing any degree of adaptation, unless it allows a final controlling role to natural selection, which its proponents appears to deny.
(4) Adaptation, nevertheless, is the most pervasive feature observed in biology.
(5) We therefore appear to lack an adequate evolutionary explanation for the most pervasive feature in biology.
What did I miss here? Probably, the detailed unpacking of how the two theories (and all the others) interrelate in real life – but there was remarkably little reference to that on the Moran thread. It surely can’t be good enough to shrug and say, “A little adaptation goes a long way.”