To a man with a hammer, said Mark Twain, everything looks like a nail. The better your hammer, I would add, the more nail-like everything looks.
In natural selection, Darwin had crafted one of the best hammers of all time. And in chapter 5 of the origin, 'Laws of Variation', you can hear him umming and aahing about various alternative mechanisms of evolutionary change before deciding that, actually, you know what this needs...hold 'er steady...Thwack!
How about, for example, the "effect of external conditions" -- "food climate &c.". Well, "Gould believes that birds of the same species are more brightly coloured under a clear atmosphere, than when living on islands near the coast."
On the other hand...Thwack!
"When a variation is of the slightest use to a being, we cannot tell how much of it to attribute to the accumulative action of natural selection, and how much to the conditions of life." And the poor match between environment and patterns of variation "incline[s] me to lay very little weight on the direct action of the conditions of life."
Well, then, how about the "Effects of Use and Disuse"? That's a bit more tricky. On the one hand, "I think there can be little doubt that use in our domestic animals strengthens and enlarges certain parts, and disuse diminishes them; and that such modifications are inherited...many animals have structures which can be explained by the effects of disuse."
But, on reflection: Thwack!
"The effects of use and disuse have often been largely combined with, and sometimes overmastered by, the natural selection of innate differences."
And how about "Correlation of growth" -- structures that arise due to something going on elsewhere in the organism? "We see that modifications of structure...may be wholly due to unknown laws of correlated growth, and without being, as far as we can see, of the slightest service to the species."
"[N]atural selection will always succeed in the long run in reducing and saving every part of the organisation, as soon as it is rendered superfluous, without by any means causing some other part to be largely developed in a corresponding degree. And, conversely, that natural selection may perfectly well succeed in largely developing any organ, without requiring as a necessary compensation the reduction of some adjoining part."
Much of the qualification in these passages sounds to me like bet-hedging. At this stage, Darwin, I think, was a rampaging adaptationist -- happily swinging his hammer confident that, while he doesn't know what causes variation, those variations are maintained and spread pretty much exclusively by natural selection.
Although Darwin doesn't hit every nail on the head. In the 'Use and Disuse' section, he suggests that cave-dwellers might lose their eyes because they don't use them, not because they are disadvantageous: "it is difficult to imagine that eyes, though useless, could be in any way injurious to animals living in darkness."
Can't you see, I thought, that making and maintaining eyes is costly, and that if you don't need them, you could use the resources for something else? Wouldn't that select against eyes? Oh well, not even Darwin's omniscient.
But then, a few pages later, in the 'Correlation of Growth' section, comes this:
"If under changed conditions of life a structure before useful becomes less useful, any diminution, however slight, in its development, will be seized on by natural selection, for it will profit the individual not to have its nutriment wasted in building up an useless structure."
Darwin goes on to talk about parasites, many of which have given up doing things that their host does for them. Why he didn't spot that the same reasoning could apply to eyes is beyond me.
Since the Origin was published, all these nails have shown a repeated tendency to work loose. Darwin himself began the backsliding in later editions of the Origin, giving more weight to Lamarckism et al. until, according to Robert Young, the book might have been called "On the Origin of Species by Means of Natural Selection and All Sorts of Other Things".
And twentieth century genetics, embryology, cell biology and so on have failed to clear things up. To take the issues in the same order as Darwin:
- The effect of external conditions has become phenotypic plasticity -- the ability of genetically identical organisms to produce different forms, depending on their environment. Some, such as Mary-Jane West Eberhard believe this is an important driver of evolution, with genetic change following where plasticity has lead.
- The possibility that traits resulting from use and disuse can be passed on -- Lamarckism -- is currently incarnated in the form of epigenetic inheritance. Here, changes to patterns of gene expression, rather than gene sequence, are inherited. There's an article by one of the idea's leading proponents, Eva Jablonka, in the current issue of Seed.
- And what are correlations of growth if not the spandrels famously pointed to by Stephen Jay Gould and Richard Lewontin -- structures that are a by-product of some other structure being selected for elsewhere?
Broadly, the key issue is that development -- the process by which bodies are made -- has never been integrated with population genetics, the science that grew out of the unification of evolution and genetic known as the modern synthesis, which explains evolution as changes in gene frequencies brought about by selection and drift. We can't link change within a body with change between generations.
This unification is among the goals of evolutionary developmental biology, evo-devo. And some biologists, many of them working in some branch of evo-devo, believe that adding things such as plasticity and epigenetics to our picture of evolution requires an extended synthesis. (I'm aiming to link to pages behind paywalls as little as possible, but it'd be weird not to mention that I reported on this meeting for Nature.)
In such a synthesis, broadly speaking, other forms of inheritance besides genes will be important, and the developmental system will be seen as an important player in evolutionary change independent from the system of inheritance.
Some population geneticists view efforts to modify the modern synthesis with the attitude of the heroine confronting the bogeyman in the ninth sequel of a horror movie franchise -- ain't you dead yet? Brian Charlesworth, for example, sees a "catalogue of failed attempts by developmental biologists to supplement or replace neo-Darwinian evolutionary biology".
Broadening the mechanisms of inheritance is not necessarily the same as ditching natural selection, as David Haig has pointed out. And regarding natural selection, for what it's worth I'm struck by how good a job adaptationist arguments, such as the models of mate choice and altruism in animal behaviour, do in explaining natural phenomena. They seem to have a clarity and generality that the other side usually lacks.
But this might be cultural -- I read the Selfish Gene at an impressionable age, and went to university in the early-90s, during the heyday of behavioural ecology. So see that as a declaration of interest, rather than an attempt to proselytise.
If I ever have a spare year, I'll get to grips with the other side, and read Stephen Jay Gould's Structure of Evolutionary Theory. Before that can happen, we'd better do the Origin's next chapter, Difficulties on Theory, on Friday.
In this chapter Darwin freely admits he does not know where variation arises, but that it is indeed there, we have already seen illustrated it in the previous chapters. He does provided 10 ideas on how he sees variation arising, but sums it all up in the final paragraph of the chapter:
"Whatever the cause may be of each slight difference in offspring from their parents - and a cause for each must exist - it is the steady accumulation, through the individual, that gives rise to all the more important modifications of structure, by which the innumerable beings on the face of this earth are enabled to struggle with each other, and the best adapted to survive."
He uses his great writing skills to lead us through a discussion of how variation arises, to admit he doesn't really know, but it is not important that we don't know, for we know it exists and is acted upon by natural selection to create new species!
The more I read the book, the more impressed I am with his skills as a writer and scientist! He may not have gotten everything correct, but that should not take away from the main ideas and arguments that have since proven to be spot on!
Darwin also freely admitted he didn't know where variation arises in chapter 1 - and I thought it a little odd that he returned to the subject for a whole chapter here. But I also thought that this chapter was more about different mechanisms of evolutionary change - again he's using 'variation' to mean both evolution's raw material and its end product.
The latter parts of the chapter, which I didn't discuss in the post, deal more with issues that we'd recognize as developmental -- such as where extreme forms come from, and what we can learn from their taxonomic distribution. And while I generally agree with your assessment of his writing, I found these bits tough going.
I am a classics major/field archaeologist dabbler, and I am excited how readable _Origin_ is proving to be. This readalong is very helpful: although I consider myself an informed citizen (I read New Scientist and Discover and the New York Times, I am not always sure where Darwin is wrong and where my understanding of the current science is wrong (and certainly where it's incomplete). I am afraid I found the earlier chapter on pigeons more interesting and accessible than this one, but again, the commentary is helpful (Thwack!).
On a literary note -- I am reading _The Origin_ in the Barnes and Noble Classics Series and yearning for hyperlinks. I would enjoy the option of reading the relevant extracts from the naturalists Darwin is citing, as well as cautionary notes ("This source was off the wall.") And I am afraid my enjoyment is greatly enhanced by travelling, as it were, through the _Origins_ with Patrick O'Brian's Stephen Maturin. Now I understand much better why he was always dissecting things.
Thanks for making it possible for people long out of college to learn something I wouldn't have appreciated nearly as much when I was 20.
I think the second half of this chapter is the most difficult to read part of the book so far -- it really seemed quite waffly and opaque to me. Often Darwin's best writing and best science interweave; this makes it all the more obvious when his good lines stand out from the writing around them. There's a brilliant paragraph in this chapter, in which he says: 'Creationists - do you think God wants you to be stupid?'
He who believes that each equine species was independently created, will, I presume, assert that each species has been created with a tendency to vary, both under nature and under domestication, in this particular manner, so as often to become striped like other species of the genus; and that each has been created with a strong tendency, when crossed with species inhabiting distant quarters of the world, to produce hybrids resembling in their stripes, not their own parents, but other species of the genus. To admit this view is, as it seems to me, to reject a real for an unreal, or at least for an unknown, cause. It makes the works of God a mere mockery and deception; I would almost as soon believe with the old and ignorant cosmogonists, that fossil shells had never lived, but had been created in stone so as to mock the shells now living on the sea-shore.
But this paragraph comes like a breath of fresh air, and you get the sense, which you don't get elsewhere, that Darwin is grandstanding. I imagined Disraeli firing back with his angel/ape quip, and the issue becoming a matter of who's got the best put-down. Most of the time, Darwin doesn't need put-downs, because he's got nature on his side.