Heritability of variability
November 12, 2020
Yesterday I wrote about Darwin’s foreshadowing of r/K selection theory.
Today I’ll argue that The Origin of Species also predicts punctuated equilibrium, even though Darwin himself is very reluctant to admit the possibility that nature makes abrupt changes.
Here’s an example of Darwin’s views on the speed of selection (emphasis mine):
That natural selection will always act with extreme slowness, I fully admit. […] But the action of natural selection will probably still oftener depend on some of the inhabitants becoming slowly modified; the mutual relations of many of the other inhabitants being thus disturbed.
He uses “slow” and its variants 135 times, about twice as often as “rapid” or “quick” (which he mainly uses to describe birthrate rather than variation). He frequently describes natural selection as “gradual.”
But yesterday I noted that he predicts that a higher number of individuals should mean more opportunities for variation. In other words — all else being equal — a larger population ought to be more variable.
Darwin is quite clear (vivid, even) about how rapidly numbers can increase under favourable conditions:
There is no exception to the rule that every organic being naturally increases at so high a rate, that, if not destroyed, the earth would soon be covered by the progeny of a single pair. Even slow-breeding man has doubled in twenty-five years, and at this rate, in less than a thousand years, there would literally not be standing room for his progeny. Linnaeus has calculated that if an annual plant produced only two seeds—and there is no plant so unproductive as this—and their seedlings next year produced two, and so on, then in twenty years there would be a million plants. The elephant is reckoned the slowest breeder of all known animals, and I have taken some pains to estimate its probable minimum rate of natural increase; it will be safest to assume that it begins breeding when thirty years old, and goes on breeding till ninety years old, bringing forth six young in the interval, and surviving till one hundred years old; if this be so, after a period of from 740 to 750 years there would be nearly nineteen million elephants alive descended from the first pair.
Other passages make it clear that he thinks variability itself should vary across varieties/species (remember, he thinks species are just differentiated varieties). And individuals, obviously, must be the bearers of any variations within a variety. He also thinks that variability is inheritable:
A large amount of inheritable and diversified variability is favourable, but I believe mere individual differences suffice for the work. A large number of individuals, by giving a better chance for the appearance within any given period of profitable variations, will compensate for a lesser amount of variability in each individual, and is, I believe, an extremely important element of success.
Some species constantly vary, apparently regardless of circumstances:
There is one point connected with individual differences, which seems to me extremely perplexing: I refer to those genera which have sometimes been called “protean” or “polymorphic,” in which the species present an inordinate amount of variation; and hardly two naturalists can agree which forms to rank as species and which as varieties. […] Genera which are polymorphic in one country seem to be, with some few exceptions, polymorphic in other countries, and likewise, judging from Brachiopod shells, at former periods of time. These facts seem to be very perplexing, for they seem to show that this kind of variability is independent of the conditions of life.
And variable species tend to stay variable:
An extraordinary amount of modification implies an unusually large and long-continued amount of variability, which has continually been accumulated by natural selection for the benefit of the species.
Moreover, species with low levels of organisation vary more than those with higher (more specialised?) levels of organisation:
As “vegetative repetition,” to use Professor Owen’s expression, is a sign of low organisation; the foregoing statements accord with the common opinion of naturalists, that beings which stand low in the scale of nature are more variable than those which are higher. I presume that lowness here means that the several parts of the organisation have been but little specialised for particular functions; and as long as the same part has to perform diversified work, we can perhaps see why it should remain variable, that is, why natural selection should not have preserved or rejected each little deviation of form so carefully as when the part has to serve for some one special purpose. In the same way that a knife which has to cut all sorts of things may be of almost any shape; whilst a tool for some particular purpose must be of some particular shape.
What I’m getting at here is that if larger numbers of individuals have more chance of variation, and variability itself can vary, and nature can produce an explosion of numbers in short order, then nature might sometimes select for variability itself in a burst-like fashion.
Unstable conditions in nature would not “cause” variability. But there must always be some background level of variation for natural selection to work at all.
During stable periods, any given mutation will be bad for the individual, who will (on average) be at a disadvantage compared to the species which is adapted for the stable environment. By surviving this far, the species is likely to have specialised for the given environment, outcompeting less specialised organisms.
If circumstances change rapidly and dramatically, then any given mutation would still probably be a bad thing for any individual organism. Across the species, though, a few might be beneficial. This is just basic natural selection.
But if we understand different species, and different individuals within a species, to have different levels of variability, it might be that by selecting for new variations (as a result of the unstable conditions), nature is (as a side effect) selecting for individuals/species that will tend to vary more in the future.
If the environmental conditions cause selection for variability itself, then this would seem to me to accelerate change.
Even in periods of exploitation, explorers must be randomly produced. But when conditions change, there may be a population explosion of explorers, who would explore the fitness landscape. Once the environment settles down again (assuming it does eventually), then a new form of exploitation may become the new equilibrium.
There is evidence, for example, of the extremely rapid optimisation of photosynthesis:
We have also shown evidence that these structural changes and duplication events were accompanied by rates of evolution of the reaction centre proteins at least 40 times greater than any rates observed in the past 2.5 billion years in any cyanobacterium, alga or plant.
I believe that what I’m describing is roughly Gould’s view, i.e., punctuated equilibrium, which Dawkins opposes. Without having read much Dawkins or any Gould, it seems to me that Gould’s view is supported by Darwin’s reasoning, even though Darwin himself wishes to disavow it.
Darwin’s belief that natural selection must proceed slowly, at least in the first half of The Origin of Species, seems to be an article of faith. Perhaps he gives more evidence for this view later.
If so, I’ll update this post. But for now, I’m wondering whether this view of evolution isn’t to make it more palatable or less disturbing to the religious; something like a belief that He doesn’t play dice.
On the other hand, perhaps I’m reasoning circularly. I suppose I’m betraying a discontinuous (“punctuated”) view of geological history independently of life, as a result of reading about extinction events. Perhaps Darwin’s view is that geological change is more gradual.