In real practical engineering terms, it would be pretty difficult to set up a room so that a true line of equilibria existed. The line is in practice likely to ‘collapse to a point’. Russell Lande’s argument too, about a line of equilibria in sexual selection, rests upon assumptions that may well not be true in nature. It assumes, for instance, that there will be a steady supply of new mutations. It assumes that the act of choosing, by a female, is entirely cost-free. If this assumption is violated, as it well may be, the ‘line’ of equilibria collapses into a single point of equilibrium. But in any case, so far we have only discussed the case where the choice discrepancy becomes smaller as the successive generations of selection go by. Under other conditions the choice discrepancy may become larger.
It is a while since we discussed the matter, so let us remind ourselves of what this means. We have a population whose males are undergoing evolution of some characteristic such as tail length in widow birds, under the influence of female preference tending to make the tails longer and utilitarian selection tending to make the tails shorter. The reason there is any momentum in the evolution towards longer tails is that, whenever a female chooses a male of the type she ‘likes’, she is, because of the nonrandom association of genes, choosing copies of the very genes that made her do the choosing. So, in the next generation, not only will the males tend to have longer tails, but the females will tend to have a stronger preference for long tails. It is not obvious which of these two incremental processes will have the highest rate, generation by generation. We have so far considered the case where tail length increases faster, per generation, than preference. Now we come to consider the other possible case, where preference increases at an even higher rate, per generation, than tail length itself does. In other words, we are now going to discuss the case where the choice discrepancy gets bigger as the generations go by, not smaller as in the previous paragraphs.
Here the theoretical consequences are even more bizarre than before. Instead of negative feedback, we have positive feedback. As the generations go by, tails get longer, but the female desire for long tails increases at a higher rate. This means that, theoretically, tails will get even longer still, and at an ever-accelerating rate as the generations go by. Theoretically, tails will go on expanding even after they are 10 miles long. In practice, of course, the rules of the game will have been changed long before these absurd lengths are reached, just as our steam engine with its reversed Watt governor would not really have gone on accelerating to a million revolutions per second. But although we have to water down the conclusions of the mathematical model when we come to the extremes, the model’s conclusions may well hold true over a range of practically plausible conditions.
Now, 50 years late, we can understand what Fisher meant, when he baldly asserted that ‘it is easy to see that the speed of development will be proportional to the development already attained, which will therefore increase with time exponentially, or in geometric progression’. His rationale was clearly the same as Lande’s, when he said: ‘The two characteristics affected by such a process, namely plumage development in the male, and sexual preference for such developments in the female, must thus advance together, and so long as the process is unchecked by severe counterselection, will advance with ever-increasing speed’.
The fact that Fisher and Lande both arrived by mathematical reasoning at the same intriguing conclusion does not mean that their theory is a correct reflection of what goes on in nature. It could be, as the Cambridge University geneticist Peter O’Donald, one of the leading authorities on the theory of sexual selection, has said, that the runaway property of the Lande model is ‘built into’ its starting assumptions, in such a way that it couldn’t help emerging in a rather boring way at the other end of the mathematical reasoning. Some theorists, including Alan Grafen and W. D. Hamilton, prefer alternative kinds of theory in which choice made by a female really does have a beneficial effect on her progeny, in a utilitarian, eugenic sense. The theory they are together working on is that female birds act as diagnostic doctors, picking out those males who are least susceptible to parasites. Bright plumage, according to this characteristically ingenious theory of Hamilton, is a male’s way of conspicuously advertising his health.
The theoretical importance of parasites would take too long to explain fully. Briefly, the problem with all ‘eugenic’ theories of female choice has always been as follows. If females really could successfully choose males with the best genes, their very success would reduce the range of choice available in the future: eventually, if there were only good genes around, there would be no point in choosing. Parasites remove this theoretical objection. The reason is that, according to Hamilton, parasites and hosts are running a never-ceasing cyclical arms race against one another. This in turn means that the ‘best’ genes in any one generation of birds are not the same as the best genes in future generations. What it takes to beat the current generation of parasites is no good against the next generation of evolving parasites. Therefore there will always be some males that happen to be genetically better equipped than others to beat the current crop of parasites. Females, therefore, can always benefit their offspring by choosing the healthiest of the current generation of males. The only general criteria that successive generations of females can use are the indicators that any vet might use — bright eyes, glossy plumage, and so on. Only genuinely healthy males can display these symptoms of health, so selection favours those males that display them to the full, and even exaggerate them into long tails and spreading fans.
But the parasite theory, though it may well be right, is off the point of my ‘explosions’ chapter. Returning to the Fisher/Lande runaway theory, what is needed now is evidence from real animals. How should we go about looking for such evidence? What methods might be used? A promising approach was made by Malte Andersson, from Sweden. As it happens, he worked on the very bird that I am using here to discuss the theoretical ideas, the long-tailed widow bird, and he studied it in its natural surroundings in Kenya. Andersson’s experiments were made possible by a recent advance in technology: superglue. He reasoned as follows. If it is true that the actual tail length of males is a compromise between a utilitarian optimum on the one hand, and what females really want on the other, it should be possible to make a male super-attractive by giving him an extra long tail. This is where the superglue came in. I’ll describe Andersson’s experiment briefly, as it is a neat example of experimental design.
Andersson caught 36 male widow birds, and divided them into nine groups of four. Each group of four was treated alike. One member of each group of four (scrupulously chosen at random to avoid any unconscious bias) had his tail feathers trimmed to 14 centimetres (about 5 1/2 inches). The portion removed was stuck, with quick-setting superglue, to the end of the tail of the second member of the group of four. So, the first one had an artificially shortened tail, the second one an artificially lengthened tail. The third bird was left with his tail untouched, for comparison. The fourth bird was also left with his tail the same length, but it wasn’t untouched. Instead, the ends of the feathers were cut off and then glued back on again. This might seem a pointless exercise, but it is a good example of how careful you have to be in designing experiments. It could have been that the fact of having his tail feathers manipulated, or the fact of being caught and handled by a human, affected a bird, rather than the actual change in length itself. Group 4 was a ‘control’ for such effects.
The idea was to compare the mating success of each bird with its differently treated colleagues in its own group of four. After being treated in one of the four ways, every male was allowed to take up its former residence on its own territory. Here it resumed its normal business of trying to attract females into its territory, there to mate, build a nest and lay eggs. The question was, which member of each group of four would have the most success in pulling in females? Andersson measured this, not by literally watching females, but by waiting and then counting the number of
nests containing eggs in each male’s territory. What he found was that males with artificially elongated tails attracted nearly four times as many females as males with artificially shortened tails. Those with tails of normal, natural length had intermediate success.
The results were analysed statistically, in case they had resulted from chance alone. The conclusion was that if attracting females were the only criterion, males would be better off with longer tails than they actually have. In other words, sexual selection is constantly pulling tails (in the evolutionary sense) in the direction of getting longer. The fact that real tails are shorter than females would prefer suggests that there must be some other selection pressure keeping them shorter. This is ‘utilitarian’ selection. Presumably males with especially long tails are more likely to die than males with average tails. Unfortunately, Andersson did not have time to follow the subsequent fates of his doctored males. If he had, the prediction would have been that the males with extra tail-feathers glued on should, on average, have died younger than normal males, probably because of greater vulnerability to predators. Males with artificially shortened tails, on the other hand, should probably be expected to live longer than normal males. This is because the normal length is supposed to be a compromise between the sexual selection optimum and the utilitarian optimum. Presumably the birds with artificially shortened tails are closer to the utilitarian optimum, and therefore should live longer. There’s a lot of supposition in all this, however. If the main utilitarian disadvantage of a long tail turned out to be the economic cost of growing it in the first place, rather than increased dangers of dying after it has grown, males who are handed an extra-long tail on a plate, as a free gift from Andersson, would not be expected to die particularly young as a result.
I have written as though female preference will tend to drag tails and other ornaments in the direction of getting larger. In theory, as we saw earlier, there is no reason why female preference should not pull in exactly the opposite direction, for instance in the direction of ever shortening, rather than lengthening, tails. The common wren has a tail so short and stubby that one is tempted to wonder whether it is, perhaps, shorter than it ‘ought’ to be for strictly utilitarian purposes. Competition between male wrens is intense, as you might guess from the disproportionate loudness of their song. Such singing is bound to be costly, and a male wren has even been known to sing himself, literally, to death. Successful males have more than one female in their territory, like widow birds. In such a competitive climate, we might expect positive feedbacks to get going. Could the wren’s short tail represent the end product of a runaway process of evolutionary shrinkage?
Setting wrens on one side, peacock fans, and widow bird and bird of paradise tails, in their gaudy extravagance, are very plausibly seen as end-products of explosive, spiralling evolution by positive feedback. Fisher and his modern successors have shown us how this might have come about. Is this idea essentially tied to sexual selection, or can we find convincing analogies in other kinds of evolution? It is worth asking this question, if only because there are aspects of our own evolution that have more than a suggestion of the explosive about them, notably the extremely rapid swelling of our brains during the last few million years. It has been suggested that this is due to sexual selection itself, braininess being a sexually desirable character (or some manifestation of braininess, such as ability to remember the steps of a long and complicated ritual dance). But it could also be that brain size has exploded under the influence of a different kind of selection, analogous but not identical to sexual selection. I think it is helpful to distinguish two levels of possible analogy to sexual selection, a weak analogy and a strong analogy.
The weak analogy simply says the following. Any evolutionary process in which the end-product of one step in evolution sets the stage for the next step in evolution is potentially progressive, sometimes explosively so. We have already met this idea in the previous chapter, in the form of ‘arms races’. Each evolutionary improvement in predator design changes the pressures on prey, thereby making prey become better at avoiding the predators. This in turn puts pressure on the predators to improve, so we have an ever-rising spiral. As we saw, it is likely that neither predators nor prey will necessarily enjoy a higher success rate as a result, because their enemies are improving at the same time. But nevertheless, both prey and predators are becoming progressively better equipped. This, then, is the weak analogy with sexual selection. The strong analogy with sexual selection notes that the essence of the Fisher/Lande theory is the ‘green beard’-like phenomenon whereby genes for female choice automatically tend to choose copies of themselves, a process with an automatic tendency to go explosive. It is not clear that there are examples of this kind of phenomenon other than sexual selection itself.
I suspect that a good place to look for analogies to explosive evolution of the sexual selection kind is in human cultural evolution. This is because, here again, choice by whim matters, and such choice may be subject to the ‘fashion’ or ‘majority always wins’ effect. Once again, the warning with which I began this chapter should be heeded. Cultural ‘evolution’ is not really evolution at all if we are being fussy and purist about our use of words, but there may be enough in common between them to justify some comparison of principles. In doing this we must not make light of the differences. Let us get these matters out of the way before returning to the particular issue of explosive spirals.
It has frequently been pointed out — indeed any fool can see — that there is something quasi-evolutionary about many aspects of human history. If you sample a particular aspect of human life at regular intervals, say you sample the state of scientific knowledge, the kind of music being played, dress fashions, or vehicles of transport, at intervals of one century or perhaps one decade, you will find trends. If we have three samplings, at successive times A, B and C, then, to say that there is a trend is to say that the measurement taken at time B will be intermediate between the measurements taken at times A and C. Although there are exceptions, everyone will agree that trends of this kind characterize many aspects of civilized life. Admittedly the directions of trends sometimes reverse (for example, skirt lengths), but this is true of genetic evolution too.
Many trends, particularly trends in useful technology as opposed to frivolous fashions, can, without much argument over value-judgements, be identified as improvements. There can be no doubt, for instance, that vehicles for getting about the world have improved steadily and without reversal, over the past 200 years, passing from horse-drawn through steam-drawn vehicles, and culminating today in supersonic jet planes. I am using the word improvement in a neutral way. I don’t mean to say that everybody would agree that the quality of life has improved as a result of these changes; personally I often doubt it. Nor do I mean to deny the popular view that standards of workmanship have gone down as mass production has replaced skilled craftsmen. But looking at means of transport purely from the point of view of transport, which means getting from one part of the world to another, there can be no disputing the historical trend towards some kind of improvement, even if it is only an improvement in speed. Similarly, over a timescale of decades or even years, there is a progressive improvement in the quality of highfidelity sound amplification equipment that is undeniable, even if you agree with me sometimes that the world would be a more agreeable place if the amplifier had never been invented. It is not that tastes have changed; it is an objective, measurable fact that fidelity of reproduction is now better than it was in 1950, and in 1950 it was better than in 1920. The quality of picture reproduction is undeniably better in modern television sets than in earlier ones, although of course the same may not be true of the quality of the entertainment transmitted. The quality of machines for killing in war shows a dramatic trend towards improvement — they are capable of killing more people faster as the years go by. The sense in which this is not an improvement is too obvious to labour.
There is no doubt about it, in the
narrow technical sense things do get better as time goes by. But this is only obviously true of technically useful things such as aeroplanes and computers. There are many other aspects of human life that show true trends without these trends being, in any obvious sense, improvements. Languages clearly evolve in that they show trends, they diverge, and as the centuries go by after their divergence they become more and more mutually unintelligible. The numerous islands of the Pacific provide a beautiful workshop for the study of language evolution. The languages of different islands clearly resemble each other, and their differences can be measured precisely by the numbers of words that differ between them, a measure that is closely analogous to the molecular taxonomic measures that we shall discuss in Chapter 10. Difference between languages, measured in numbers of divergent words, can be plotted on a graph against distance between islands, measured in miles, and it turns out that the points on the graph fall on a curve whose precise mathematical shape tells us something about rates of diffusion from island to island. Words travelled by canoe, island-hopping at intervals proportional to the degree of remoteness of the islands concerned. Within any one island words change at a steady rate, in very much the same way as genes occasionally mutate. Any island, if completely isolated, would exhibit some evolutionary change in its language as time went by, and hence some divergence from the languages of other islands. Islands that are near each other obviously have a higher rate of word flow between them, via canoe, than islands that are far from each other. Their languages also have a more recent common ancestor than the languages of islands that are far apart. These phenomena, which explain the observed pattern of resemblances between near and distant islands, are closely analogous to the facts about finches on different islands of the Galápagos Archipelago which originally inspired Charles Darwin. Genes island-hop in the bodies of birds, just as words islandhop in canoes.
The Blind Watchmaker Page 29