by Livio, Mario
Another fascinating (although controversial) example of natural selection is the evolution of the peppered moth. Prior to the industrial revolution, the light colors of this moth (known among biologists as Biston betularia betularia morpha typica) provided ample camouflage against the background of its habitat: lichens and trees. The industrial revolution in England brought with it immense levels of pollution that destroyed many lichens and blackened many trees with soot. Consequently, the white-bodied moths were exposed suddenly to massive predation, which led to their near extinction. At the same time, the melanic, dark-colored variety of the moth (carbonaria) started to flourish around 1848, because of its much improved camouflage characteristics. As if to demonstrate the importance of “green” practices, the white-bodied moths started reappearing again once better environmental standards had been adopted. While some studies of the peppered moth and the phenomenon described above (“industrial melanism”) have been criticized by a number of creationists, even some of the critics agree that this is a clear case of natural selection, and they argue only that this does not provide proof of evolution, since the net result is merely of one type of moth morphing into another rather than into an entirely new species altogether.
Another common, more philosophical, objection to natural selection is that Darwin’s definition of it is circular, or tautological. Put in simple terms, the adverse judgment goes something like this: Natural selection means “survival of the fittest.” But how do you define the “fittest”? They are identified as those that survive best; hence, the definition is a tautology. This argument stems from a misunderstanding, and it is absolutely false. Darwin did not use “fitness” to refer to those who survive but to those who, when compared with other members of the species, could be expected to survive because they were better adapted to the environment. The interaction between a variable feature of an organism and the environment of that organism is crucial here. Since the organisms compete for limited resources, some survive and some don’t. Furthermore, for natural selection to operate, the adaptive characteristics need to be heritable, that is, capable of being genetically passed on.
Surprisingly, even the famous philosopher of science Karl Popper raised a suspicion of tautology against evolution by natural selection (albeit a more subtle one). Popper basically questioned natural selection’s explanatory power based on the following argument: If certain species exist, this means that they were adapted to their environment (since those that were not adapted became extinct). In other words, Popper asserted, adaptation is simply defined as the quality that guarantees existence, and nothing is ruled out. However, since Popper published this argument, a number of philosophers have shown it to be erroneous. In reality, Darwin’s theory of evolution rules out more scenarios than it leaves in. According to Darwin, for instance, no new species can emerge without having an ancestral species. Similarly, in Darwin’s theory, any variations that are not achievable in gradual steps are ruled out. In modern terminology, “achievable” would refer to processes governed by the laws of molecular biology and genetics. A key point here is the statistical nature of adaptation—no predictions can be made about individuals, just about probabilities. Two identical twins are not guaranteed to produce the same number of offspring, or even to both survive. Popper, by the way, did recognize his error in later years, declaring, “I have changed my mind about the testability and the logical status of natural selection; and I am glad to have an opportunity to make a recantation.”
Finally, for completeness, I should mention that although natural selection is the main driver of evolution, other processes can bring about evolutionary changes. One example (which Darwin could not have known about) is provided by what has been termed by modern evolutionary biologists genetic drift: a change in the relative frequency in which a variant of a gene (an allele) appears in a population due to chance or sampling errors. This effect can be significant in small populations, as the following examples demonstrate. When you flip a coin, the expectation is that heads will turn up about 50 percent of the time. This means that if you flip a coin a million times, the number of times you’ll get heads will be close to a half million. If you toss a coin just four times, however, there is a nonnegligible probability (of about 6.2 percent) that it will land heads each time, thus deviating substantially from the expectation. Now imagine a very large island population of organisms in which just one gene appears in two variants (alleles): X or Z. The alleles have an equal frequency in the population; that is, the frequency of X and Z is 1/2 for each. Before these organisms have a chance to reproduce, however, a huge tsunami wave washes the island, killing all but four of the organisms. The surviving four organisms could have any of the following sixteen combinations of alleles: XXXX, XXXZ, XXZX, XZXX, ZXXX, XXZZ, ZZXX, XZZX, ZXXZ, XZXZ, ZXZX, XZZZ, ZZZX, ZXZZ, ZZXZ, ZZZZ. You will notice that in ten out of these sixteen combinations, the number of X alleles is not equal to the number of Z alleles. In other words, in the surviving population, there is a higher chance for a genetic drift—a change in the relative allele frequency—than for keeping the initial state of equal frequencies.
Genetic drift can cause a relatively rapid evolution in a small population’s gene pool, which is independent of natural selection. One oft-cited example of genetic drift involves the Amish community of eastern Pennsylvania. Among the Amish, polydactyly (extra fingers or toes) is many times more common than in the general population of the United States. This is one of the manifestations of the rare Ellis-van Creveld syndrome. Diseases of recessive genes, such as the Ellis-van Creveld syndrome, require two copies of the gene to cause the disease. That is, both parents have to be carriers of the recessive gene. The reason for the higher-than-normal frequency of these genes in the Amish community is that the Amish marry within their own group, and the population itself originated from around two hundred German immigrants. The small size of this community allowed researchers to trace back the Ellis-van Creveld syndrome to just one couple, Samuel King and his wife, who arrived in 1744.
There are three points that need to be emphasized about genetic drift. First, the evolutionary changes that are due to genetic drift occur entirely as a result of chance and sampling errors—they are not driven by selection pressure. Second, genetic drift cannot cause adaptation, which remains entirely the province of natural selection. In fact, being entirely random, genetic drift can cause certain properties to evolve whose usefulness is otherwise very puzzling. Finally, while genetic drift clearly occurs to some degree in all populations (since all the populations are finite in size), its effects are most pronounced in small, isolated populations.
These are, very concisely, some of the key points of Darwin’s theory of evolution by natural selection. Darwin revolutionized biological thinking in two major ways. He not only recognized that beliefs held for centuries could be false but also demonstrated that scientific truth can be achieved by the patient collection of facts, coupled with bold hypothesizing about the theory that binds those facts together. As you must have realized, his theory does a superb job in explaining why life on Earth is so diverse and why living organisms have the characteristics they have. The nineteenth-century English suffragist and botanist Lydia Becker beautifully described Darwin’s achievement:
How seemingly unimportant are the movements of insects, creeping in and out of flowers in search of the nectar on which they feed! If we saw a man spending his time in watching them, and in noting their flitting with curious eyes, we might be excused for imagining that he was amusing himself by idling an hour luxuriously in observing things which, though curious, were trifling. But how mistaken might we be in such an assumption! For these little winged messengers bear to the mind of the philosophical naturalist tidings of mysteries hitherto unrevealed; and as Newton saw the law of gravitation in the fall of the apple, Darwin found, in the connection between flies and flowers, some of the most important facts which support the theory he has promulgated respecting the modification of specific forms in animate
d beings.
Indeed, Darwin was to the nineteenth century what Newton was to the seventeenth, and Einstein to the twentieth. It is curious that the theory of evolution constituted one of the most dramatic revolutions in the history of science. In the words of biologist and science historian Ernst Mayr, it “caused a greater upheaval in man’s thinking than any other scientific advance since the rebirth of science in the Renaissance.” The question, then, is: Where was Darwin’s blunder?
CHAPTER 3
YEA, ALL WHICH IT INHERIT, SHALL DISSOLVE
Life’s perhaps the only riddle
That we shrink from giving up!
—WILLIAM SCHWENCK GILBERT,
THE GONDOLIERS
The title of this chapter is taken partly from William Shakespeare’s The Tempest, but as we shall soon see, it poetically captures the essence of Darwin’s blunder. The source of the blunder was the fact that the prevailing theory of heredity in the nineteenth century was fundamentally flawed. Darwin himself was aware of the existing shortcomings, as he confessed candidly in The Origin:
The laws governing inheritance are quite unknown; no one can say why the same peculiarity in different individuals of the same species, and in individuals of different species, is sometimes inherited and sometimes not so; why the child often reverts in certain characters to its grandfather or grandmother or other much more remote ancestor; why a peculiarity is often transmitted from one sex to both sexes, or to one sex alone, more commonly but not exclusively to the like sex.
To say that the laws of inheritance were “quite unknown” was probably the most glaring understatement of the entire book. Darwin had been educated according to the then widely held belief that the characteristics of the two parents become physically blended in their offspring—as in the mixing of paints. In this “paint-pot theory,” the heredity contribution of each ancestor was predicted to be halved in each generation, and the offspring of any sexual partners were expected to be intermediates. In Darwin’s own words: “After twelve generations, the proportion of blood, to use a common expression, of any one ancestor is only 1 in 2,048.” That is, as with gin and tonic, if you keep mixing the drink with tonic, you eventually no longer taste the gin. Somehow, in spite of apparently understanding this inevitable dilution, Darwin still expected natural selection to work. For instance, in his example of wolves preying on deer, he concluded, “If any slight innate change of habit or of structure benefited an individual wolf, it would have the best chance of surviving and leaving progeny. Some of its young would probably inherit the same habits or structure, and by the repetition of this process, a new variety might be formed.” But the simple fact that this expectation was absolutely untenable under the assumption of a blending theory of heredity did not occur to Darwin. The inconsistency was first noted by the Scottish engineer Fleeming Jenkin.
Jenkin was a multitalented individual whose pursuits ranged from drawing portraits of passersby to designing undersea telegraph cables. His criticism of Darwin was fairly straightforward. Jenkin argued that natural selection would be totally ineffective in “selecting” a single variation (a rare novelty that arose by chance, which he referred to as a “sport”; today we would call it a mutation), because any such variation would be swamped and diluted by all the normal types in the population and obliterated entirely after a few generations.
Darwin could not be faulted for not knowing any better than the heredity theory that was scientifically accepted at his time. Consequently, I do not consider his adopting the idea of blending inheritance as a blunder. Darwin blundered in having completely missed the point (at least initially) that his mechanism of natural selection simply could not work as envisioned, under the assumption of blending inheritance. Let us examine this serious blunder and its potentially devastating consequences in more detail.
Swamping
Fleeming Jenkin published his criticism of Darwin’s theory as an anonymous review of the fourth edition of On the Origin of Species. The article appeared in the North British Review in June 1867. While the essay attacked the theory of evolution on several grounds, I shall concentrate here on the one argument that exposed Darwin’s blunder. To demonstrate his point, Jenkin assumed that each individual has one hundred offspring, but of those, on the average, only one survives to reproduce. He then discussed an individual with a rare mutation (“sport”) that has the advantage of having twice the chance of survival and reproduction as any other. Appropriately for the rigorous engineer that he was (he received no fewer than thirty-seven patents between 1860 and 1886), Jenkin’s approach was quantitative—he wanted to actually calculate the effect of such a “sport” on the general population:
It will breed and have a progeny of say 100; now this progeny will, on the whole, be intermediate between the average individual and the sport. [Since the sports are rare, a sport is expected to mate with an average individual.] The odds in favour of one of this generation of the new breed will be, say, 1.5 to 1 [under the assumption of blending], as compared with the average individual; the odds in their favor will therefore be less than that of their parent; but owing to their greater number, the chances are that about 1.5 of them would survive. Unless these breed together, a most improbable event, their progeny would again approach the average individual; there would be 150 of them [1.5 times 100], and their superiority would be say in the ratio of 1.25 to 1 [again because of blending]; the probability would now be that nearly two of them would survive [1 percent of 1.25 times 150] and have 200 children, with an eighth superiority. Rather more than two of these would survive; but the superiority would again dwindle, until after a few generations it would no longer be observed, and would count no more in the struggle for life, than any of the hundred trifling advantages which occur in the ordinary organs.
Jenkin argued that even under the most extreme form of selection, one could not expect the complete transformation of a well-established characteristic, such as skin color, into a new one, if that new characteristic had been introduced into the population only once. To illustrate this swamping effect, Jenkin chose a startlingly prejudicial example of a white man with superior characteristics shipwrecked on an island inhabited by blacks. The racist and imperialistic tone of the passage utterly shocks us today, but it probably was commonplace in late-Victorian Britain: Even if this person “would kill many blacks in the struggle for existence” and “would have a great many wives and children,” and “in the first generation there will be some dozens of intelligent young mulattoes,” Jenkin argued, “can any one believe that the whole island will gradually acquire a white, or even a yellow population?”
As it turned out, Jenkin actually made one serious logical mistake in his calculations. He assumed that each sexual pair had one hundred offspring, of whom, on the average, only one offspring survived to reproduce. However, since only females can reproduce, it follows that out of each mating couple, two offspring must on the average survive (one male and one female); otherwise the size of the population would be halved in each generation—a recipe for rapid extinction. Surprisingly, only Arthur Sladen Davis, an assistant mathematics master at Leeds Grammar School, discovered this obvious error, and he explained it in a letter to the journal Nature in 1871.
Davis showed that when a correction is made to keep the population roughly constant in size, the effect of a sport does not die out (as Jenkin contended), but, in fact, although diluted, it becomes distributed throughout the entire population. For instance, a black cat introduced into a population of white cats would (under the assumption of blending inheritance) on the average produce two gray kittens, four lighter grandkittens, and so on. Successive generations would become progressively lighter, but the dark hue would never disappear. Davis also concluded correctly that “though any favourable sport occurring once, and never again, except by inheritance, will effect scarcely any change in a race, yet that sport, arising independently in different generations, though never more than once in any one generation, may effect a very consid
erable change.”
In spite of Jenkin’s mathematical error, his general criticism was correct: On the supposition of blending inheritance, even under the most favorable conditions, a black cat occurring once could not turn an entire population of white cats black, no matter how advantageous the black color might have been.
Before we scrutinize the question of how Darwin could have missed this seemingly fatal shortcoming of his theory of natural selection, it would be helpful to understand the blending theory of heredity from the perspective of modern genetics.