Brilliant Blunders: From Darwin to Einstein - Colossal Mistakes by Great Scientists That Changed Our Understanding of Life and the Universe (6 page)

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 animated 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?

Life’s perhaps the only riddle

That we shrink from giving up!

—WILLIAM SCHWENCK GILBERT,
THE GONDOLIERS

T
he 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.

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 considerable 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.

In the context of our current understanding of genetics, the molecule known as
DNA
(
deoxyribonucleic acid
) provides the mechanism responsible for heredity in all living organisms. Very roughly speaking, DNA is made up of
genes,
which contain the information that codes for proteins, and of some noncoding regions. Physically, DNA is located on elements called
chromosomes,
of which each individual organism in sexual species has two sets, one inherited from the mother (the female) and one from the father (the male). Consequently, each individual has two sets of all of its genes, where the two copies of a gene may be identical, or slightly different. The different forms of a gene that can be present at a particular location on a chromosome are the variants referred to as alleles.