Brilliant Blunders: From Darwin to Einstein - Colossal Mistakes by Great Scientists That Changed Our Understanding of Life and the Universe (7 page)

The modern theory of genetics originated from the mind of an unlikely explorer: a nineteenth-century Moravian priest named Gregor Mendel. He performed a series of seemingly simple experiments in which he cross-pollinated thousands of pea plants that produce only green seeds with plants that produce only yellow seeds. To his surprise, the first offspring generation had only yellow seeds. The next generation, however, had a 3:1 ratio of yellow to green seeds. From these puzzling results, Mendel was able to distill a
particulate,
or
atomistic,
theory of heredity. In categorical contrast to blending, Mendel’s theory states that genes (which he called “factors”) are discrete entities that are not only preserved during development but also passed on
absolutely unchanged
to the next generation. Mendel further added that every offspring inherits one such gene (“factor”) from each parent, and that a given characteristic may not manifest itself in an offspring but can still be passed on to the following generations. These deductions, like Mendel’s experiments themselves, were nothing short of brilliant. Nobody had reached similar conclusions in almost ten thousand years of agriculture. Mendel’s results at once disposed of the notion of blending, since already in the very first offspring generation, all the seeds were not an average of the two parents.

A simple example will help to clarify the key differences between Mendelian and blending heredity, in terms of their effects on natural selection. Even though blending inheritance clearly never used the concept of genes, we can still employ this language while preserving the essence of the process of blending. Imagine that organisms that carry a particular gene
A
are black, while the bearers of gene
a
are white. We will start with two individuals, one black and one white, each one having two copies of the respective gene (as in
figure 4
). If no gene dominates over the other, then in both blending heredity and Mendelian heredity, the offspring from such a couple would be gray, since they would have the gene combination (or
genotype
)
Aa.
Now, however, comes the key difference. In the blending theory, the
A
and the
a
would physically blend to create a new type of gene that gives its carrier the color gray. We can call this new gene
A
⁽¹⁾
. Such
blending would not occur in Mendelian heredity, where each gene would keep its identity. As
figure 4
shows, in the grandchildren’s generation, all the offspring would be gray under blending heredity, while they could be black (
AA
), white (
aa
), or gray (
Aa
) under Mendelian heredity. In other words, Mendelian genetics pass down extreme genetic types from one generation to the next, thereby efficiently maintaining genetic variation. In blending heredity, on the other hand, variation is inevitably lost, as all the extreme types vanish rapidly into some intermediate mean. As Jenkin observed correctly, and the following (highly simplified) example will clearly demonstrate, this feature of blending heredity was catastrophic for Darwin’s ideas on natural selection.

Figure 4

Imagine that we start with a population of ten individuals. Nine have the gene combination
aa
(and are therefore white), and one has the combination
Aa
(say, by some mutation), which renders it gray. Suppose further that being black is advantageous in terms of survival and reproduction, and that even having a somewhat darker color is
better than being entirely white (although the advantage decreases with decreasing darkness).
Figure 5
attempts to follow schematically the evolution of such a population under blending heredity. In the first generation, the blending of
A
with
a
will produce the new “gene”
A
⁽¹⁾
, which, when mating with
aa
will yield
A
⁽¹⁾
a
, which will blend again to produce the gene
A
⁽²⁾
, corresponding to an even lighter and less advantageous color. You can easily see that after a large number (
n
) of generations, the most that can happen is that the population will be transformed into one with the combinations
A
⁽ⁿ⁾
A
⁽ⁿ⁾
, which will be only slightly darker than the original white population. In particular, the color black will become extinct even after the first generation, since its gene will be blended out of existence.

But under Mendelian heredity (
figure 6
), since the
A
gene is preserved from one generation to the next, eventually two
Aa
’s will mate and produce the black
AA
variety. If black confers an advantage in the environment, then given enough time, natural selection could even turn the entire population black.

The conclusion is simple: For Darwin’s theory of evolution by natural selection to really work,
it needed Mendelian heredity. But in the absence of yet-undiscovered genetics, how did Darwin respond to Jenkin’s criticism?

Darwin was a genius in many ways, but he definitely was not a sharp mathematician.
In his autobiography, he acknowledged, “I attempted mathematics, and even went during the summer of 1828 with a private tutor (a very dull man) to Barmouth, but I got on very slowly. The work was repugnant to me, chiefly from my not being able to see any meaning in the early steps of algebra . . . I do not believe that I should ever have succeeded beyond a very low grade.” That being the case, arguments in
The Origin
are generally qualitative rather than quantitative, especially when it comes to the production of evolutionary change. In the few places where Darwin attempted to do simple calculations in
The Origin
, he managed occasionally to botch them.
No wonder, then, that in one of his letters to Wallace, after reading Jenkin’s rather mathematical criticism, he confessed,
“I was blind and thought that single variations might be preserved much oftener than I now see is possible or probable.” Still, it would have been amazing to think that Darwin had been totally unaware of the potential swamping effect of blending heredity until he read Jenkin’s article. And indeed he wasn’t. As early as 1842, twenty-five years before the publication of Jenkin’s review, Darwin had already observed,
“If in any country or district all animals of one species be allowed freely to cross, any small tendency in them to vary will be constantly counteracted.”
In reality, Darwin even relied to some extent on swamping to produce populational integrity in the face of the tendency of individuals to depart from their type due to variations. How did he then fail to understand how difficult it would be for a “sport” (a single variation) to fight off the averaging force of blending? Darwin’s blunder and his slowness to recognize the point raised by Jenkin probably reflected on one hand his conceptual difficulties with heredity in general, and on the other, his residual overattachment to the idea that variations had to be scarce. The latter may have partially been a consequence of his general theory of reproduction and development, in which he assumed that only developmental stress triggered variations. Darwin’s bafflement with heredity ran much deeper, as can be seen from the following inconsistency. At one point in
The Origin
, Darwin noted:

When a character which has been lost in a breed, reappears after a great number of generations, the most probable hypothesis is, not that the offspring suddenly take after an ancestor some hundred generations distant, but that in each successive generation there has been a tendency to reproduce the character in question, which at last, under unknown favourable conditions, gains an ascendancy.

This notion of some latent “tendency” departed manifestly from normal blending heredity, and in many ways it was close in spirit to Mendelian heredity. Yet it apparently did not occur to Darwin, at
least initially, to invoke this idea of latency in his struggle to respond to Jenkin. Instead, Darwin decided to change the emphasis from the role he had previously assigned to single variations to that of
individual differences
(the wide spectrum of tiny differences occurring frequently, which was supposed to be distributed continuously throughout the population), in supplying the “raw materials” for natural selection to effect. In other words, Darwin now relied on an entire continuum of variations for the production of evolution by natural selection over many generations.

In a letter to Wallace on January 22, 1869, the distressed Darwin wrote, “I have been interrupted in my regular work in preparing a new edition of the ‘Origin,’ which has cost me much labour, and which I hope I have considerably improved in two or three important points. I always thought individual differences more important than single variations, but now I have come to the conclusion that they [individual differences] are of paramount importance, and in this I believe I agree with you. Fleeming Jenkin’s arguments have convinced me.” To reflect his new emphasis, Darwin amended the fifth edition and subsequent editions of
The Origin
by changing singulars referring to individuals into plurals, as in “any variation” turning into “variations,” and “an individual” into “individual differences.” He also added a few new paragraphs in the fifth edition, two of which, in particular, are of great interest. In one, he admitted openly:

I saw, also, that the preservation in a state of nature of any occasional deviation of structure, such as a monstrosity, would be a rare event; and that, if preserved, it would generally be lost by subsequent intercrossing with ordinary individuals. Nevertheless, until reading an able and valuable article in the “North British Review” (1867), I did not appreciate how rarely single variations, whether slight or strongly marked, could be perpetrated.

In the other paragraph, Darwin presented his own brief summary of Jenkin’s swamping argument. This paragraph is fascinating
because of two apparently small yet extremely significant differences from Jenkin’s original text. First, Darwin assumes here that a pair of animals has two hundred offspring,
of which
two
survive to reproduce. In spite of his nonmathematical background, therefore, Darwin appears to have anticipated already in 1869 the correction to Jenkin pointed out in A. S. Davis’s letter to
Nature
in 1871: For the population not to disappear, two offspring, on the average, must survive. Second, and even more intriguing, Darwin assumes in his summary that only half of the offspring of the “sport” inherit the favorable variation. Note, however, that this assumption is contrary to the predictions of blending heredity! Unfortunately, Darwin was still unable at that time to elaborate on the possible consequences of a nonblending theory of heredity, and he accepted Jenkin’s conclusions without any further discussion.