In an earlier post in this series, I listed the three stages involved in natural selection, each of which seemed to have seemingly small probabilities. In the previous post, I showed how because of the large numbers of organisms and long time scales involved, the first item got converted into a very high probability event.

The next item in the list, the issue of how a mutation with a small advantage in the properties of an organism can end up with that property dominating the species, was both Darwin’s greatest challenge and his greatest triumph.

The triumph came from a crucial insight that Darwin had concerning the importance of varieties within species. Recall that Platonic ideas were dominant at that time, and that laid the emphasis on the idealized forms of things. So for example while a real triangle drawn on paper would contain imperfections, these were considered incidental, the drawing being a mere approximation to the idealized triangle that one could envision in some abstract space.

In Darwin’s time, the biological equivalent of this thinking was that while it was plain to see that (say) chickens were different from one another in small ways, these differences were not considered important. They were considered mere approximations to an idealized chicken that represented the essence of chickenhood (so to speak), and it was the latter that was important.

But Darwin realized that the variety that he saw in species, rather than distracting from an understanding of the ideal, was important in its own right. In fact, he recognized that the diversity within a species was so vast that it was often hard to say what was a variety within a species and what was a different species altogether. As he wrote, “[I]t will be seen that I look at the term species, as one arbitrarily given for the sake of convenience to a set of individuals closely resembling each other, and that it does not essentially differ from the term variety, which is given to less distinct and more fluctuating forms” (Charles Darwin, On the Origin of Species (1859) p. 52). It was this wide variety that allowed some animals to survive better than others and was the driver of natural selection. The existence of variety lay at the heart of his theory.

(The problem with all classification schemes is that it is often impossible to specify both necessary and sufficient conditions to make unerring judgments as to which category some organism belongs. The fact that there is often no sharp line that can be drawn between varieties within species and differences between species should put to rest the artificial distinction made by intelligent design creationists who say they have no trouble with what they call ‘microevolution’ (what they define as change within a species) but cannot accept ‘macroevolution’ (the creation of new species). This is a distinction without much merit.)

But Darwin faced a serious problem. Even though people might accept his idea that one variety of an species might be more suited to survive than another, the lack of a real understanding at that time of the mechanism of inheritance worked against him. It was believed that sexual reproduction resulted in features being mixed (called ‘blending inheritance‘), with the child of parents being intermediate in terms of properties such as height, skin color, etc. Hence even if an occasional particular mutation had better chances for survival, it was believed that its advantageous properties would soon get diluted and disappear by mating with those animals that did not have this same property. This is the well-known phenomenon of ‘regression towards the mean,’ first articulated by the polymath Francis Galton, a cousin of Darwin.

In artificial breeding one could avoid this blending outcome by simply restricting the breeding of animals to those organisms with the desired properties and thus preserve and enhance desired changes. But in the wild, organisms would mate more indiscriminately and this raised the question of how advantageous mutations could be preserved.

Around the same time that Darwin’s theory was already reeling from estimates of a short age of the Earth from William Thomson (aka Lord Kelvin), Fleeming Jenkins wrote a long article in 1867 criticizing Darwin’s theory on precisely the blending inheritance issue. In addition, the co-discoverer of natural selection Alfred Wallace (who had initially been seen as an even more zealous advocate of natural selection than Darwin) had become interested in spiritualism and in 1869 unexpectedly published a paper in which he asserted that natural selection, although it could explain everything else, couldn’t account for the human brain, and he even went so far as to espouse an early version of intelligent design creationism saying that while the living world is governed by laws, “an Overruling Intelligence has watched over the action of those laws, so directing variations and so determining their accumulation” in order to produce the wonderful thing that is the human brain (David Quammen, The Reluctant Mr. Darwin (2006), p. 215). (The idea that the workings of the human brain, and that the mind and consciousness lie outside the realm of natural selection and the laws of biology, is something that persists down to this day, a topic I will examine in the future when I look at what we are now learning about the nature of consciousness.)

There was nothing much that Darwin could do about Thomson’s criticism but hope that someone else would prove the physicist wrong, which did happen with the discovery of radioactivity in the first decade of the 1900s. There was also nothing that Darwin could do about Wallace going against one of the fundamental precepts of natural selection, although Darwin felt that the whole idea of natural selection was meaningless if an outside ‘intelligence’ could drive organisms towards a pre-ordained result. Darwin simply wrote “No!!!” in the margins of Wallace’s paper.

As for Jenkins’s criticisms, Darwin had not been unaware that this would be a problem for his theory and had tried to anticipate it by suggesting that successful mutations would take hold in only those cases where the mutations appeared concurrently in numerous individuals and that these would then breed with each other, allowing that variety to grow and take root in the population. (Quammen p. 212)

Darwin’s defense was not very persuasive but it was all he had. Although the real defense against Jenkins’s critique was already at hand in the form of Mendel’s theory of genetics (which showed that genes are discrete entities that remain intact on breeding and do not get blended away), Mendel’s work was not widely known at that time and Darwin’s theory had to wait until its rediscovery in 1900 to fully overcome objections of the type put forward by Jenkins.

Darwin the man and the scientist are fascinating character studies. He was painstakingly thorough in his work and conscientious about the need to amass evidence to buttress the main argument he was making. But once he felt convinced by the evidence that the theory of natural selection was sound, he was determined. While he was willing to give ground on the periphery of his theory, he was firm in his commitment to its core ideas, and one of these was that his theory would make no sense if you allowed an outside agency (an ‘intelligence’ or whatever name you gave to a god-like power) to intervene in the process at any time in any way. He was a methodological naturalist, a necessary condition for any good scientist.

But it is a very thin line that separates methodological naturalism from philosophical naturalism (or atheism) and this, at heart, which is why Darwin’s theory is so subversive to beliefs about god.

Next in this series: The debate over natural selection in Darwin’s own time

POST SCRIPT: Science? Evidence? Who cares?

In congressional testimony this week, outgoing US Surgeon-General Richard Carmona spoke of how in the Bush administration, ideology trumped science every time, with constant political interference muzzling him on scientific issues like embryonic stem cell research. He said, “Anything that doesn’t fit into the political appointees’ ideological, theological or political agenda is ignored, marginalized or simply buried.”

His testimony reminded me of this Tom Tomorrow cartoon from February 27, 2007.

Meanwhile, Secretary of Homeland Security Michael Chertoff’s extraordinary statement, also this week, that he felt ‘in his gut’ that a terrorist attack might occur in the US this summer reminded me of astronomer Carl Sagan’s reply when an interviewer pressed him for his ‘gut feeling’ as to whether there was life elsewhere in the universe. Sagan replied, “But I try not to think with my gut. Really, it’s okay to reserve judgment until the evidence is in.” (Richard Dawkins, The God Delusion, p. 47.)

Chertoff would be well-advised to follow Sagan’s advice.