Jeff's Lunchbreak

This entry is part of a series. For a bit of an introduction and an index of all entries in the series, go here.

This installment covers Chapter 15, Chromosomes and Genes.

Darwin used artificial selection (breeding) as an analogy to natural selection. Here, McCann tried to cast doubt on that analogy.

Artificial selection selects exceptional, most widely divergent characters which appear only in a few individuals, whereas natural selection is a selection of slight differences appearing simultaneously in many individuals. Artificial selection often leads to morbid or exaggerated development, to a sickly disposition, to an undermining of the whole constitution, whereas natural selection effects no injury to the whole constitution but on the contrary strengthens and betters it. Artificial selection results in lack of stability. Natural selection remains constant.

"In the light of this truth," says Morgan ("Evolution and Adaptation," 1903) "the relation between the two selective theories may appear quite different from the interpretation that Darwin gives it." (McCann 195)

Artificial selection is still a very good analogy to natural selection. In fact, they're pretty much the same thing. It's just that the selection pressures can so heavily favor single traits in artificial selection, or become extremely relaxed for traits that domestic animals no longer need. For example, let's look at modern chickens. They're extremely exaggerated compared to wild chickens. They're little more than meat factories with the bare minimum of life support to grow that meat (more info). But selection explains exactly why they got that way. Farmers chose the most muscle bound chickens to breed, so they created the most offspring. Scrawnier chickens were sent straight to the slaughterhouse without a chance to make any baby chicks. Farmers have just become a new selection pressure compared to what wild chickens see.

I don't want to criticize McCann too much for misunderstanding genetics, since it was still a fairly young science in his time, but here is a perfect case of what the modern synthesis would bring to the theory of evolution.

Here the biometrician and the Mendelist part company. The biometrician says: "Selection is the process of accumulating infinitesimal differences through gigantic periods of time." The Mendelist says: "Selection is a process of combining and sorting out genes." The biometrician says: "Selection is creative, actually producing new characters." The Mendelist says: "Selection merely assorts, and such effects of variation as are sometimes said to be found are merely due to new combinations of characters that were already present." (McCann 196)

When McCann quotes an actual scientist immediately following the above paragraph, he can make a good point.

De Vries himself says ("Darwin and Modern Science," p. 70): "Natural selection acts as a sieve; it does not single out the best variations but it simply destroys the larger number of those which are from some cause or another, unfit for their present environment. In this way it keeps the strains up to the required standard, and in special circumstances may even improve them." (McCann 197)

We now know that both points of view presented above are right. Random mutation is the ultimate source of the genetic variation in a population, and mutation occurs all the time. Current estimates for human mutation rates are around 100 mutations per individual. Obviously, most of those are neutral, but some will be advantageous or deleterious, and even that will depend on environment.

So, thanks to mutation, you have populations of organisms with quite a bit of genetic diversity. Natural selection acts on that existing diversity - it doesn't create new characters. Go back to my hypothetical population from a previous entry, where an environmental change favors animals that can browse from the tops of trees. Natural selection never creates an individual with a longer neck. Random mutations create animals with longer and shorter necks than their parents, and natural selection describes how those animals with the longer necks survive and reproduce more. After many generations, the population will be composed of animals with much longer necks than the founding population, but it was random mutation that created the longer necks in each generation.

To anyone who's spent any time following creationism, the argument that genetic mutation can only cause the loss of information, and not create anything new, is a familiar argument. After reading McCann, I now realize that this argument is nearly as old as genetics itself.

But science is not looking for losses along the path of evolution. On the contrary science insists she is looking for gains, additions.

It is for this reason that so many scientists are reluctant to admit that characters which look like additions in domesticated or cultivated forms are really due to the LOSS of something which in the past has prevented the appearance of the hidden factor. (McCann 197-198)

I'll once again use Richard Lenski's experiment, where e. coli developed mutations that gave them the ability to digest a new food source (citrate), as an example of how random mutations can result in new functions. I'll also link to an article by Richard Dawkins, The Information Challenge, which explains the processes of how information can be added to the genome.

With the way so many people today use the term 'Darwinism' synonymously with evolution, it's interesting to see it used in a slightly different (and probably more accurate) context.

"Klebs, the eminent plant physiologist," says Kellogg, "keenly criticizes the mutation theory. Copeland finds in the mutations of De Vries nothing radically different either in character or behavior from the Darwinian fluctuating variations." (See "Darwinism Today," 1907, pp. 372-373). Having abandoned Darwin and come to De Vries, there would thus seem to be a desire to return to Darwin. On this point Kellogg is clear and emphatic. He says, under the caption, "The Deathbed of Darwinism," in the introduction to "Darwinism Today," 1907: "... Numerous books and papers are appearing now in such numbers and from such a variety of reputable sources as to reveal the existence among biologists and philosophers of a widespread belief in the marked weakening, at least, if not serious indisposition of Darwinism. A few of these books and papers from scientific sources even suggest that their writers see shadows of a deathbed. (McCann 199)

So, Darwinism here isn't simply evolution. It's contrasting Darwin's evolutionary theories with De Vries' evolutionary theories.

With how much we take for granted our knowledge of genetics today, it's interesting to see this.

The remarkable fact has now been established that every species of plant or animal has a fixed and characteristic number of chromosomes. In many of the lower animals the number of chromosomes to the cell has been determined positively. With respect to man the number is now thought to be twenty-four. Wieman (1917) asserts that the number in both negro and white spermatogonia is twenty-four, thereby agreeing with Duesberg's (1906) count. (McCann 199-200)

The vast majority of people have 24 chromosomes. However, the number's not exactly fixed. Mutations involving fusions are so common that they have their own name, Robertsonian Translocations. Those people only have 23 chromosomes, and they get along just fine, though their children carry a higher risk of Down Syndrome and other similar disorders.

If only McCann could have actually seen the research he was wondering about.

At this writing, 1921, no scientist may foretell what a contrasted examination of the chromosomes of the chimpanzee, gorilla, orang, gibbon and man will reveal, yet the old dogmatic certitude of the evolutionists, who have heeded none of the bewildering complexities involved in this study, persists, as if it were indeed a thing upon which the freakish Trinil Ape-Man, Piltdown Ape-Man and Neanderthal Ape-Man might look with profound contempt. (McCann 201)

There's a very good comment in Pharyngula, that I often link to in these discussions (I have a local copy on this site it's so good). It discusses the very thing McCann mentioned - "a contrasted examination of the chromosomes of the chimpanzee, gorilla, orang, gibbon and man". We're related. Chimpanzess and bonobos are the most closely related of the great apes, and are next most closely related to humans. Our DNA is more similar to a chimp or bonobo than is that of a gorilla or orangutan.

We do have a different number of chromosomes than the other great apes, but when you look at our chromosome 2, you find telomeres and centromeres exactly where you'd expect if two chromosomes had fused together in one of our ancestors. And when you compare our chromosome 2 side by side with chromosomes 2p and 2q from the other great apes, you see marked similarities.

There's no doubt that all of us great apes are related.

Proceed to Chapter 16