Daily Archives: 21 December 2013

December 21, 1889 (a Saturday)

Sewall Wright

Sewall Wright

On this date, the American mathematician and biologist Sewall Green Wright (he later dropped the middle name) was born. He was one of the founders, along with R. A. Fisher and J. B. S. Haldane, of modern theoretical population genetics. He researched the effects of inbreeding and crossbreeding with guinea pigs and, later on, the effects of gene action on inherited characteristics. The synthetic theory of evolution as described by Sewall Wright synthesizes (combines) the principles of natural selection outlined by Charles Darwin with the principles of genetics. Wright explained evolution in terms of changes in gene frequencies.

The classic example which supports this theory is that of the peppered moth in England. The moth can be either dark or light colored. Scientists have determined that body color in the peppered moth is controlled by a single gene with two alleles: the allele for dark body color is dominant and the allele for light body color is recessive. Prior to the industrialization of central England, the light-colored allele was most prevalent. The light-colored moths would hide on the white-barked trees and avoid bird predation. But the pollution generated by the new industries stained the light-colored trees dark. Gradually the light-colored moth was attacked and that allele became much less prevalent. In its place, the dark-colored allele became the most predominant allele because moths that carried that allele could camouflage themselves on the stained trees and avoid being eaten by their bird predators. Clearly the population had evolved to a better adaptive condition.

Wright is perhaps best known for his concept of genetic drift, formerly known as the “Sewall Wright effect.” Genetic drift results when small populations of a species are isolated and due to pure chance, the few individuals who carry certain relatively rare genes may fail to transmit them. The genes may therefore disappear and their loss may lead to the emergence of new species, although natural selection has played no part in the process. Genetic drift can be summarized as “bad luck, not bad genes.”

Wright had long been concerned with cases in which genes interacted in ways not predictable from their individual effects. He believed that evolutionary creativity often depended on putting together favorable combinations of genes that were individually deleterious. But natural selection will not ordinarily incorporate such genes in a large, sexually reproducing population. Wright’s answer was his “shifting balance theory“, which holds that the best opportunity for adaptive evolution lies in the population structure.

Wright thought that many, if not most, species were subdivided into small populations that exchanged only a few migrants with each other and thus were not completely isolated. Because of the small size of each of these populations, genetic drift would have a significant effect on the genetic composition of each, thus allowing the populations to differentiate genetically by an appreciable amount. In this way, each of the populations would act as a small experiment in evolution.

Wright’s shifting balance theory consists of three distinct phases:

  • Phase 1, the exploratory phase, is characterized by the action of genetic drift in a local population. One or more may drift into an advantageous gene combination.
  • In phase 2, a new advantageous combination of genes is naturally selected in one or more populations.
  • Finally, in phase 3, those populations will then increase or, more likely, send out migrants to adjacent local populations, introducing the advantageous gene combination of the immigrants. As a result of this process, eventually all of the populations attain the favorable gene combination.

Although Wright’s theory remains controversial, it has been very popular and influential in the biological community. It is one of the things that biologists argue over. They do not argue over whether or not evolution occurs; that evolution occurs is a biological fact.

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December 21, 1835 (a Monday)

Charles Darwin by G Richmond.

On this date, HMS Beagle arrived at New Zealand with Charles Darwin on board. He was not too impressed with the natives (Maori), whom he viewed with suspicion (they practiced cannibalism before the missions arrived).

I should think a more warlike race of inhabitants could not be found in any part of the world than the New Zealanders. Their conduct on first seeing a ship, as described by Captain Cook, strongly illustrates this: the act of throwing volleys of stones at so great and novel an object, and their defiance of “Come on shore and we will kill and eat you all,” shows uncommon boldness. This warlike spirit is evident in many of their customs, and even in their smallest actions. If a New Zealander is struck, although but in joke, the blow must be returned; and of this I saw an instance with one of our officers.

(…)

The first European impression of Māori people, by Isaac Gilsemans (the artist on Abel Tasman's voyage to New Zealand in 1642).

The first European impression of Māori people, by Isaac Gilsemans (the artist on Abel Tasman’s voyage to New Zealand in 1642).

Looking at the New Zealander, one naturally compares him with the Tahitian; both belonging to the same family of mankind. The comparison, however, tells heavily against the New Zealander. He may, perhaps, be superior in energy, but in every other respect his character is of a much lower order.

(…)

Whilst at New Zealand we did not hear of any recent acts of cannibalism; but Mr. Stokes found burnt human bones strewed round a fire-place on a small island near the anchorage; but these remains of a comfortable banquet might have been lying there for several years. It is probable that the moral state of the people will rapidly improve.

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December 21, 1767 (a Monday)

William Paley

William Paley

On this date, William Paley was ordained as an Anglican priest. He was a prolific author, but his most influential contribution to biological thought was his book Natural Theology: or, Evidences of the Existence and Attributes of the Deity, Collected from the Appearances of Nature, first published in 1802. He introduced one of the most famous metaphors in the philosophy of science, the image of the watchmaker:

. . . when we come to inspect the watch, we perceive. . . that its several parts are framed and put together for a purpose, e.g. that they are so formed and adjusted as to produce motion, and that motion so regulated as to point out the hour of the day; that if the different parts had been differently shaped from what they are, or placed after any other manner or in any other order than that in which they are placed, either no motion at all would have been carried on in the machine, or none which would have answered the use that is now served by it. . . . the inference we think is inevitable, that the watch must have had a maker – that there must have existed, at some time and at some place or other, an artificer or artificers who formed it for the purpose which we find it actually to answer, who comprehended its construction and designed its use.

Even Charles Darwin commented on Paley in his Autobiography:

In order to pass the B.A. examination, it was, also, necessary to get up Paley’s Evidences of Christianity, and his Moral Philosophy… The logic of this book and as I may add of his Natural Theology gave me as much delight as did Euclid. The careful study of these works, without attempting to learn any part by rote, was the only part of the Academical Course which, as I then felt and as I still believe, was of the least use to me in the education of my mind. I did not at that time trouble myself about Paley’s premises; and taking these on trust I was charmed and convinced of the long line of argumentation.

Natural theology had dominated English thinking for nearly two centuries, Paley’s arguments going back to authors such as John Ray, and have had a long intellectual history, surviving to the present day in many a piece of creationist rhetoric. Although totally discredited in modern science, natural theology was important scientifically because it guided researchers to the fundamental question of how life works. Even today, when scientists discover a new kind of organ or protein, they try to figure out its function.