Lecture 16
Back Home Next

 

All living things evolved from a common ancestor
The theory of Natural Selection: Lecture Outline 16

Cast of Characters
Malthus, Thomas Robert (1766 - 1834)
George Cuvier (1769 - 1832)
Jean Baptist Lamarck (1744 - 1829)
Robert Fitz-Roy (1805 - 1865)
Charles Darwin (1809 - 1882)
Alfred Russel Wallace (1823 - 1913)

Malthus, Thomas Robert (1766 - 1834)

English economist, sociologist, and pioneer in modern population study. In "An Essay on the Principle of Population" (1798; 1803) he contended that poverty and distress are unavoidable because population increases faster than the means of subsistence.

George Cuvier (1769 - 1832)

Comparative anatomist; catastrophist. Rejected evolution; believed in successive special creations. (Note this is pre-Darwin).

Jean Baptiste Lamarck (1744 - 1829)

Late eighteenth-early nineteenth century evolutionist. Although known for his belief in the inheritance of acquired characteristics, he was the first evolutionist.

Robert FitzRoy (1805 - 1865)

Robert FitzRoy was the 23 year old commander of the H.M.S. Beagle. He hoped the naturalist on board his ship would find evidence to substantiate the biblical, Genesis account of the flood and the first appearance of all created things on the earth. (Britain's Archbishop Ussher calculated that the world was created on a Sunday morning at 9:00 AM, October 25, 4004 BC.) 22 year old Charles Darwin, "a clergyman-to-be, amateur naturalist", seemed to be the right man for this task.

Charles Darwin (1809 - 1882)
     A) Voyage of the Beagle - 1831-1836.

Charles Darwin was the son and grandson of successful doctors. His grandfather, Erasmus Darwin, was an early evolutionist. Darwin was sent to the University of Edinburgh to study medicine, however, after two years he dropped out and transferred to Cambridge to prepare for a life as a country parson. There he was greatly influenced by Henslow, a professor of botany, and Sedgwick, a professor of geology. Henslow recommend him to Captain Fitz-Roy to be the "unpaid naturalist" on the British naval vessel Beagle going to chart the coast of South America. The voyage took five years to complete from 1831 to 1836. He began the voyage believing in the "fixity of species". 

     B) The theory of Natural Selection.

Darwin made many observations which later led to his acceptance of evolution. His observations on the Galapagos islands were especially significant. There he saw the evidence of change in isolated populations. Turtles, mocking birds and finches were unique to each island even though they were obviously derived from the same mainland species. He saw first hand that environments different from the mainland had resulted in modified turtles and birds. However, the idea of evolution by natural selection did not mature until after his return. The ornithologist, John Gould, pointed out to Darwin that the mockingbirds which Darwin had collected from different Galapagos Islands were so distinct from one island to another as to represent different species. This revelation seems to have led Darwin to doubt the fixity of species and to set about gathering evidence on the "transmutation of species". The theory of Natural Selection crystallized on September 28, 1838, 2 years after he returned from the voyage of the Beagle. He recounts in his autobiography that, " I happened to read for amusement Malthus on Population, and being well prepared to appreciate the struggle for existence which everywhere goes from long-continued observation of the habits of animals and plants, it at once struck me that under these circumstances favourable variations would tend to be preserved and unfavourable ones destroyed.".

He wrote up his theory in an "abstract" in 1844 which he circulated among a special group but the manuscript was set aside with instructions to his wife that it be published posthumously. He was apparently aware of the impact it would have and he was unwilling to face the controversy. In a letter to his wife he requests that "in case of my sudden death .... you will devote 400 [Pounds] to its publication, and further, will yourself, or through Hensleigh (Wedgwood), take the trouble in promoting it. .... With respect to editors, Mr. (Charles) Lyell would be the best if he would undertake it. I believe he would find the work pleasant and he would learn some facts from it. Dr. (Joseph Dalton) Hooker would also be very good."

Twenty years pass before Darwin goes public in 1858 in a paper presented to the Linnean Society of London and in 1859 with the publication of "The Origin of Species". "The Origin" contains two principle ideas:

     (1) All organisms descend with modifications from common ancestors. (This was not a new idea and is not theory but a fact.)
     (2) The chief agent of modification is the action of natural selection on individual variation. (This was the theoretical insight and new.)

He was forced to publish his theory when he received a manuscript from Alfred Russel Wallace, a British naturalist, proposing the same theory (see below). At the urging of his friends, Charles Lyell and Joseph Hooker, he prepared an extract from his 1844 manuscript which was presented together with Wallace’s report, before the Linnean Society on July 1, 1858.

(1) "On the Tendencies of Species to form Varieties". (Wallace)
(2) "Perpetuation of Varieties and Species by Natural Means of Selection". (Darwin)

In 1859 he published "The Origin of Species by Means of Natural Selection or The Preservation of Favoured Races in the Struggle for life." The book sold out "its first printing of 1,250 copies in one day and set in motion a controversy that has still not entirely subsided" (Futuyma).

Alfred Russel Wallace (1823 - 1913)

On June 18, 1858, Darwin received a manuscript in the mail from a collector working in Malaysia. In February 1858 Alfred Russel Wallace was ill on a small volcanic island in the Moluccas, the Spice Islands between New Guinea and Borneo. He had a fever and during the night got an insight by recalling Malthus' book on Populations, the same book that Darwin credits with giving him the his insight for Natural Selection. He writes: "It occurred to me to ask the question, Why do some die and some live? And the answer was clearly, that on the whole the best fitted lived. ........ Then I at once saw, that the ever present variability of all living things would furnish that material from which, by mere weeding out of those less adapted to the actual conditions, the fittest alone would continue the race. There suddenly flashed upon me the idea of the survival of the fittest."

Darwin's reaction: "I never saw a more striking coincidence; if Wallace had my MS sketch written out in 1844, he could not have made a better short abstract."

The Modern Synthesis

Darwin marshaled evidence from all relevant sources of information: (1) The fossil record. (2) Geographic distribution of species. (3) Comparative Anatomy and Embryology and (4) Modifications of domesticated organisms (i.e. artificial selection). His deductive reasoning in arriving at his theory of evolution by natural selection can be summarized as "three facts and two deductions":

Fact 1. All organisms tend to increase in geometric proportion.
Fact 2. In spite of this tendency, the numbers of individuals of a given species remain more or less constant.
Deduction 1. Thus there is a struggle for existence.
Fact 3. All organisms vary considerably.
Deduction 2. Since there is a struggle for existence and since not all organisms are alike, more of those with favorable variations will survive and produce more offspring than those with unfavorable variations (i.e. survival of the fittest).

After the rediscovery of Mendelian genetics at the beginning of the twentieth century, a fourth fact and third deduction were added. The incorporation of modern genetics in the theory of natural selection is known as the Modern Synthesis.

Fact 4. Some variation is inherited and will be available for transmission to later generations.
Deduction 3. Therefore, there will be a differential transmission of inherited variation.