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The Darwin-Wallace Concept of Evolution by Natural Selection

As many more individuals of each species are born than can possibly survive; and as, consequently, there is a frequently recurring struggle for existence, it follows that any being, if it varies however slightly in any manner profitable to itself, under the complex and sometimes varying conditions of life, will have a better chance of surviving, and thus be naturally selected. From the strong principle of inheritance, any selected variety will tend to propagate its new and modified form.
Darwin, 1859, p. 5.

This is one way that Charles Robert Darwin (1809-1882) stated his theory of evolution by natural selection. His book was entitled On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life, and the word evolution does not occur in it. The book appeared in 1859, shortly after a joint reading at the Linnean Society of London in 1858 of papers prepared on the topic by both Darwin and Alfred Russell Wallace (1823-1913). Wallace, during his travels in the East Indies, formulated a theory of natural selection that was essentially the same as Darwin's. Wallace recorded later that the idea of survival of the fittest came to him during a period of illness in the Moluccas in February, 1858. The theory was finally thought out during a fever, written in several evenings, and sent to Darwin by the next mail. Darwin received the manuscript on June 18 and immediately recognized his own theory. In a letter he wrote the geologist Lyell that very day, he states: "I never saw a more striking co­incidence." Lyell and the botanist Hooker arranged the joint reading of Wallace's essay and one by Darwin. While we should perhaps speak of the Darwin-Wallace concept of natural selection, major recognition

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is usually given to Darwin. Wallace himself gave such recognition to Darwin. Considering Darwin's early attainment of the concept (1835—1837), his success with the "Origin" book, and his phenomenal scientific productivity in fields of scientific investigation other than evolution, the universal recognition given him seems not unwarranted.

Of historical interest is the fact that Darwin's book was published in London with a first edition of 1,250 copies, nearly all of which were bought on the first day of issue (November 24, 1859). There is good evidence that the Darwin-Wallace concept of evolution by natural selection had already permeated the scientific community before Darwin's book was published. The papers by the two evolutionists read before the Linnean Society on July 1, 1858, had been published jointly on August 20, in the Society's journal. And in October, 1859, a month before publication of the "Origin," the first acknowledged application of the new theory appeared. H. B. Tristram used it to explain the particular colors of birds adapted to desert and other environmental conditions. That this first application of their concept (appearing in The Ibis, journal of the British Ornithologists' Union) deals with adaptation in birds is indicative of a lead that ornithology took, and until recently continued to take, in the task of elucidating evolutionary processes.

In general, remarkable parallels exist for the respective experiences of Darwin and Wallace prior to reaching a comparable conclusion. For example, both traveled extensively in the tropical latitudes: Darwin in South America and the Galapagos Archipelago; Wallace in the Amazon basin and the East Indies. Each read and ultimately reacted to the "Essay on Population" by Malthus, which contained, along with the idea of geometric increase in population in a limiting environment, the expression struggle for existence. We may guess that each coupled in his mind two observations or sets of information: (1) an awareness of the past and present distributions and morphological characteristics of plants and animals suggesting nonrandom or orderly trends correlated with attributes of the environment; (2) an understanding of the array of individual variations from which the "environment selects" those better fitted for contributions of offspring to the next generation. So to speak, they placed differential reproduction and survival on a Malthusian background of organisms producing more offspring than can survive, and then developed the concept of evolution by natural selection.

It is of value here to summarize the Darwin-Wallace concept of how evolution occurs by natural selection:

1. Organisms produce far more offspring than can ever survive to become reproducing individuals.

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2. Because the numbers of individuals in species are for the most part constant, there must be a high death rate.

3. Individuals are not identical, but vary in their characteristics,

4. Those variants that naturally fit their environments will be favored over those less fit. By inheritance those individuals better fit will pass their characteristics on to the next and future generations in greater numbers than those less fit.

5. The following generations of individuals will exhibit, and continue to improve, the adaptations realized in gradual changes by their ancestors.

Although this listing of steps in evolution by natural selection approximately summarizes the statements of both Darwin and Wallace, their respective theories—as presented in 1858, and again in 1859 by Darwin—differ in points of emphasis. Darwin emphasized clearly, following Malthus, that while individuals and populations have a remarkable potential for increase in numbers, in reality they remain on the average fairly constant in numbers. He thus concluded that natural populations are kept in check "by recurrent struggles against other species or against external nature." At this point, Darwin developed his idea of survival of the fittest in the following manner: "Yearly more are bred than can survive; the smallest gain in balance, in the long run, must tell on which death must fall, and which shall survive. Let this work of selection on one hand, and death on the other, go on for a thousand generations. Who will pretend to affirm that it would produce no effect, when we remember what, in a few years [artificial selection effected in cattle and sheep] by this identical principle of selection."

In such ways Darwin concluded that advantageous, heritable variations—however small—are preserved and in turn accumulate in species. He reasoned that with time and change in local environment, or with dispersal and encounter of new environments, species depart increas­ingly from their ancestral or parental form. Today we refer to such changes as phyletic evolution. Darwin distinguished no qualitative dif­ference between artificial selection and natural selection. In contempo­rary terms, one can say that he used knowledge of results of artificial selection by man to explain evolution by natural selection.

Wallace also emphasized that although species have tremendous potential for increase in numbers, they still maintain constant or near- constant population sizes. He concluded that most natural populations have their potential for increase in numbers checked by food shortages and other limiting factors. From this, he inferred that the abundance or rareness of a species results from the capacities of its members to com­bat their limiting factors. Like Darwin, he believed that the external condition, or the environment, is usually limiting or harsh, and that this

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results in a situation wherein individuals or species compete for food, space, special habitats, mates, or other factors in limited supply.

We must disregard here the many other contributions to biology and evolution made by these two investigators. Among these are Darwin's theory of sexual selection and Wallace's studies in zoogeography. Dar­win in particular was ahead of his time in his far-reaching thought, and many of his scientific predictions were to be realized only in our century. The following two instances illustrate this point.

Major Leonard Darwin in conversation with E. B. Ford noted that his father once said that by choosing the right material it might be possible to detect evolutionary changes in natural populations within a period of fifty years.¹ Ford in his own studies, and in summarizing those of others (for example, H. B. D. Kettlewell's investigation of industrial melanism), has superbly documented this prediction by Darwin. Such examples are discussed in Chapter 4, along with the research of R. K. Selander and R. F. Johnston, demonstrating rapid ecogeographic adaptation in the house sparrow introduced into North America during the last century.

In another instance, quoted (p. 271) by Sir Gavin de Beer in 1956, Darwin wrote "It is often said that all the conditions for the first pro­duction of a living organism are now present, which could ever have been present. But if (and oh! what a big if!) we could conceive in some warm little pond, with all sorts of ammonia, and phosphoric salts, light, heat, electricity, etc., present, that a proteine compound was chemically formed ready to undergo still more complex changes, at the present day such matter would be instantly devoured or absorbed, which would not have been the case before living creatures were formed." Thus in 1871 Darwin anticipated by some eighty years the possibility of a chemical evolution of proteins. In 1951, S. L. Miller demonstrated that an electrical discharge in a nonliving environment of hydrogen, water, ammonia, and methane resulted in the production of various organic compounds, including acetic acid, formic acid, and most of the amino acids.

Darwin's and Wallace's concept of evolution by natural selection was not without errors or weaknesses. In most instances this was unavoid­able, a result of lack of knowledge of biological systems at the time of their study. To cite a specific case, in 1858 and 1859 chromosomes and genes were completely unknown. Darwin and Wallace knew that some variations were inherited simply from observing the passage of traits of parents to their offspring. But they had no convincing mechanism to ex­plain how such heritable characters were passed. Darwin at various times followed Lamarckian theory and presented concepts of pangenesis (where gemmules are thought to be given off by all parts of the body and transported by the blood stream to the germ cells) and blending

¹ Leonard Darwin (1850-1943) was Darwin's eighth child. Edmund Brisco Ford (1901-1988), ecological geneticist.

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inheritance. At that time the particulate inheritance of Mendelian genetics was yet to come (see Chapter 2). Still another criticism of Darwin's book is that, despite the "Origin" title, it had little to do with the mechanism of origin of new species from parental species. The role of geographic isolation, like the role of mutation and recombination in particulate genetic variation, was to become widely understood only in the present century.

It would be difficult to summarize the achievements of Darwin and Wallace, or the impact of their work on late nineteenth- and twentieth-century science. For the purposes of this introductory book, however, it should be noted that both men considered adaptation to environment to represent evolution by natural selection of the individual. In other words, they considered the individual to be the unit of natural selection. Their theory of natural selection assumed unequivocally that species are mutable; thus they challenged, among other ideas, the belief that species were instantaneously created. Although a few before them had taken the view that species were subject to change, Darwin and Wallace were more successful in formulation of theory and accumulation of evidence. In ways sufficiently clear and complete to attract others to their position, they altered a generation's way of thinking. Contemporary science is indebted to Darwin and Wallace for the principle of mutability of species. Concepts of natural selection and evolution follow from the idea of gradual accumulation of small, but favorable changes. Students believing or interested in the premise that the history of science and of great ideas is the history of great men can do no better than to study the many writings of these two evolutionists. The effects of Darwin's book transcended biology and entered the realms of philosophy, sociology, and political theory. Karl Marx wanted to dedicate a part of Das Kapital to Darwin, but Darwin declined the honor.

Further Reading

Barzun, J. Darwin, Marx, Wagner. Boston: Little, Brown, and Company, 1941.

Bell, P. R. Darwin's Biological Work. London: Cambridge University Press, 1959.

Carter, G. S. A Hundred Years of Evolution. New York: The Macmillan Company, 1957.

Darwin, C. The Autobiography of Charles Darwin (1877). Edited with restoration of original omissions, notes, and an appendix by Nora Barlow. New York: Harcourt, Brace and Company, 1959.
--------- . The Life and Letters of Charles Darwin. Two volumes and an autobiographical chapter edited by Francis Darwin. New York: D. Appletonand Company, 1897.