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Evolutionary Restraints

THE UNIVERSITY OF CHICAGO PRESS

Chapter One

Debates over the nature and power of Darwin's primary mechanism of evolutionary change, natural selection, began with the publication of On the Origin of Species and continue into the present. In the sixth chapter of the first edition, Darwin addressed various difficulties his theory faced. These challenges included an oblique reference to the idea that is the subject of this story—the notion that natural selection may act at a level above that of the individual. As the study of biology has developed in the twentieth century, applications of natural selection to levels below the individual, particularly the level of the gene, have become increasingly common and comparatively unproblematic. Applications of Darwin's mechanism in the opposite direction were initially accepted as unexamined claims about the "good of the species," but as these genetic explanations became more prevalent, the higher-level explanations were increasingly seen as suspect.

Sterile Hybrids and Neuter Castes

The idea that natural selection acted at a level above that of the individual was a challenge to Darwin's theory from the beginning. Darwin himself recognized the difficulty of explaining the existence of the neuter castes of social insects by individual selection, as well as the fact of hybrid sterility. By the end of the century, the neuter insects became the locus of an important debate over the inheritance of acquired characteristics between the neo-Darwinians, represented by August Weismann, and the neo-Lamarckians, led by Herbert Spencer. Darwin's own explanation, as presented below, was based on natural selection acting on traits that were beneficial to the colony. With regard to hybrid sterility, Darwin acknowledged that sterility could be of no possible benefit to the hybrid individual and so must be incidental to other acquired differences. The benefit of the sterility of hybrids accrues not to the individual, according to Darwin, but rather to the species whose integrity is maintained (see fig. 1).

Despite the conceptual ambiguity presented here, historians of science have let this potentially fertile problem lie fallow. The only explicit historical treatment of group selection in Darwin's work was a 19 0 article in the Annals of Science by philosopher of biology Michael Ruse. His "Charles Darwin and Group Selection" was written in the midst of the sociobiology debate of the mid-1970s and early 19 0s. Ruse argued that despite the claims of sociobiology's detractors, Darwin was not sympathetic to the idea of group selection except in a very few particular cases. Ruse achieved this narrow interpretation of Darwin by using modern definitions of individual and group selection that do not apply in the context of the nineteenth century. Individual selection, according to Ruse, is "selection which in some sense affects an individual's reproductive interests. This could be directly through the individual, or indirectly in some way: For instance, by kin selection, where an individual's interests are furthered through close relatives."

The inclusion of kin selection here is dubious. Although Darwin was certainly aware that leaving progeny was important to the struggle for existence, kin selection in the modern sense quantifies relatedness and the benefits of aiding kin in a way that was not possible before the development of classical genetics. Furthermore, it is formally selection between groups of kin.

I believe that Ruse's restrictive reading of Darwin is off the mark. This is evident from various passages from both the Origin and the Descent that more accurately present Darwin's own position with regard to selection acting at a level above the individual. The following often-quoted passage, from chapter 3 of the Origin, titled "The Struggle for Existence," illustrates the breadth of action that Darwin assigned to the mechanism of natural selection: "I should premise that I use the term struggle for existence in a large and metaphorical sense, including dependence of one being on another and including (which is more important) not only the life of the individual, but success in leaving progeny."

Later in the Origin, Darwin's statements on higher-level selection dealt mostly with the social insects. Darwin recognized the difficulty that the neuter insects, with their distinct morphology and habits, presented for his theory, and in typical Darwinian style he did his best to explain and defuse this potentially devastating case.

How the workers have been rendered sterile is a difficulty; but not much greater than that of any other striking modification of structure; for it can be shown that some insects and other articulate animals in a state of nature occasionally become sterile; and if such insects had been social and it had been profitable to the community that a number should have been annually born capable of work, but incapable of procreation, I can see no very great difficulty in this being effected by natural selection.

The passage above illustrates Darwin's commitment to the mechanism of selection despite the lack of a clear theory of heredity. The question of the evolution of the neuter insects became fundamental to the ongoing debate over the inheritance of acquired characteristics. Both Lamarckian supporters of use inheritance and neo-Darwinian selectionists used the case to show the insufficiency of the other side's theory. I offer the following lengthy quotation from the Origin to demonstrate Darwin's position with regard to the evolution of various castes among the social insects.

I believe that natural selection, by acting on the fertile parents, could form a species which should regularly produce neuters, either all of large size with one form of jaw, or all of small size having jaws of widely different structure; or lastly, and this is our climax of difficulty, one set of workers of one size and structure, and simultaneously another set of workers of a different size and structure; a graduated series having been first formed, as in the case of the driver ant, and then the extreme forms, from being the most useful to the community, having been produced in greater and greater numbers through the natural selection of the parents which generated them; until none with an intermediate structure were produced.

Thus as I believe, the wonderful fact of two distinctly defined castes of sterile workers existing in the same nest, both widely different from each other and from their parents, has originated. We can see how useful their production may have been to a social community of insects, on the same principle that the division of labour is useful to civilised man.

Even though these passages come from the chapter in the Origin titled "Instinct," there is no explicit reference to the inheritance of instinct. Rather, Darwin described the morphological traits of the neuter castes and explained them in terms of their usefulness to the community. In an excellent recent review of this problem, historian Thomas Dixon has argued that Darwin's "community selection explanation of the evolution of well adapted neuter insects provided one important example of a case that could not be explained by Lamarckian inheritance of modifications produced by use and disuse." Dixon's analysis, consistent with the account I develop here, challenges the characterization of Darwin by scholars such as Helena Cronin, who present "those twentieth-century biologists who invoked group selection as departing from 'the individual-level orthodoxy of Darwin, Wallace and their contemporaries.'" In the case of the social insects, however, instinct was clearly recognized as an important factor in the evolution of the social systems. This idea was more carefully developed in The Descent of Man, which I will discuss below, but the following quotation gives some indication of Darwin's position in the Origin with regard to instinct in the social insects: "Thus, I believe it has been with social insects: a slight modification of structure, or instinct correlated with the sterile conditions of certain members of the community has been advantageous to the community: consequently the fertile males and females of the same community flourished, and transmitted to their fertile offspring a tendency to produce sterile members having the same modification."

It is clear from a careful reading of the selections from Darwin's work that he indeed conceived of the mechanism of natural selection as functioning at the level of the community. This is especially clear in the case of the social insects but was also part of Darwin's theory with regard to communal organisms such as the Portuguese man-of-war and the coral polyp and also true of higher social animals.

The final passage from the Origin comes from chapter 6, "Difficulties on Theory." In this chapter Darwin discussed various phenomena that he recognized as potentially contradictory to his theory. Through the broad application of the mechanism of natural selection—that is to say, application to the group rather than the individual—Darwin's theory can encompass even the most self-destructive of "adaptations." "We can perhaps understand how it is that the use of the sting should so often cause the insect's own death: for if on the whole the power of stinging be useful to the community, it will fulfill all the requirements of natural selection, though it may cause the death of some few members."

Social Insects and Social Instincts

In The Descent of Man, we see Darwin shifting emphasis from the social insects to the social instincts. Generally he continued to use social insects for the model of the evolution of social instincts, but he also included the social behavior of primates and other higher animals. This shift in emphasis acted as an accelerant to the debates generated by Darwinian theory. In The Descent of Man, Darwin was explicitly drawing the connection between the moral faculties of man and the social instincts of the lower animals.

Darwin's most straightforward presentation of the evolution of the social instincts came in chapter 3, "Moral Sense." In this passage Darwin argued that the inheritance of the social instincts was of the utmost importance to the later development of the human society and, furthermore, that the development of these instincts was for the good of the community over and above the advantage to the individual. "Finally, the social instincts which no doubt were acquired by man, as by the lower animals, for the good of the community, will from the first have given him some wish to aid his fellows, and some feeling of sympathy."

Here we see Darwin's explanation of the human need to offer aid to another in terms of the selective benefit this behavior confers on the community, in the same way that the existence of the sterile caste maintains the selective advantage of the hive. In the next chapter, "On the Manner of Development of Man from Some Lower Form," Darwin pointed out that in the case of the social animals, selection acting at the level of the community could have indirect effects on individuals: "With strictly social animals, natural selection sometimes acts indirectly on the individual, through the preservation of variations which are beneficial only to the community. A community including a large number of well-endowed individuals increases in number and is victorious over other and less well-endowed communities; although each separate member may gain no advantage over the other members of the same community."

Darwin went on to illustrate the point with the example of the social insects, describing pollen-collecting behavior and the sting of worker bees in addition to the jaws of the soldier ants. According to Darwin's argument, these apparatuses and behaviors were of no direct advantage to the individual; rather, they served the community and were maintained by natural selection acting on the level of the community.

The final passage from the Descent that I will include here illustrates the importance Darwin assigned to the social instincts. "All this implies some degree of sympathy, fidelity and courage. Such social qualities, the paramount importance of which to the lower animals is disputed by no one, were no doubt acquired by the progenitors of man in a similar manner, namely, through natural selection, aided by inherited habit."

It follows that if these instincts are as important to the evolution of social groups as Darwin insisted, and if the selection of these instincts often occurs at a level above that of the individual, then higher-level selection is an important factor in Darwinian evolutionary theory. In this way Darwin explicitly equated selection among communities with natural selection—a very different stance from that of later authors, like George C. Williams, who equate only individual selection with natural selection (as I will discuss in chapter 6). Despite Ruse's claim to the contrary, it is apparent that for Darwin there was at least some possibility for his mechanism to function above the level of the individual. At best, Ruse is using a modern definition of kin selection retroactively in evaluating the past. As worst, he is misdefining kin selection in order to "rescue" Darwin from the "error" of group selection.

Darwin's ambiguity on this question of levels at which the mechanism might work contributed to an intellectual environment where claims about "the good of the species" or "the benefit of the community" were accepted unexamined. From the end of the nineteenth century through the first decades of the twentieth, biologists' characterization of this adaptation or that behavior, which had clearly evolved for the benefit of the species or community or group, were generally accepted.

Early Context: The Darwinian Revolution?

Despite a generation of scholarship dedicated to examining the state of Darwinian theory at the end of the nineteenth century, there remained deep disagreements on some very basic points. To what extent was Darwin's work an echo of the Victorian capitalist ethos? What was the actual influence of Malthus on Darwin? What is the meaning of evolution? Was Darwinian theory progressive? These questions, among many others, have fueled the Darwin industry for decades.

Peter Bowler's The Non-Darwinian Revolution: Reinterpreting a Historical Myth provides an excellent analysis of the complexity of the situation with regard to evolutionary theory in the period under consideration. Bowler has spent his career examining the varied evolutionary ideas that were being espoused throughout the community of evolutionary thinkers. Bowler's point (and one that is well taken) is that most evolutionists at the end of the nineteenth century and the beginning of the twentieth were not Darwinians, often despite their claims to the contrary.

In The Non-Darwinian Revolution, Bowler described the continuing influence of Lamarck's ideas about the importance of use and disuse, Ernst Haeckel's idea of recapitulation—which was closely linked to the idealist and transcendentalist origins of the developmental view of nature—and other scientific and philosophical concepts to illustrate the intellectual confusion that reigned during this period. Bowler also dedicated a chapter to social Darwinism, which he argued was often closer to social Lamarckism. (Bowler argued that if social thinkers wanted people to strive to get ahead, Darwinian theory gave them no grounds to make any effort. Either they had the advantageous traits or they did not. The Lamarckian notion of inheritance of acquired characteristics made quite a bit more sense.) Hence the ultra-social Darwinist Herbert Spencer, as classically presented in Richard Hofstadter's Social Darwinism in American Thought, is a misconception. According to Bowler, Spencer's ideas about evolution's improving society were often more Lamarckian than Darwinian.

By 1909, the centennial of Darwin's birth and fifty years after the Origin, almost everyone agreed that life evolved, but there was no such agreement on a mechanism. There remained the unsolved problems of variation and heredity, and further complicating matters, physicists' and geologists' estimates of the age of the earth were not providing sufficient time for Darwin's gradualistic account. Even the concept of species was still vague, which created problems for evolutionary theory at the most fundamental level. Meanwhile, new fields in the life sciences arose as older areas of study were modified or abandoned. Weismann and Hugo de Vries, although they themselves did not do research in the field, saw the solution to the problem of inheritance in colloidal chemistry, the study of large molecules that were the basis of life and of how to identify, understand, and analyze them. Physiology, and the work of Jacques Loeb among others, was reshaping biology along mechanistic and experimental lines. Major advances had occurred in plant hybridization techniques. The opening of the American West led to the discovery of major fossil fields and fueled rapid growth of paleontology. The biometricians, led by Darwin's cousin Francis Galton, developed techniques for analyzing inheritance and correlating traits.

(Continues...)

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Excerpted from Evolutionary Restraints by MARK E. BORRELLO Copyright © 2010 by The University of Chicago. Excerpted by permission of THE UNIVERSITY OF CHICAGO PRESS. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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