Section I: Chapter 3
A History of Evolutionary Theory Part 2: Darwin
Natural selection was one of three selective processes which Darwin hypothesized as the engine driving evolution. The theories behind these three processes: natural selection, pangenesis and sexual selection were introduced in 1859, 1868, and 1871 respectively. The introduction of natural selection in 1859 as the prime force behind the evolution of species appeared in the book, The Origin of Species. For many its publication had the immediate effect of moving the discussion from whether evolution occurred to how evolution occurred. Still, it would be almost 80 years before natural selection would become the default choice for evolutionary theorists.
Natural selection states that since species produce far more individuals than can survive to procreate, and that those individuals have different features, features randomly produced and inheritable, then the individuals that manage to grow to adulthood and procreate will populate the environment with progeny with those same features. These features can be physiological or behavioral and can involve such traits as maturation speed, running speed, dietary requirements, size, aggressive qualities, cooperative tendencies, color, etc. In Darwin's words, "...multiply, vary, let the strongest live and the weakest die." (Darwin, Charles (1968 (1859)) Origin of the Species. Penguin, London p. 263)
Among the variety of early supporters of natural selection, one of the greatest controversies in the years following The Origin of the Species revolved around how much of a role Lemarck's theories played in the process of evolution. Spencer, for example, a widely respected popular writer, respected by Darwin in fact, supported natural selection but believed a heavier emphasis should be laid upon Lamarkian interpretations of evolution.
"But the fact we have to note is that while Mr. Darwin thus took account of special effects due to special amounts and combinations of agencies in the environment, he did not take account of the far more important effects due to the general and constant operation of those agencies." (Spencer, Herbert (1895) Factors of Organic Evolution. D. Appleton, New York. p. 434)
Darwin, as a believer in Lamarckian processes, was not the most extreme of supporters for natural selection (Richards, 1992). Though Darwin wrote that natural selection worked upon random variation, even in the Origin of Species, 1st edition, Darwin noted that natural selection worked upon, "Variability from the indirect and direct action of the external conditions of life, and from use and disuse..." (p. 459). Wallace, for example, was more "Darwinist" than Darwin. Lamarckian theories addressed a number of seeming anomalous situations that natural selection did not seem robust enough to tackle. Primary among these issues was Mivart's insight that natural selection, by positing that variation is random, did not attempt to theorize possible processes leading to the appearance of progeny with features pre-selected for an environment. In other words, processes other than what we today understand as mutation may influence evolution. The Lamarckians believed they saw evidence of these processes though they could not put forth a satisfactory hypothesis to explain the evidence. Darwin's attempt in this area, pangenesis, by his own admission, failed to satisfactorily explain the evidence that he perceived existed that supported use and disuse, and direct environmental influences on the creation of variation. Feelings still run high in this area.
"The Lamarckian hope that a discovery is just around the corner, that somewhere, perhaps in some recondite corner of the immune system, acquired characteristics will turn out to be inherited, is in vain. In our world or epigenetic recipe embryology, the inheritance of acquired characteristics is impossible." (Cronin, Helena (1992) The Ant and the Peacock: Cambridge Univ. Press, Cambridge p. 44)
An issue born with the birth of the theory of natural selection was identifying at which level evolution operates. Darwin emphasized selection on the individual level while Wallace focussed selection on the species level. Controversy has followed the issue ever since. Dawkins (1976) and Cronin (1992) sport a strong gene position. They emphasize that the only selective agent that can effectively work in a identifiable self interest is the gene. Wilson (1998) eloquently concurs. At the other end of the argument Gould (1998) argues that natural selection operates at multiple levels, with permeable level walls, with the different levels characterized by specific features. For Darwin, without the theoretical tools that Mendelian genetics provides, selection occurred at a species level.
Numerous other discussions involving varying nuances of natural selection occurred. Some issues would not be resolved until the merger of Mendel's rediscovered experiments on dominant and recessive genes with August Weismann's conjectures (Eldredge, 1999) concerning the impregnability of the wall of the cell germ (Eiseley, 1958). The Mendel/Weisman synthesis specifically addressed those theorists concerned that any individual with a useful trait would quickly find the trait breed out of evidence, swamped, by those members of his species with a different trait.
Of concern to many theorists, with respect to natural selection, were Lord Kelvin's calculations that the earth could not be more than 100 million years old. When radiation was discovered in 1896, the age of the earth suddenly expanded to support Darwin's belief that far more time was required than Lord Kelvin's estimates (based on the brief time it would take the earth to cool from an original scalding state) giving life enough time for natural selection to operate (Bowler, 1984).
One of the most contentious issues that Darwin faced, one still used by creationists to support their position that evolution does not exist, was the incompleteness of the fossil record. Darwin, as a student of Lyell's, favored mostly a slow and steady scenario for species evolution. Though the fossil record has marvellously filled in many evolutionary blank areas, startling gaps remain. The consistency of the blank areas, in other words the regularity of species appearing in the record with no intermediary fossils between it and its most recent ancestor, suggests a dynamic not evident if natural selection is accepted as the lone selective process. New nuances of natural selection have been hypothsized to explain this anomaly. Theorists from all perspectives have weighed in and presented their position on this aspect of evolution. This is not an arcane issue, and will be addressed later in this work when details of shift theory explore gaps in the fossil record.
In 1871, Darwin published, The Descent of Man. In this book sexual selection is described in detail. Darwin (1871) divides sexual selection into 1) female sexual selection or female choice where the female in a species picks a male for a specific quality, for example colorful feathers or a beautiful song. 2) male combat sexual selection called male-male competition where males compete physically for the opportunity to mate with a female, for example by having larger antlers or a more massive bulk. Fox (1983) suggests that there is a strong element of female choice in both alternatives, with male-male competition being a form of female choice where a variety of successful aggression is sought after. Perper (1989) notes that in humans, what may look like male-male competition may indeed be, in actuality, female choice. By describing both processes as sexual selection, Darwin makes clear how the lines between the two may, at times, be blurred.
Sexual selection was also controversial, yet was never focussed on by theorists early in the century as a selective process as powerful as natural selection. Many theorists today agree with Darwin that sexual selection may be the primary selective process in human evolution. There are theorists that believe that sexual selection is the most powerful force in species evolution (Miller, 1994). Human hairlessness and language, Darwin believed, could be explained by sexual selection. Also, males and females could pick different qualities in each other resulting in various anomalous human features. In The Descent of Man, Darwin pointed to Africa as the likely origin of our species, and to apes as our closest relatives. Wallace, Darwin's co-creator of natural selection, was a staunch opponent of sexual selection. Many theoreticians ignored the theory altogether. Fisher, in 1930, rescued the theory from oblivion with his theory of runaway sexual selection that mathematically supported Darwin's conjectures. Only beginning in the 1970's (Trivers, 1972) were male-male competition and female choice supported again as important processes in evolutionary theory.
This discussion of Darwin's theory of sexual selection will focus on the following issues: the accelerated nature of sexual selection; the focus on novelty in sight, sound and behavior; the impact of female choice on human evolution and the results of mutual selection in humans; human brain size increases; and Darwin's blind spot as a child of Victorian England. Some of these issues, such as runaway sexual selection, will be addressed in more depth in the next chapter when we discuss later theorists' elaborations on Darwin's works. Female sexual selection or female choice, as one of the foundation principles of shift theory, will be returned to several times throughout the course of this work as we discuss the origin and evolution of culture and the etiology of neurological disease, certain cancers, and psychological disorders.
Darwin (1871) emphasized that sexual selection was qualitatively different from natural selection in that it could act far faster and could explain leaps of transformation that natural selection might not be able to approach. He compared sexual selection to the artificial selection of domesticated animals, both processes exhibiting a relative lightening speed of change. Darwin followed Lyell's non cataclysmic orientation in suggesting that evolution generally followed a slow gradual path, but in his discussions of sexual selection (Cronin, 1992) the restraints were off.
In addition to speed of change, sexual selection offers an explanation for some mysteries of evolution that natural selection, for many, had not satisfactorily explained. How birds could have incrementally acquired feathers and flown, when any half way point in the process may have offered few benefits, has been an issue from Mivart to Gould. Wesson (1991) has written that feather evolution may have progressed because the female of the ancestor bird species was picking males with beautiful plumage and selected for big fat colorful feathers that later were found useful for flight (it has been suggested that their origin came for their ability to retain heat).
Sexual selection and its propensity to operate with speed and extreme uniqueness has also been noted when it comes to human beings. Darwin (1871) wrote that humans may have been prepared for language by sexual selection reinforcing the most eloquent (non verbal) vocalists thereby creating a song-making ability in humans that could later be bridged into language (Darwin, 1871). Darwin believed that the many radical shifts in evolution may have been sexually selected. Other writers have noted that sexual selection in combination with culture, may be the engine behind the extreme speed in human evolution, especially in the last 40,000 years (Laland, 1995).
Darwin opens a discussion, in The Descent of Man, which flirts with the very foundation of evolutionary theory. Darwin (1871) stated point blank that animals, particularly the females, have an aesthetic sense (Darwiin, 1871). We might say that they are attracted to novelty or variations on a theme. At the foundation of female choice is this attraction to novelty which can be explained because it reinforces the priorities of natural selection. In other words, natural selection may have selected for female sexual selection. These propensities are deeply ingrained (Ridley, 1993; Small, 1993) and in many species may have placed the female in the position of guiding the direction of evolution (Small, 1993). This thirst for novelty (Miller, 1994) may also be what characterizes human evolution.
Female sexual selection is driven by an attraction to novelty (Darwin, 1871), and novelty is characterised by variations in the way things look, sound, smell or feel which can be manifested physiologically and behaviorally. In recounting his observations of sexual displays, Darwin concentrates on changes in looks and sounds paying less attention to behavior (Miller, 1994), Yet he noticed that in their displays, animals emphasized either sight or sound, rarely excelling in both (Darwin, 1971).
Darwin (1871) assigned the insights he derived from his observations of nature to human beings. He listed several anomalous human features derived from sexual selection. The human lack of hair he ascribed to male selection of less hairy females (Darwin, 1871), a trait that became generalized to the species. Crook (1972) concurs in this assessment. Hulse (1978) gives as an example the relative lightness of skin color at different social stratas in Japanese culture as evidence of males picking females with the specific qualities that Darwin alluded to.
In, perhaps, one of the strangest turns in the history of evolutionary theory, Wallace (1895), when confronted with human lack of hair and expanded brain size in human beings, utterly rejected sexual selection and turned to intervention by God to explain the anomalous characteristics. Wallace insisted that natural selection was the only selective process. This compelled him, in the end, to look to deistic intervention to explain human evolution.
Human sexuality was not easily discussed in Darwin's day. Freud's works would not appear for several decades. A number of contemporary theorists have focussed on human sexual features, both physiological and behavioral, as the results of sexual selection. Many of these hypotheses are built upon a principle that Darwin alluded to, but did not pursue, that females in human evolution hypothetically could be responsible for particularly important human features. Darwin very hesitantly suggested that human evolution progressed, before the establishment of contemporary polygynous and monogamous patriarchal social structures, under promiscuous social structures characterized by a high degree of female choice (Smith, 1976). In some very succinct ways, human sexual anatomical and behavioral characteristics offer more than just clues as to how human being evolved, they suggest the very process itself. At the core of that process is female choice.
Fisher (1992) writes that female choice in human evolution is evidenced by the size of the male penis. Cronin (1983), and Small (1993) describe how penis size and testicle size are impacted by female choice in other species. There is a close correlation between testicle size and penis size and promiscuous social structure as seen in chimpanzees where males with larger testicles and penises produce and distribute larger quantities of sperm creating an advantage over the other males (Margulis & Sagan, 1991), If you are a male living in a hierarchically organized social system, a gorilla harem for example, then copious amount of sperm production or a long penis does not increase your chances of producing progeny. Your ability to prevent other males from sexual contact with females, by force or intimidation, positively influences your chances of reproductive success. Gorillas retain a diminutive penis and testicle size. Human males evidence sexual anatomical characteristics suggestive of a promiscuous social structure (Wilson & Daley, 1992). There are exceptions such as the asian highly stratified river/irrigation cultures who have had a highly hierarchical, patriarchal, monogamous social structures for thousands of years (Miller, 1994).
Though Darwin hesitated to pursue the implications, human evolution may have progressed following the dynamic that comes with the benefits of female choice. Trivers (1985) notes that female choice tends to reinforce certain characteristics in the male population. Tanner (1981) applies this principle to human evolution and comes up with a compelling argument that males disposed to making sure that the children are taken care of would be selected by the females as copulation partners in the presence of big-headed babies requiring years of care. Human anatomy evidences this suggestion. Male anatomy with a large penis and big testicles suggests a non-hierarchical promiscuous social structure, usually characterized by female sexual selection. We will go into detail on this issue in Section III.
Darwin identified numerous anomalies among the physical features that make up human beings. The differentiaton of the races, for example he ascribed to sexual selection. One dynamic of sexual selection he suggested, was peculiar to human beings. Darwin (1871) believed that human beings might be the only species where females choose males, and males choose females for different characteristics. The wide variation in humans features may be due to mutual sexual selection. Miller (1994) outlines the evidence in support of this supposition. Later in this work we will explore how human evolution from matrifocal promiscuous social structures over most of the course of human evolution to the more recent surge in patrifocal monogamous and polygynous social structures explains how these features appeared.
After discussing the possibility that matrifocal social structures may lay at the foundation of human evolution, referring to the work of Morgan, Darwin goes on...
"Nevertheless, from the strength of the feeling of jealousy all through the animal kingdom, as well as from the analogy of the lower animals, more particularly of those which come nearest to man, I cannot believe that absolutely promiscuous intercourse prevailed in times past, shortly before man attained to his present rank in the zoological scale." ..."We may indeed conclude from what we know of jealousy of all male quadrupeds, armed as many of them are, with special weapons for battling with their rivals, that promiscuous intercourse in a state of nature is extremely improbable." (Darwin, C. (1871) The Descent of Man . John Murray: London p. 611-12)
Tanner (1981) passed through the door that Darwin opened, a passageway still unexplored. Most contemporary theorists are still carrying a burden that comes with viewing the world through the eyes of a particular social structure. Hrdy (Smith, 1984) suggests an alternative point of view, one that opens up theoretical vistas offering useful models. Darwin's theory of female choice, when combined with other theoretical tools, offers solutions to the question; What exactly is "human", and how did we become it? In sections II and III we will step through the Darwin's doorway and continue to explore the passageway Tanner first wandered into twenty years ago.
Darwin believed that human language capabilities could be explained by sexual selection, the courtship ritual selecting for the best singers. Lovelock (1979) wrote that whales may have evolved their behemoth brain by having had the best singers chosen as procreation partners. Neuroscientist T. J. Crow, 1995) goes on to suggest the human brain structure, lateralization in general and specific skill and talent structures specifically, seems sexually selected. And, finally, there are those specialists such as Jones (1995) that believe that sexual selection in combination with neoteny may be responsible for brain size increases in human evolution.
It is the proposition of this work that Darwin opened a door that he could not pass through. Information available to us, not available to Darwin, makes possible an understanding of the exact relationship of the three selective processes that Darwin believed lay at the foundation of species evolution. Brain size increases, the origins of language, human sexual anomalies, and that truly strangest of all features, culture, are understandable by combining Darwin's genius with the results of contemporary studies. Darwin was also bound by the cultural mores of his time. We are still encumbered by these restraints. It is no mistake that most of the finest current theoretical innovations in anthropology and neuropsychology are being done by the females of our species.
Proceed to Chapter 4