On a bit of a writing junket for his book, Mankind Evolving, in 1963 Theodosius Dobzhansky put an essay in Current Anthropology titled "Anthropology and the Natural Sciences -- The Problem of Human Evolution."
He spends the first half of the essay expositing a dual inheritance theory of human evolution, with both biological and cultural systems included, and puts forward an argument for a synthetic vision of the cultural and biological approaches to studying humanity.
Like any well-written essay, it is really in the second half that this argument builds up steam. This passage concerning the recent force of natural selection as a result of cultural changes is a good example:
The radical changes in the ways of life of our generation compared to those of our parents and grandparents must have been largely cultural rather than genetic. This only proves again the absence of one-to-one correspondence between genetic and cultural changes; this does not prove that the biological evolution of mankind has stopped or that it is irrelevant to the cultural evolution. It is admittedly difficult to prove that mankind has changed biologically since, let us say, the days of the ancient Greeks and Romans, if by "proof" you mean demonstration of sizeable gene differences. We cannot test the genes of Pericles or Caesar or their contemporaries. But neither was Darwin able to "prove" organic evolution in this sense. The evidence is indirect, inferential, but nevertheless, I think, conclusive. Paradoxically, it is precisely because we know that mankind changes so greatly culturally that we can be so confident that it changes to some extent also genetically. When the environment changs, the only other necessary condition for the occurrence of genetic evolutioanry chagne can be defined. This is the presence in human popluations of genetic variants, some of which confer upon their carriers a higher fitness in the older, and other variants in the incoming environments. Despite all the inadequacies of our present knowledge of human genetics, this can scarcely be doubted. What is more, since the environment in which man lives is in the first place his sociocultural environment, the genetic changes induced by culture must affect man's fitness for culture and hence may affect culture. The process thus becomes self-sustaining. Biological changes increase the fitness for, and the dependence of their carriers on culture, and therefore stimulate cultural developments; cultural developments in turn instigate further genetic changes. This amounts to a positive feedback relationship between the cultural and the biological evolutions. The positive feedback explains the great evolutionary change, so great that it creates the illusion of an unbridgeable gap between our animal ancestors and ourselves.... Those who believe that man no longer evolves biologically might contend that our species has entered upon such a period. Here we must, however, proceed with the greatest caution. The potentialities for rapid evolution of the human species have not been depleted, since hte environment continues to change and the genetic variance remains plentiful. Mankind assuredly continues to evolve, both culturally and biologically (Dobzhansky 1963:147, emphasis added).
This really became the conventional view within a certain school of human evolutionists (which ultimately encompassed my own training), although not always so clearly expressed. It is behind the notion that culture is the "human niche," although not going so far as the ultimate expression of the "niche" idea, which came to hold that no other cultural species could have existed alongside ours.
I would say that Dobzhansky represents the rational conclusion about human evolution from the perspective of the neo-Darwinian synthesis. After having begun from the premise that morphological evolution must be explicable in population genetic terms, there is a problem for the synthetist: human behavior (and morphology itself to the extent that it is influenced by behavior) includes a learned component that can induce behavioral stability across genetic variants in time or space. In population genetic terms, this is an environmental effect, but it is clearly a determining effect for much of what interests us about humans. And culture is not readily incorporated into a synthesis in which the population genetic mechanisms are the basis of change.
So Dobzhansky attempts to bring the fields of biological evolution and cultural "evolution" together with three basic assumptions:
(1) Human exceptionalism -- "Man is the sole product of evolution who has achieved the knowledge that he came into this universe out of animality by means of evolution" (p. 148). The effect of this is to cordon off cultural evolution as a special mechanism applying to humans.
(2) Separate causes. By assuming that the causes of cultural evolution are different from the causes of biological evolution, Dobzhansky envisages a feedback process between the two. If they had the same causes (i.e., maximization of Darwinian fitness), then cultural evolution could simply supplant biological evolution. Nor would there be any possibility of a "conflict" between the two, as he goes on to discuss.
(3) Biological variation is value-neutral. This is the anti-racist assumption that marks a strong difference between Dobzhansky and many of his contemporaries.
These latter two claims both reflect the orthogonality of cultural and biological variation. In other words, the factors that cause and maintain cultural variability are not the same as the factors that cause and maintain biological variability, and the two are (potentially) independent. (We may observe that to the extent that a "feedback" process occurs, it is by inducing some correlation between the factors underlying cultural and biological variation). The two assumptions are marked by the following quote from page 147:
The chief reasons why so many people are loath to admit the genetic variability of socially and culturally significant traits are two. First, human equality is stubbornly confused with identity, and diversity with inequality, as though to be entitled to an equality of opportunity, people would have to be identical twins. Human diversity is not incompatible with equality. Secondly, it is futile to look for one-to-one correspondence between cultural forms and genetic traits. Cultural forms are not determined by gnes, but their emergence and maintenance are made possible by the genetically conditioned human diversity. The division of labor in human societies is primarily a cultural rather than a genetic phenomenon, but could it be sustained in a population consisting of persons genetically as similar as identical twins? This is not entirely a vain question, since at least one great geneticist has recently envisaged the possibility of bringing about such genetic uniformity.
I find it interesting that in comparison with most exponents of dual inheritance theory, Dobzhansky does not accept a fourth assumption that cultural evolution applies to rapid change and genetic evolution applies to slow change -- in other words, an assumption of "different rates." He knew all too well that the pace of genetic evolution is often rapid, and follows the implication of that view to quite logically conclude that cultural evolution could induce rapid biological evolution within humans, even asserting the logical necessity of accepting genetic evolution of humanity "since the Greeks and Romans."
After the passage cited above, Dobzhansky goes on to describe the perils of recent cultural and biological evolutionary trends in our species, which have tended toward population explosion and the potential hazards therein. Then he proceeds to point out that worries that evolution has been "suspended" are wrongly directed:
Neither do I need to retell here the story of the alleged relaxation or suspension of natural selection in civilized mankind. THe dangers from this source, although not necessarily exaggerated, have often been presented in a wrong perspective. A notion, which is less frequently stated explicitly than implied in many writings, is that the progress of mankind would be safe and even irresistable if only the natural selection were permitted to operate unobstructed by civilization and its amenities. This notion does not stand critical examination. Natural selection does not even insure that the species on which it acts will survive, let alone that it will improve, in any sense of the word "improvement." Dinosaurs became extinct, despite their evolution having been piloted by natural selection, quite unhampered by culture, medicine, or charity (Dobzhansky 1963:148).
Of all the major population geneticists of the synthesis, Dobzhansky was probably the least sympathetic with the aims and practices of the eugenicists. And he spends several paragraphs in this paper describing -- in not so many words -- why the problems addressed by eugenicists were wrongly posed.
But even in spite of this skepticism of aims, Dobzhansky is more or less accepting of the potential for human controls to deliberately control natural selection on humankind -- as he wrote (p. 148): "Man, if he so choose, may introduce his purposes into his evolution."
The paper ends with this (I would offer, "apocalyptic") vision of anthropology:
Being an anthropologist only by avocation, I may perhaps venture to claim for anthropology more than most anthropologists dare claim for themselves. The ultimate function of anthropology is no less than to provide the knowledge requisite for the guidance of human evolution. Human evolution has arrived at a crossroad from which there is no turning back and no escape. Our animal past is irretrievably lost -- we could not go back to it even had we wished. The choice is between a twilight, cultural as well as biological, or a progressive adaptation of man's genes to his culture, ans od man's culture to his genes. But to fulfill its function, anthropology cannot belong entirely either to biological or to social sciences or to humanities. It must, in the fullness of time, become a synthesis of all three (148).
Well, I think he probably could have predicted that the mantle of planning human destiny was not really what anthropology was about, even in 1963. In fact, reflecting on that date, this is probably about as close to a Kennedy-esque vision of anthropology as ever was expressed!
Dobzhansky T. 1963. Anthropology and the natural sciences - The problem of human evolution. Curr Anthropol 4:138+146-148.