Tuatara: Volume 26,Issue 2, November 1983
The Role of Population Genetics in Our Understanding of Evolution
The Role of Population Genetics in Our Understanding of Evolution
Saiff and Macbeth (see p.73 of this journal) suggest that we need no longer teach population genetics in courses on evolutionary theory. In their discussion they quote Lewontin, 1974, Spiess, 1977 and Roughgarden 1979 as expressing sorrow that population genetics has not (at least as yet) provided the hoped for explanation of the origin of species. And thus, argue Saiff and Macbeth “there is no need to teach population genetics in introductory courses on evolution, although advanced courses may include it as a matter of history .
I feel this argument deserves an answer, in part because Macbeth's book “Darwin Retried (1971) served for some time as a valuable stimulus to controversy in our evolution courses. Secondly I believe that the article gives a mistaken view of the nature of population genetics which I would like to correct. And finally, I object to an argument that takes and uses the words of notable scientists out of context.
Population genetics is the study in theory, in the laboratory and in the field, of genes and chromosomes as they occur and vary in members of populations. We have overwhelming evidence that genes and chromosomes, together with envoronmental influence, control the development of organisms. We observe that different species are different because they develop differently. It is assumed therefore that population genetics could supply answers as to how these differences came about. I say answers since different groups of organisms have almost certainly been influenced by very different processes in their evolutionary history. In this context it is important to remember that population genetics is an area of study rather than a particular body of theory. A novelist attempting the perfect description of a sunset will hope with time to improve his words, so as to convey his picture more vividly to his readers. Population geneticists want to describe what is happening to genes and chromosomes in populations to a point where people, understanding the description, will know both what to expect next and how things got to their present state. We are not at that point yet except for a very few cases. To include more cases our theories will have to change and adapt.
One difficulty until recently has been an artificial separation of our study of environmental effects (ecology) from our study of hereditary control (genetics). There have been good historical reasons for this separation and as well some practical reasons. Genetics in its early stages was most easily studied with a reductionist, laboratory approach, looking for the effects of individual genes in controlled populations. Ecology, on the other hand, has seemed to call for a more holistic, field observation based approach. However, of late, this distinction in breaking down and a more inclusive field, often referred to as ecological genetics, is developing. The process is slow, like any integration program, as Merrell discussed in detail in the first chapter of his recent book (1981, pp. 3-12), and in many cases texts which claim to be ecological genetics are actually a volume on ecology and another on genetics within the same cover. Merrel seems largely to overcome this problem in his own book. To suggest that the population genetics section of this growing discipline should be dropped because its integration with ecology is not yet complete would seem page 74 premature to say the least. An analogy would be a metorologist bemoaning the fact that, despite the exquisite detail of thermodynamic theory (which purports to describe the movement of particles including those of gases and water which make up the atomosphere) we are still unable accurately to predict the weather. All of us who have had picnics rained out, yachts blown over and so on, will readily join his lament. However, I doubt we would actually suggest that Newton should no longer be taught to our budding meteorologists. Rather we might like to see an increased effort to develop thermodynamic theories which would be more useful when dealing with such complex systems.
Similarly we might suppose that the failings of population genetic theory in explaining evolution fully, argue for more effort rather than less.
I also have some objection to the way Saiff and Macbeth (1983) take remarks out of context. Thus Roughgarden (1979), for all his disappointment with the subject, devotes 169 pages to explaining basic population genetics and 330 pages to showing how it can be applied to evolutionary theory. Lewontin (1974) calls his book “The genetic basis of evolutionary change. He covers basic evolutionary genetics and uses his vast knowledge of ecology to make a major contribution to the synthesis of the two fields. His title and the general tenor of his work leave no doubt that he believes he is studying the basic questions of evolution. Spiess (1977) is more concerned with the dynamics of genes in populations in the shorter term, but states clearly that with restriction in gene flow, divergence of the genome is likely to occur and (this) constitutes a mechanism that is an essential ingredient for the evolution of divergence at the species level (p. 666). He then goes on to complain that we do not know enough about the process.
The way in which Saiff and Macbeth quote the authors mentioned gives the false impression that they are saying population genetics is no use, when they are actually saying it needs improving, as do most, if not all, scientific models.
Saiff and Macbeth in suggesting deleting population genetics from evolution courses, seem to have an altruistic motive of saving teachers from a difficult and dull subject. They claim this lack of ease in teaching the subject “without fear of contradiction . My own experience is that students are genuinely interested in why 97% of adult Thais cannot tolerate lactose (milk sugar) compared to only 4% of adult Australian aboriginals (Sutton, 1980, p. 495), or why some hangingfly males have more offspring by pretending to be female (Thornhill, 1980). Only a few of the less numerate students are put off by the simple mathematics required at an introductory level.
Finally, I suspect that some current applications of population genetics theory to evolutionary theory are inappropriate. This me ans we should re-examine our assumptions at regular intervals and change them as our understanding improves. It does, however, not suggest we should scrap the whole area of study.
Lewontin, R. C. 1974. The genetic basis of evolutionary change. Columbia University Press, N.Y. and London, xiii + 346 pp.
Macbeth, N. 1971. Darwin Retried. Dell Publishing Co., N.Y., 178 pp.
Merrell, D. J. 1981. Ecological Genetics. Longman, London. xii + 500 pp.
Roughgarden, J. 1979. Theory of Population Genetics and Evolutionary Ecology: An Introduction. Macmillan, N.Y., × + 634 pp.
Saiff, E. I. and N. Macbeth 1983. Population Genetics and Evolutionary Theory. Tuatara. 26(2): 73-74.
Spiess, E. B. 1977. Genes in Populations. John Wiley, N.Y. xi + 780 pp.
Sutton, H. E. 1980. An Introduction to Human Genetics (3rd Ed.). Saunders College, Philadelphia, xiii + 592 pp.
Thornhill, R. 1980. Sexual Selection in the Black-tipped Hangingfly. Scientific American. 242 (6): 138-145.