Category Archives: Evolution

On the Conditions for Natural Selection

Olivia Judson’s piece in today’s New York Times makes the important (and chastening) point that too many contemporary biologists have not actually read Darwin in the original.  As usual, she is terrific and generally right on.  I do have one small issue with a comment she makes:

Natural selection will operate whenever all of three conditions are met. These are: (1) some of the differences between individuals are inherited differences, not due to differences in their environments; (2) more individuals are born than can survive; and (3) part of the reason at least some of the survivors make it is owing to the traits — a longer-than-average beak, say — that they inherited from their parents. For natural selection, then, what is important is that some differences are inherited; and this, Darwin could show.

The problem with this statement is that it’s not actually correct.  But it’s incorrect in an interesting way, given Judson’s point that there is value in understanding the history of our science.  In the Origin, Darwin was clearly influenced by another foundational document, Thomas Mathus’s Essay on the Principle of Population. Indeed, the Malthusian insight that populations, human or otherwise, can easily outstrip their resource base proved to be a critical link in Darwin’s formulation of the idea of natural selection.

The key about Judson’s statement is that point (2) is not necessary.  If it were true, it would imply that natural selection can not act in growing populations.  The necessary and sufficient conditions for natural selection acting on a trait are: (i) variation in the trait, (ii) heritability of the trait, and (iii) differential reproductive success as a function of the trait. So a trait can increase even in the absence of differential mortality as long as there is differential fertility.  Darwinian fitness is a relative concept. A trait with higher fitness is one that increases in frequency relative to other traits.  This increase in relative frequency can come about because of differential mortality or differential fertility or a combination of the two.  

It turns out that models of selection are more mathematically tractable when one assumes only viability selection and so this is what is most commonly employed in theoretical work.  This relates to the fact that fertility selection works at the level of the breeding pair and not the individual.  The equations describing selection in this context thus become rather more complex.  But this is more technical than most sane people would care to get.  A good review of models of fertility selection can be found in Feldman et al. (1983).

Judson is essentially correct though.  We should all have a better understanding of the history of Evolutionary Biology (and, I should add, Anthropology).  A major part of this is reading the classics: Darwin, Malthus, Wallace, Fisher, Dobzhansky, Wright, Lorenz, Tibergen, Hinde… I could go on for a while, of course.  When I teach my class in life history theory, almost all of the readings are (20th century) classics.  My experience in the field is that lots of people cite Hamilton (1966) or Cole (1954), for instance, but very few people have actually read them.

References

Cole, L. C. 1954. The Population Consequences of Life History Phenomena. Quarterly Review of Biology 29 (2):103-137.

Feldman, M. W., F. B. Christiansen, and U. Liberman. 1983. On Some Models of Fertility Selection. Genetics 105 (4):1003-1010.

Hamilton, W. D. 1966. The Moulding of Senescence by Natural Selection. Journal of Theoretical Biology 12:12-45.

Do These Points Form a Curve?

I was interested to browse through a paper by Buunk et al. in the most recent issue of Evolution and Human Behavior in which the authors report the results of psychological experiments exploring the differential relationship between height and sexual jealousy in women and men. The authors predicted that (self-reported) sexual jealousy would decline with increasing height in men and that women of average height would report the lowest levels of sexual jealousy. The theory driving these predictions is that higher-status, more attractive individuals should be less jealous on average because they are better able to prevail over would-be competitors and, presumably, if they experience partner infidelity, they can always find another partner. The authors cite the abundant evidence for increased social dominance in taller men and suggest the relationship between women’s attractiveness and height is quadratic, with women of average height being most attractive. One hundred women and 100 men were asked question, “In general, how jealous are you in your current relationship?” Responses fell on a six point scale ranging from (1) “not jealous” to (6) “morbidly jealous”. The authors’ results apparently support their hypotheses. So here are the two figures that they use to show that (1) jealously declines linearly with height in men and (2) is quadratic for women, with average-height women least jealous. The first figure is for men:

Buunk et al. (2008) Figure 1

The second figure is for women:

Buunk et al. (2008) Figure 2

Hmmm. I don’t know if I would rest much on the interpretation of that figure as being “quadratic.” It seems entirely possible that the curve is driven simply by the sparseness of the tails. There are fewer women of extreme height, either tall or short and this allows a few influential points to leverage the line up at the ends. Think about the upper 95% confidence interval of a linear regression line. Doesn’t look that different from their figure 2, no? This makes me wonder how robust the relationship is. For example, if we were to bootstrap replicate samples (with replacement) and re-fit the quadratic form, how many would have a significant at some conventional level (e.g., p<0.05)? There is also the question of whether this quadratic curve fits better than a linear relationship. One could test the two nested models using a likelihood ratio test.

Then there is the question of confounding variables. At the very least, it seems that one would want to control for age of the actors, duration of relationship, and quite possibly other measures of wealth or status. It seems reasonable to posit that being extremely wealthy would modify the degree of sexual jealousy experienced by a man of average height, for instance.

This is why I remain a skeptic of evolutionary psychology…

Reference

Buunk, A.P. J. H. Park, R. Zurriaga, L. Klavina and K. Massar (2008) Height predicts jealousy differently for men and women. Evolution and Human Behavior. 29(2):133-139.

The Requirements for an Ecological Anthropology Curriculum

A question was posted today on the ecological anthropology listserv: What are the basic requirements for an ecological anthropology graduate program? I don’t claim to be qualified to say what these are for the field as a whole, but I am qualified to say what we have decided on in setting up our new ecological and environmental anthropology Ph.D. program at Stanford. Here I include an edited version of the reply I sent to the thread.

At the risk of essentializing, there are, broadly speaking, two general classes of ecological anthropologists: (1) those who use human relationships with the environment as a lens through which to study problems in cultural anthropology (e.g., agency, social structure, the construction of meaning, etc.), and (2) ecologists who study humans as their primary organism. The majority of practitioners currently falling under the latter category are probably human behavioral ecologists, though I can think of some notable exceptions to this. This is the approach our program emphasizes.

In addition to departmental requirements, EE students are required to take the following:

  • Evolutionary Theory
  • Research Methods in Ecological Anthropology
  • Data Analysis in the Anthropological Sciences

All students need to know how to integrate theory, method, and application, but the specific nature of the courses in which they learn that doesn’t matter that much. Therefore, we require three courses from a list of theory-driven graduate classes, including (but not limited to):

  • Advanced Ecological Anthropology
  • Human Behavioral Ecology
  • Conservation and Evolutionary Ecology
  • Demography and Life History Theory
  • Environmental Change and Emerging Infectious Disease

Required classes deal with what you know, but equally important is how you know. We expect our students to engage in research from the outset of their graduate studies. Students attend weekly lab meetings. These can be within the Anthropology department (e.g., Rebecca Bird and I run a joint meeting or we have a joint spatial interest meeting this quarter) or in other departments (e.g., Biology, Woods Institute).  Students also attend a colloquium (comprised of visiting speakers) one quarter out of the year.

We’re big on methods, but we don’t legislate what methods students learn (other than research design and statistics).  Most students are interested in remote sensing and GIS, but we also have students working on social network analysis, demographic methods,  and advanced statistical methodology.

So, that’s our idea for a graduate program.  We will have  a proper web page describing the program in detail some time in the future.

On Modules and Medical Materialism

Something about a recent post on the subjective experience of migraines by Siri Hustvedt got me to thinking about problems in the evolution of the human mind. I suppose this is because I am currently teaching a class on evolutionary theory for graduate students in the the Anthropological Sciences program and we have been thinking a lot about the intellectual legacy of sociobiology and, especially, Evolutionary Psychology (EP). EP is a currently popular school of thought for understanding the human mind. One of the central tenets of contemporary EP is the idea of extensive modularity — that the brain is a collection of special purpose “organs” designed to deal with problems that our ancestors habitually dealt with in our hunter-gatherer past.

The philosopher, David Buller has leveled what I see as a pretty devastating critique on this fundamental idea in contemporary EP. He suggests that the end product of brain mechanisms cannot, in themselves, be seen as adaptations since the development of these mechanisms is dependent on an environmentally-induced phenotype. Brain ontogeny is characterized by by a variety of additive and subtractive events in which new connections are formed and excess cells are pruned (possibly in a Darwinian fashion) subject to environmental input. In this view, it is the developmental processes, not the final products, that are the object of selection, a perspective that follows Terry Deacon‘s argument in the Symbolic Species. Buller suggests that it is the brain’s plasticity that is the adaptation and not specialized information-processing modules.

There is a lot to ruminate on here, particularly regarding the genetic architecture of putative modules and how selection is likely to be meted out with respect to cognitive traits. But that will need to wait for another day…

References

Buller, D. J. 2006. Evolutionary psychology: A critique. In Sober, E. (ed.), Conceptual Issues in Evolutionary Biology, 3rd Edition, pp. 197-216. Cambridge: MIT Press.

Tooby, J., and L. Cosmides (1992). The Psychological Foundations of Culture. In Jerome H. Barkow, et al. (eds.), The Adapted Mind: Evolutionary Psychology and the Generation of Culture, pp. 19-136. New York: Oxford University Press.

Evolution of a Bourgeois Temperment?

So, I’m teaching a graduate-level class in evolutionary theory this quarter. Given my druthers, I would have run a rather technical class in which we would discuss quantitative genetics, optimality models, game theory, multi-level evolution… Stuff like that. Well, we’ve done a bit of that but, due to popular demand, I actually took out two weeks on game theory and optimality models, and instead we are reading Gregory Clark‘s new book, A Farewell to Alms, in which argues that the Industrial Revolution may have its roots in quite recent biological evolution. Nicholas Wade wrote a review of the book in the New York Times that a number of students and I found intriguing. In this review, Wade quotes Clark as saying,

Through the long agrarian passage leading up to the Industrial Revolution, man was becoming biologically more adapted to the modern economic world.

We’ll see… Regardless of what I think of the book (which I’ve not yet read, but will do so along side the students starting next week), it seemed like an interesting case on which to bring to bear our new-found analytical skills in evolutionary theory. More later…