Quasi-independence, fitness, and advantageousness

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Abstract

I argue that the idea of ‘quasi-independence’ [Lewontin, R. C. (1978). Adaptation. Scientific American, 239(3), 212–230] cannot be understood without attending to the distinction between fitness and advantageousness [Sober, E. (1993). Philosophy of biology. Boulder: Westview Press]. Natural selection increases the frequency of fitter traits, not necessarily of advantageous ones. A positive correlation between an advantageous trait and a disadvantageous one may prevent the advantageous trait from evolving. The quasi-independence criterion is aimed at specifying the conditions under which advantageous traits will evolve by natural selection in this type of situation. Contrary to what others have argued [Sterelny, K. (1992). Evolutionary explanations of human behavior. Australian Journal of Philosophy, 70(2), 156–172, and Sterelny, K., & Griffiths, P. (1999). Sex and death. Chicago: University of Chicago Press], these conditions must involve a precise quantitative measure of (a) the extent to which advantageous traits are beneficial, and (b) the degree to which they are correlated with other traits. Driscoll (2004) [Driscoll, C. (2004). Can behaviors be adaptations? Philosophy of Science, 71, 16–35] recognizes the need for such a measure, but I argue that she does not provide the correct formulation. The account of quasi-independence that I offer clarifies this point.

Section snippets

1.

In a recent study of parallel evolution in sticklebacks, a case is described in which selection acts on two traits, armor strength and speed (Colosimo et al., 2005). A stickleback gains an advantage by having strong armor, since strong armor provides more protection than weak against being eaten. Swimming fast also provides an advantage, since fast fish are better able to avoid predators than slow ones. If armor strength and speed were independent of each other, we might expect natural

An objection

If the degree of negative correlation between advantageous traits is not perfect, then it seems obvious that both advantageous traits are guaranteed to evolve, contrary to what I have just claimed. After all, if the fittest trait combinations are available, they will both eventually reach fixation. I have two replies to this concern. First, we should view the predictions of the model I’ve developed as being limited to a one-generation snapshot. In each generation, an advantageous trait will

Sticklebacks and the quasi-independence criterion

The specific question we set out to answer, recall, was this: how much positive correlation between the advantageous trait of having strong armor and the disadvantageous trait of swimming slowly is consistent with strong armor’s evolving to replace weak, and with fast’s evolving to replace slow? One part of the answer is that there must be less than perfect positive correlation. However, this is a necessary condition, not a sufficient condition. Under perfect correlation, only one of the two

Quasi-independence, sociobiology, and evolutionary psychology

According to Sterelny, 1992, Sterelny, 1995, and Sterelny and Griffiths (1999, p. 320) the idea that a trait cannot be an adaptation unless it satisfies the QIC explains why sociobiology (SB) is a hopeless research program, while evolutionary psychology (EP) is not. One important difference between these research programs is that SB seeks adaptationist explanations for behaviors, while EP proposes such explanations for the psychological mechanisms that produce those behaviors. Why should we

Conclusion

A trait cannot be an adaptation unless it causally contributes to fitness in the way in which advantageous traits do. However, traits that causally contribute to fitness are not automatically fitter than traits that causally detract from fitness. There are a number of factors that can prevent an advantageous trait T from evolving by natural selection. Whether the developmental connection between T and other traits is one of them depends, in ways I have tried to make clear, on the nature of this

Acknowledgements

For many helpful comments on earlier drafts on this paper, I’d like to thank Elliott Sober. I’d also like to thank Ellery Eells, Patrick Forber, Andrew Hamilton, Daniel Hausman, Richard Lewontin, Jay Odenbaugh, and Joel Velasco.

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