A constrained view of constraints.

Pigliucci and Kaplan1xThe fall and rise of Dr Pangloss: adaptationism and the Spandrels paper 20 years later. Pigliucci, M. and Kaplan, J. Trends Ecol. Evol. 2000; 15: 66–70Abstract | Full Text | Full Text PDF | PubMed | Scopus (79)See all References1 examine the impact of the famous Spandrels paper2xThe spandrels of San Marco and the Panglossian paradigm: a critique of the adaptationist programme. Gould, S.J. and Lewontin, R.C. Proc. R. Soc. London Ser. B. 1979; 205: 581–598Crossref | PubMedSee all References2, with the goal of encouraging a new ‘synthesis of constraints (spandrelism) and selection (panglossianism)’. My goal is to show that this dichotomy between constraints and selection emerges from a constrained view of constraints. I will not assert that this view is wrong, but rather, that we need to be aware of its parochial perspective. For many of us, selection is a consequence of constraints and the challenge is to explain them.I will begin with some minor asides. First, the authors fail to cite the brilliant critique of the Spandrels paper by Dave Queller3xThe spaniels of St Marx and the Panglossian paradox: a critique of a rhetorical programme. Queller, D.C. Q. Rev. Biol. 1995; 70: 485–489CrossrefSee all References3. Second, the paper includes some strange ideas, such as ‘the currently available theory leads us away from the idea of survival of the fittest and towards a model of survival of the barely tolerable’. They infer this by analogy to the concept of satisficing, which describes a pattern in institutional decision making4xForaging Theory. Stephens, D.W. and Krebs, J.R. See all References4. In practice, firms do not gather all the information and take all the necessary time to wring every last marginal bit of profit out of every decision. This is not optimal if time and information have no marginal costs, but it might be optimal if they do have costs and these cost constraints are incorporated into our analysis of the firm’s decision process. We have to incorporate these constraints properly before we can determine whether the process is, in fact, optimizing.What are constraints? Stephens and Krebs4xForaging Theory. Stephens, D.W. and Krebs, J.R. See all References4 point out that ‘one person’s constraint may be another’s design problem’. The behaviour I study as a potentially adaptive variable is a constraint to my colleagues in community ecology; my dependent variables are their parameters.When Pigliucci and Kaplan assert that ‘selection and constraints [are] the two major deterministic players on the evolutionary stage’, they are apparently assuming that selection is unconstrained. How can selection be unconstrained, when constraints are what shape fitness surfaces? It is just that these constraints are implicit and unexplained. The surface describes the consequences of constraints without explaining them5xThe causes of natural selection. Wade, M.J. and Kalisz, S. Evolution. 1990; 44: 1947–1955CrossrefSee all References5.Pigliucci and Kaplan only recognize genetic constraints to the evolutionary response across generations, which are external to the fitness surface. For instance, if fitness y relates to a trait x as: y=ax+b, neither the linear form of the function nor the actual values of the parameters would be treated as constraints. If we knew that x must be less than 42, that would be a constraint, regardless of whether we could explain why. This is a workable perspective, but it is not universally shared. For instance, in Arnold’s6xConstraints on phenotypic evolution. Arnold, S.J. : 258–278See all References6 exposition of constraints, he states that ‘the strength and pattern of selective constraints can be measured as the slope and curvature on an adaptive landscape’. The phrase ‘selective constraints’ implies that Pigliucci and Kaplan are presenting a parochial perspective. The apparent dichotomy between constraints and selection is an artifact of that perspective. From my perspective the challenge is not to bridge a vast chasm between constraints and selection, but rather to understand the constraints that create and shape selection. I hope this contributes to the authors’ goal of ‘a more balanced and pluralistic approach to the study of evolutionary biology’.