Explaining how and explaining why: Developmental and evolutionary explanations of dominance

There have been two different schools of thought on the evolution of dominance. On the one hand, followers of Wright [Wright S. 1929. Am. Nat. 63: 274-279, Evolution: Selected Papers by Sewall Wright, University of Chicago Press, Chicago; 1934. Am. Nat. 68: 25-53, Evolution: Selected Papers by Sewall Wright, University of Chicago Press, Chicago; Haldane J.B.S. 1930. Am. Nat. 64: 87-90; 1939. J. Genet. 37: 365-374; Kacser H. and Burns J.A. 1981. Genetics 97: 639-666] have defended the view that dominance is a product of non-linearities in gene expression. On the other hand, followers of Fisher [Fisher R.A. 1928a. Am. Nat. 62: 15-126; 1928b. Am. Nat. 62: 571-574; Bürger R. 1983a. Math. Biosci. 67: 125-143; 1983b. J. Math. Biol. 16: 269-280; Wagner G. and Burger R. 1985. J. Theor. Biol. 113: 475-500; Mayo O. and Reinhard B. 1997. Biol. Rev. 72: 97-110] have argued that dominance evolved via selection on modifier genes. Some have called these "physiological" versus "selectionist," or more recently [Falk R. 2001. Biol. Philos. 16: 285-323], "functional," versus "structural" explanations of dominance. This paper argues, however, that one need not treat these explanations as exclusive. While one can disagree about the most likely evolutionary explanation of dominance, as Wright and Fisher did, offering a "physiological" or developmental explanation of dominance does not render dominance "epiphenomenal," nor show that evolutionary considerations are irrelevant to the maintenance of dominance, as some [Kacser H. and Burns J.A. 1981. Genetics 97: 639-666] have argued. Recent work [Gilchrist M.A. and Nijhout H.F. 2001. Genetics 159: 423-432] illustrates how biological explanation is a multi-level task, requiring both a "top-down" approach to understanding how a pattern of inheritance or trait might be maintained in populations, as well as "bottom-up" modeling of the dynamics of gene expression.