should one think about the formative role of genes? In this essay I discuss some results of my own struggles with this problem that may help other nongeneticists think about genes. The companion volume to this one, Cells and Surveys (National Research Council, 2001) contains a chapter (Wallace, 2001) on genetic markers in population surveys of human traits whose language could serve as a model of meticulous accuracy in the discussion of genetic data. This chapter is intended as a kind of reader’s guide for how to relate genes to phenotypic traits in general, in order to better interpret research results and public discussions that attempt to relate genes to particular human characteristics. For a more thorough discussion, see West-Eberhard (2003, Part I).

To understand how any complex apparatus works, it helps to know how it was put together. For traits of organisms, this means understanding how they develop and how they have evolved. Sometimes evolution is depicted as a process of random genetic mutation and selection. Advances in the molecular genetics of gene expression and development, as well as in phylogenetic methods that permit more accurate histories of organismic change, support a different view: novel traits originate via the developmental reorganization of ancestral phenotypes, not just by a series of random mutations and their cumulative new effects. That is, the traits one observes have been assembled via the reorganization of older traits, with old genes used in new combinations. Furthermore, developmental reorganization can be initiated by environmental factors, as well as by mutations. In keeping with the universally acknowledged importance of environment in development, environmental induction can play an important role in the reorganizational origins of novel traits (for a summary and extensive documentation see West-Eberhard, 2003, Chapters 9-18, on evolution by developmental reorganization; Chapters 6, 20, 26 on the role of environmental factors).

These findings are relevant to the search for genetic markers—genetic loci whose different alleles correlate strongly with, and can therefore be used to predict, variation in human traits. First, due to change by reorganization of gene expression, related organisms or populations can have markedly distinctive characteristics, or “phenotypes,” without having a large number, or any, distinctive new genes or genetic alleles (alternative DNA sequences at the same chromosomal locus). This is illustrated by the small genetic distance between humans and chimpanzees despite considerable differences in their behavioral and morphological phenotypes (King and Wilson, 1975). Second, the reuse of the same genes in different