A single mode of canalization

outcomes, such as discrete tissue types and organs, with few intermediate forms. The term ‘CANALIZATION’ (see Glossary) was coined to describe this phenomenon of discrete developmental outputs [1,2], and has been extended to include the ability of such systems to withstand genetic or environmental perturbations. Canalization has historically been inferred from the observation of organisms under genetic or environmental conditions that result in an increased range of phenotypic variation. This variation is then shown to have a partially heritable basis, indicating the presence of genetic and environmental variation that was masked under normal conditions (Box 1). Several terms have been used to describe this property, such as HOMEORHESIS [3] and phenotypic or DEVELOPMENTAL BUFFERING or stability (the history and usage of these terms is reviewed in [4]). Here, we use these terms entirely synonymously in the following manner: a homeorhetic (or canalizing) allele is one that reduces the PHENOTYPIC VARIANCE of a trait across genetic backgrounds and environments relative to a nonhomeorhetic allele. Similarly, a canalized trait (or a trait exhibiting homeorhesis) is one that demonstrates a restricted range of variation across genetic backgrounds and environments relative to a noncanalized trait. Canalization is therefore recognized as a property of organisms that influences their variability, or their propensity to vary [5]. Although the evolutionary fate of mutations that contribute directly to phenotypic differences has been studied extensively, the evolution of alleles that constrain or promote phenotypic variability is less well understood. Several theoretical studies have recently looked at the subject of variability and evolution, focusing on the evolution of mutation rates [6] as well as on the evolution of canalization [7–9]. Within the neodarwinian framework, the causes of variation are independent of the consequences TRENDS in Ecology & Evolution Vol.17 No.10 October 2002