Evolution of Phenotypic Robustness

Abstract

Evolutionary biology, in the neo-Darwinian tradition, is based on the study of genetic and phenotypicvariation and its fate in populations. Thus the observation that genetic variability of a trait is itselfinuenced by the genotype has obvious theoretical implications (Waddington, 1957; Stearns, 1994). It isin this context that the robustness of phenotypic traits was ⬚rst conceptualized as canalization and becamethe focus of a signi⬚cant research e⬚ort (reviewed in Scharloo, 1991). With increasing awareness of theintricate molecular mechanisms maintaining the life of cells, the ubiquity of bu⬚ering and compensatorymechanisms came into focus (Wilkins, 1997; Gerhart and Kirschner, 1997; Rutherford, 2000). Recentyears have seen a conuence of the classical concept of canalization and new research in molecular biologythat resulted in a sharp increase in the interest in canalization and related phenomena. One can speakof an emerging ⬚eld of biological robustness research that is able to draw on a sophisticated arsenal oftechnical and theoretical tools that were developed over the last ten years (de Visser et al., 2003).The present chapter aims at summarizing the principal ⬚ndings of the classical and the newer literatureon robustness of phenotypes and to identify the issues that require attention in future research. We willstart with an attempt to formalize the notion of phenotypic robustness to clarify the criteria that need tobe met in order to experimentally demonstrate phenotypic robustness. Then we review the experimentalevidence about environmental and genetic robustness and ⬚nd that genetic robustness is particularlydi⬚cult to demonstrate. Finally an overview of theoretical models for the evolution of canalization isprovided which shows that the existing literature is strongly biased towards a few scenarios. We concludethat there are major unsolved questions both in the experimental demonstration of canalization as wellas in understanding the evolutionary dynamics of canalization.