Biological adaptation in light of the Lewontin-Williams (a)symmetry

Abstract

Neo-Darwinism characterizes biological adaptation as a one-sided process, in which organisms adapt to their environment but not vice versa. This asymmetric relationship-here called Williams' asymmetry-is called into question by Niche Construction Theory, which emphasizes that organisms and their environments often mutually affect each other. Here, we clarify that Williams' asymmetry is specifically concerned with (quasi)-directed modifications toward phenotypes that increase individual fitness. This directedness-which drives the adaptive fit between organism and environment-entails far more than the mere presence of cause-effect relationships. We argue that difficulties with invoking fitness as the guiding principle of adaptive evolution are resolved with an appropriate definition of fitness and that objections against Williams' asymmetry reflect confusions about the nature of biological adaptation.

Figure 1

Schematic representation of adaptive evolution and other causal forces. Organisms can influence their habitat and vice versa, leading to ecoevolutionary feedback. In contrast to other causal forces, however, adaptive evolution shapes organisms toward matching their environment, in the sense of exhibiting traits that confer high inclusive fitness in that environment. On this view, the environment to which organisms adapt is not identical with the physical habitat, but instead corresponds to the totality of conditions (including the focal species’ current traits) that determine the mapping of fitness on phenotypes. This mapping (here symbolised by a sieve) biases gene propagation toward genes whose effect is to build better‐adapted organisms. The (quasi‐) directionality inherent in this process enables trends of cumulative improvement, which underlie much of evolution's creative potential.