Organisms can respond plastically to novel conditions to generate functional variation. The specific adaptive phenotypes generated need not be the direct targets of past selection. Rather, they are often the expression of the more general ability of developmental processes to accommodate novel inputs adaptively, enabling functionally integrated responses to a broad range of conditions (West-Eberhard 2003; Kirschner & Gerhart 2007).
Moreover, through niche construction, environments can be changed by organisms to suit themselves. Consider the example of the earthworms which, despite living on land for millions of years, have retained the characteristic physiology of the freshwater animals from which they evolved (Turner 2000). For instance, earthworms produce large quantities of dilute urine, a trait that on land leaves them highly vulnerable to desiccation. Earthworms can only survive because they process the soil in ways that allow them to draw water into their bodies more effectively. They construct a simulated aquatic environment on land.
The key point is that the adaptive complementarity of earthworms and soils results to a large extent from the worms changing the soil through niche construction, rather than natural selection changing the worms to a typical terrestrial physiology.
Traditionally, the latter route has not been recognized as an alternative to adaptation – after all, the earthworm’s niche-constructing traits are adaptations too. However, the conventional stance assumes a particular model of biological causation, which advocates of the niche-construction perspective reject (Laland et al. 2011).
From the niche-construction perspective, the current selective environment was itself brought about through earlier niche construction, which caused the bout of selection. The attribution of all causal significance to selection fails to capture the true reciprocal nature of causation in this system.
Central to these debates are different assumptions concerning the independence or interdependence of the causes of phenotypic variation, differential fitness and inheritance, which are Lewontin’s (1970) three conditions for evolution by natural selection (Walsh 2015; Uller & Helanterä 2019). Traditionally, evolutionary biologists have assumed these processes are quasi-independent, but in practice they are often causally intertwined.
