Abstract
The great diversity of forms and functions in nature has fascinated generations of scientists and naturalists. With his seminal book On the Origin of Species (1859), Darwin postulated that populations change over time and evolve via natural selection on heritable phenotypic variability. Still, the origin of selectable variation remained a great puzzle to him until the end of his life, and this is expressed in his statement, “there is hardly any question in biology of more importance than this of nature and causes of variability” (quoted in chapter 2 by Costa and from1). Since then, the field of evolutionary biology has advanced greatly on resolving this issue, especially after the discovery of genes and how they tend to combine with environmental effects to create phenotypic variation. Nevertheless, even though it is generally recognized that the environment provides a major contribution to the phenotype (i.e. most traits have modest but non-zero heritability)2, how such environmental effects are relevant for evolutionary processes remains controversial.