Figure 1.

Illustration of genetic, environmental, and epigenetic factors acting at various stages of an organism’s life history. (For simplicity, genotype-by-environment interactions are not depicted.) Epigenetic mechanisms are one of several developmental processes that are influenced by environment, especially during so-called critical periods (Waterland and Garza 1999). Mechanisms of epigenetic effects, which alter gene expression without changing DNA sequences, include DNA methylation and histone modification. “Ontogeny” can be used to describe the entire sequence from fertilization (conception) through development, growth, sexual maturation, aging, and senescence. Additional genes (not shown) may act not only at specific ages or stages but generally across most or all of ontogeny. Similarly, some environmental factors (e.g., pH of a lake in which fish live) may last for the entire lifecycle (subject to seasonal cycles). For many animals, immediate maternal environmental effects stop at weaning (e.g., for mice in a lab setting), but this is not necessarily the case for humans of for other animals in the wild, especially for species in which offspring tend to inherit their parents’ home ranges or territories.