Darwin described how animal populations have two adaptation strategies: natural selection based on genetic variation acting over many generations, and developmental plasticity acting within the lifetime of an individual. The contribution of these processes can be difficult to distinguish at the individual level, and the relevance of human developmental plasticity to aging and health in later life is only now being recognized. The formal definition of developmental plasticity is ‘the ability of a single genotype to produce more than one alternative form of structure, physiological state or behaviour in response to environmental conditions’. As an alternative to adaptation to changing environments through genetic diversification, living things have evolved plastic responses. This enables the production of phenotypes that are better suited to their environment than would be possible if the same phenotype was produced regardless of environmental conditions