thumb|upright=1.35|The Baldwin effect compared to [[Lamarckism|Lamarck's theory of evolution, Darwinian evolution, and Waddington's genetic assimilation. All the theories offer explanations of how organisms respond to a changed environment with adaptive inherited change.]]
In evolutionary biology, what is now called the Baldwin effect describes the ways agency, imitation and learned behaviour can pioneer evolutionary change. It was first christened as such in the 1950s by George Gaylord Simpson, one of the architects of the modern synthesis, to bring attention to a process highlighted in the previous century by James Mark Baldwin.
Inspired to challenge late Victorian neo-darwinism by Darwin's own use of his theory of natural selection (in On the Origin of Species) to reframe the laws of use and disuse in terms of transitional habits—giving several examples of the ways different organisms' change of habits, as in flying squirrels and flightless beetles, have altered their anatomies' subsequent evolutionary fates—Baldwin and others re-emphasised that an organism's ability to learn new behaviours (e.g., to colonise new habitat or acclimatise to a new stressor) may affect its reproductive success and may, therefore, subsequently affect the genetic makeup of its species through natural selection, if supported by heritable traits. The Baldwin effect posits that, if such new habits prove advantageous, subsequent selection will reinforce those habits and any other structures they affect so that they will become instinctive or in-born over many generations. This process may appear similar to non-Darwinian Lamarckism, a view which proposes that living things may directly inherit their parents' acquired characteristics. But, in contrast to Lamarck, and echoing Darwin's argument about transitional habits in On the Origin of Species, Baldwin proposed that, only if supportable by heritable traits, can changed behaviour lead to adaptive evolutionary change.
The Baldwin effect has been independently proposed several times. It is generally recognized by proponents of the modern synthesis. And it has become a central plank of 21st century evolutionary biologies which challenge the 20th century's modern synthesis by retheorizing the leading role played by organisms' agency in the origin of species (see Extended Evolutionary Synthesis)
"A New Factor in Evolution"
By highlighting the Baldwin effect when he did, Simpson invited evolutionary biologists to reconsider the earlier claim made by Julian Huxley and others that, viewed through the prism of the modern synthesis, Darwin’s theory denied any role for the “purposiveness of organic structure” in the evolution of adaptations (most famously in Evolution: The Modern Synthesis). Baldwin’s 1896 paper "A New Factor in Evolution" had christened the 'new factor' Organic Selection, a label which became the title of his second paper on the topic published in 1897. These two papers specified three ways in which an organism might through its own changes become adaptively modified during its life-history: bodily or ‘physico-genetically’, neurally or ‘neuro-genetically’, and through 'conscious agency' or ‘psycho-genetically’. As the historian of science Robert Richards put it in 1987: