The Baldwin effect, also known as Baldwinian evolution or ontogenic evolution, is a theory of a possible evolutionary process that was originally put forward in 1896 in a paper, "A New Factor in Evolution," by American psychologist James Mark Baldwin. The paper proposed a mechanism for specific selection for general learning ability. As Robert Richards explains:
If animals entered a new environment — or their old environment rapidly changed — those that could flexibly respond by learning new behaviors or by ontogenetically adapting would be naturally preserved. This saved remnant would, over several generations, have the opportunity to exhibit spontaneously congenital variations similar to their acquired traits and have these variations naturally selected. It would look as though the acquired traits had sunk into the hereditary substance in a Lamarckian fashion, but the process would really be neo-Darwinian. (p. 399)
Selected offspring would tend to have an increased capacity for learning new skills rather than being confined to genetically coded, relatively fixed abilities. In effect, it places emphasis on the fact that the sustained behavior of a species or group can shape the evolution of that species. The "Baldwin effect" is better understood in evolutionary developmental biology literature as a scenario in which a character or trait change occurring in an organism as a result of its interaction with its environment becomes gradually assimilated into its developmental genetic or epigenetic repertoire (Simpson, 1953; Newman, 2002). In the words of Daniel Dennett,
Thanks to the Baldwin effect, species can be said to pretest the efficacy of particular different designs by phenotypic (individual) exploration of the space of nearby possibilities. If a particularly winning setting is thereby discovered, this discovery will create a new selection pressure: organisms that are closer in the adaptive landscape to that discovery will have a clear advantage over those more distant. (p. 69, quoting Dennett, 1991)
An update to the Baldwin Effect was developed by Jean Piaget, Paul Weiss, and Conrad Waddington in the 1960s-1970s. This new version included an explicit role for the social in shaping subsequent natural change in humans (both evolutionary and developmental), with reference to alterations of selection pressures.
Suppose a species is threatened by a new predator and there is a behavior that makes it more difficult for the predator to kill individuals of the species. Individuals who learn the behavior more quickly will obviously be at an advantage. As time goes on, the ability to learn the behavior will improve (by genetic selection), and at some point it will seem to be an instinct.
Baldwin gives the following case involving cooperation: "Animals may be kept alive let us say in a given environment by social cooperation only; these transmit this social type of variation to posterity; thus social adaptation sets the direction of physical phylogeny and physical heredity is determined in part by this factor" (Baldwin, 1896, p. 553).
The appearance of lactose tolerance in human populations with a long tradition of raising domesticated animals for milk production has been suggested as another example. This argument holds that a feedback loop operates whereby a dairy culture increases the selective advantage from this genetic trait, while the average population genotype increases the collective rewards of a dairy culture.
The opposite of the Baldwin effect is 'shielding'. Modern medicine, for example, could artificially control a harmful pathogen, thus preventing any genetic immunity against it from being selected for. Here, learned behavior that improves fitness also prevents genetic adaptation.
The Baldwin effect theory has been controversial, with scholars being split between "Baldwin boosters" and "Baldwin skeptics". The theory was first called the "Baldwin effect" by George Gaylord Simpson in 1953 (p. 3). Simpson "admitted that the idea was theoretically consistent, that is, not inconsistent with the Modern Evolutionary Synthesis" (p. 4), but he doubted that the phenomenon occurred very often, or if so, could be proven to occur. In his discussion of the reception of the Baldwin-effect theory Simpson points out that the theory appears to provide a reconciliation between a neo-Darwinian and a neo-Lamarckian approach and that “Mendelism and later genetic theory so conclusively ruled out the extreme neo-Lamarckian position that reconciliation came to seem unnecessary”. Ernst Mayr, in Animal Species and Evolution, contends that the Baldwin-effect theory is untenable because (1) the argument is stated in terms of the individual genotype, whereas what is really exposed to the selection pressure is a phenotypically and genetically variable population, (2) it is not sufficiently emphasized that the degree of modification of the phenotype is in itself genetically controlled, and (3) it is assumed that phenotypic rigidity is selectively superior to phenotypic flexibility. Julian Huxley, however, in Evolution: The Modern Synthesis, gives more credit to the theory, which he considers important and to appear to have been unduly neglected by evolutionists.  In 1987 Geoffrey Hinton and Steven Nowlan demonstrated by computer simulation that learning can accelerate evolution, and they associated this with the Baldwin effect. 
Paul Griffiths suggests two reasons for the continuing interest in the Baldwin effect. The first is the role mind is understood to play in the effect. The second is the connection between development and evolution in the effect. Baldwin’s account of how neurophysiological and conscious mental factors may contribute to the effect  brings into focus the question of the possible survival value of consciousness.
There have been a number of arguments against the effect. For example, it has been argued that the change from learning to instinct might not constitute an improvement, because only very stable environments where change is extremely slow would favour innate traits as opposed to the plasticity of learning (especially social learning, which doesn't have such high costs as individual learning by trial-and-error). The very mechanism of the transition has also been questioned, as genetic variations which "tend to decouple [...] behaviour from environmental signals" might be "distant from those genotypes that mediate plastic, learned response".
Still, "it is striking that a rather diverse lot of contemporary evolutionary theorists, most of whom regard themselves as supporters of the Modern Synthesis, have of late become 'Baldwin boosters'" (p. 4). These Baldwin boosters
are typically evolutionary psychologists who are searching for scenarios in which a population can get itself by behavioral trial and error onto a "hard to find" part of the fitness landscape in which human brain, language, and mind can rapidly coevolve. They are searching for what Daniel Dennett, himself a Baldwin booster, calls an "evolutionary crane," an instrument to do some heavy lifting fast. (p. 4)
- Genetic assimilation
- James Mark Baldwin
- Phenotypic plasticity
- Weismann barrier
- Baldwin, J. Mark; A New Factor in Evolution, The American Naturalist, Vol. 30, No. 354 (June 1896), 441-451
- Osborn, Henry F.; Ontogenic and Phylogenic Variation, Science, New Series, Vol. 4, No. 100 (November 27, 1896), 786-789
- Baldwin, J. Mark; Organic Selection, Science, New Series, Vol. 5, No. 121 (April 23, 1897), 634-636
- Hall, Brian K.; Organic Selection: Proximate Environmental Effects on the Evolution of Morphology and Behaviour, Biology and Philosophy, 16: 215-237, 2001
- Bateson, Patrick; The Active Role of Behaviour in Evolution, Biology and Philosophy, 19: 283-298, 2004
- Sterelny, Kim; The Baldwin Effect and Its Significance: A Review of Bruce Weber and David Depew (eds) Evolution and Learning: The Baldwin Effect Reconsidered, Cambridge, MA: MIT Press, 2003, pp x, 341 To appear in: Evolution and Development
- Simpson, G. Gaylord; The Baldwin effect, Evolution, 7:110–117, 1953
- Newman, Stuart A.; Putting Genes in their place, Journal of Biosciences, 27:97-104, 2002
- Richards, Robert J.; Darwin and the Emergence of Evolutionary Theories of Mind and Behavior, Chicago: The University of Chicago Press, 1987.
The 10,000 Year Explosion: How Civilization Accelerated Human Evolution by Gregory Cochran and Henry Harpending (Oct 19, 2010)
- Richards, Robert J. (1987). Darwin and the Emergence of Evolutionary Theories of Mind and Behavior. Chicago: The University of Chicago Press. ISBN 0-226-71199-4.
- Dennett, Daniel (2003), "The Baldwin Effect, a Crane, not a Skyhook" in: Weber, Bruce H.; Depew, David J. (2003). Evolution and learning: The Baldwin effect reconsidered. Cambridge, MA: MIT Press. pp. 69–106. ISBN 0-262-23229-4.
- Burman, J. T. (2013). Updating the Baldwin Effect: The biological levels behind Piaget’s new theory. New Ideas in Psychology, 31(3), 363-373. doi:10.1016/j.newideapsych.2012.07.003
- Levy, Steven; Artificial Life - The Quest for a New Creation (1992), Jonathan Cape, pp. 268-269
- Depew, David J. (2003), "Baldwin Boosters, Baldwin Skeptics" in: Weber, Bruce H.; Depew, David J. (2003). Evolution and learning: The Baldwin effect reconsidered. Cambridge, MA: MIT Press. pp. 3–31. ISBN 0-262-23229-4.
- Simpson, George Gaylord (1953). "The Baldwin effect". Evolution 7(2): 110–117.
- Mayr,Ernst (1963). Animal Species and Evolution. Cambridge, MA: Harvard University Press. ISBN 0-674-03750-2.
- Huxley, Julian (1942). Evolution: The Modern Synthesis. London: George Allen & Unwin Ltd.
- Hinton, Geoffrey E.; Nowlan, Steven J. (1987). "How learning can guide evolution". Complex Systems 1: 495–502.
- Maynard Smith, John (1987). "When learning guides evolution". Nature 329: 761–762.
- Puentedura, Ruben R. (2003). "The Baldwin effect in the age of computation". In Weber,Bruce H.; Depew, David J. Evolution and Learning: The Baldwin Effect Reconsidered. Cambridge, MA: MIT press. pp. 219–234.
- Griffiths, Paul E. (2003). "Beyond the Baldwin effect: James Mark Baldwin’s ‘social heredity,’ epigenetic inheritance, and niche construction". In Weber, Bruce H.; Depew, David J. Evolution and Learning: The Baldwin Effect Reconsidered. Cambridge, MA: MIT press. pp. 193–215.
- Baldwin, J. Mark (1896). "Heredity and instinct". Science 3: 438–441, 558–561.
- Baldwin, J. Mark (1896). "Consciousness and evolution". The Psychological Review 3: 300–309.
- Baldwin, J. Mark (1896). "A new factor in evolution". The American Naturalist 30: 441–451, 536–553.
- Lindahl, B. I. B. (2001). "Consciousness, behavioural patterns and the direction of biological evolution: implications for the mind–brain problem". In Pylkkänen, Paavo; Vadén, Tere. Dimensions of Conscious Experience. Amsterdam: John Benjamins Publishing Company. pp. 73–99. ISBN 90-272-5157-6.