The Origin of Instinct: How Darwin solved the greatest conceptual challenge of evolutionary theory

As a theory of evolution, the origin of instinct by means of natural selection was one of Darwin's most significant theoretical challenges. The narrative of how Darwin articulated this role combines his understanding of the advancements of other scientists with his own observations and research, leading to his development of the concept of group selection. In The Descent of Man (1871) Darwin defined group selection as selection that produces characteristics that help others, including non-relatives, in an individual's group (Ruse 36).

Darwin first theorized that habit, defined as an acquired mode of behavior, provided a naturalistic explanation for the instincts of animals. He thought that instinct resulted from acquired habits and "at each...exercise of instinct the Deity supplied the animal mind with the necessary "knowledge and design"" ("Instinct and Intelligence in British Natural Theology: Some Contributions to Darwin's Theory of the Evolution of Behavior" 18). Darwin saw several limitations to this association of acquired habit and his theory of natural selection.  First, it did not always appear that habits were inherited. And although acquired habits seemed to operate with domestic animals, it did not entirely explain the natural selection of behavior in colonial insects with sterile workers.

Initially Darwin's observations of domestic animals and colonial insects led him to believe that organisms acclimated themselves to their environment through habit, eventually resulting in determinate instincts. In the example of the worker bee, selection favored the survival of those queens that breed the most workers (Coe 270). Although individual sterile organisms of a colony appeared to jeopardize the theory of natural selection because they had no direct descendents to receive their variation, Darwin believed that these types of colonies had a selective advantage because proportionately more energy was spent in physical labor instead of reproduction. Neuter insects were "annually born, capable of work, but incapable of reproduction" (Coe 268). Though a well-developed concept, it still did not account for habit as the mechanism of instinct (Coe 270).

Early in the development of his evolutionary theory, Darwin did not have an adequate explanation for the inheritance of instinct in sterile colonial insects. He even commented in the Origin of Species (1859), "many instincts are so wonderful that their development will probably appear to the reader a difficulty sufficient to overthrow my whole theory" (Darwin 317). Two essays written in 1842 and 1844 influenced Darwin to alter his theory of behavior to the concept of unconscious intention. Using the explanation of group selection, a heritable trait may confer little or no benefit to an individual, but advances the adaptability of the group (The Cambridge Companion to Darwin 115). This solution meant that natural selection could also operate on units larger than the individual.

Darwin would later abandon habit as the principle mechanism of instinct, to concentrate more on group selection as its likely source ("Instinct and Intelligence in British Natural Theology: Some Contributions to Darwin's Theory of the Evolution of Behavior" 19). He developed group selection to account for colonial insects with sterile workers, where the sex of the offspring was determined by the food the larva receives, its environment, and the size of its cell ("Instinct and Intelligence in British Natural Theology: Some Contributions to Darwin's Theory of the Evolution of Behavior" 6).

Animal behavior was the subject of Lord Brougham's Dissertations (1839) ,which stimulated Darwin to consider the role of natural selection in the evolution of instinct ("Instinct and Intelligence in British Natural Theology: Some Contributions to Darwin's Theory of the Evolution of Behavior" 19). Brougham's view that instinct was a behavioral trait and that animals were unconscious of their actions convinced Darwin that his theory of evolution did not need a creationist foundation.

Brutes act from a principle, a thinking principle, a mental principle, something different from their bodies and from     surrounding objects, but that they act towards an end of which they are ignorant, and accomplish that end without design, though very possibly they may also in so acting accomplish some intermediate end of which they are aware, and which they intend to attain. (Brougham, 221)

As Brougham's ideas influenced Darwin's views of animal behavior, William Youatt's The Horse (1831) discussed the similarities between natural and man-made artificial selection.  Youatt described a horse breed that was created as a result of the mass killing of undersized horses during the reign of Kings Henry VII and VIII. Darwin was convinced that wild animals were subject to natural selection in a process similar to the domestic selection of animals, such as English horses.   

The Origin of Species continues to generate popular and scientific controversy to this day.  Darwin himself shaped his evolutionary theory according to contemporary controversies, proposing the concept of group selection in response to the inadequacy of habit as the principle mechanism to explain the heritability of instinct. Darwin "concluded that natural selection might act on the parents & continually preserve those which produced more & more aberrant offspring, having any structures or instincts advantageous to the community" (The Cambridge Companion to Darwin 101).

Alter, Stephen G. "Separated at Birth: The Interlinked Origins of Darwin's Unconscious Selection Concept and the Application of Sexual Selection to Race." Journal of History of Biology 40 (2007): 231-258.

Brougham, Henry Lord. Natural Theology: Comprising a Discourse of Natural Theology,Dialogues on Instinct, and Dissertations on the Structure of the Cells of Bees and Fossil Osteology. London: Richard Griffin and Company, 1856.

Coe, Charles Clement. Nature Versus Natural Selection: An Essay On Organic Evolution. London: Swan Sonnenschein & Co, 1895.

Hodge, J. and G. Radick, eds. The Cambridge Companion to Darwin. Cambridge: Cambridge UP, 2003.

Richards, Robert J. Darwin and the Emergence of Evolutionary Theories of Mind and Behavior. Chicago: University of Chicago Press, 1989.

Richards, Robert J. "Instinct and Intelligence in British Natural Theology: Some Contributions to Darwin's  Theory of the Evolution of Behavior." Journal of the History of Biology 14.2 (1981): 193-230.

Ruse, Michael. The Darwinian Paradigm: Essays on Its History, Philosophy, and Religious Implications. New York: Routledge, 1993.