2. Evolution in the History of Population Thought

This chapter places evolutionary demography in the history of population thought, and more particularly in relations between demography and evolutionary population biology. Darwin conceived evolution as a dynamics of variation arising from the behaviour of populations at intra- and inter-species levels. While Malthus’s principle of population was an important early stimulus, Darwin resolved the core problem in evolution -- how mechanisms of variation combine to produce divergence of character -- by analogy to Smith’s account of the division of labour. With the benefit of hindsight, we can describe Darwinian population thinking as the first general methodology in which it became possible to combine bottom-up observation including enumeration of local population dynamics with top-down statistical methods. The two components entail different concepts of population, which may be characterised broadly as ‘open’ and ‘closed’. Their combination shows that evolutionary theory is rooted in the same sources of population thinking that gave rise to demography: the former lie in Classical population thinking and early modern population arithmetics, and the latter in 19th-century statistics and probability.

Hereditary influences remained a ‘black box’ in Darwin’s theory, which only began to be unpacked with the rediscovery of Mendel’s research. The second half of the chapter traces the central role which demographic methods played in topical and analytical developments of the first half of the 20th century, including both the formulation and critique of eugenics, the emergence of population ecology, and the rise of the mathematical theory of population genetics. There is an irony here: even as demographic methods came to play an integral role, mainstream demographers became less and less involved. The ‘separatism’ of demography and evolutionary biology often remarked in the post-war era thus has deeper roots. These lie partly in topical issues, like reactions against eugenics, but more importantly in a conceptual shift in how we understand relationships between ultimate and proximate mechanisms of population change, and its implications for analysis and modelling. Evolutionary theory entails a balance of methods and insights drawing on both population concepts, which demography has not yet achieved. The concluding section provides examples of how current evolutionary demography is now integrating these developments into demographic explanation.