Beyond birth order: The biological logic of personality variation among siblings

Notwithstanding their relatedness, siblings vary as much as strangers with respect to personality traits. Attempting to explain this paradox across many publications, Frank J. Sulloway invokes evolutionary theory, specifically emphasizing Malthusian competition and referencing the concept of adaptive radiation, which produced beak variation among Darwinian finches as they spread across the Galapagos Archipelago. However, Sulloway understands birth order and other familial dynamics to create personality variation among siblings, using evolutionary concepts only as illustrative comparisons. The present paper argues that Sulloway mistook a literal truth for an analogy. Sibling personality variation does not mirror a process of evolution, it is a process of evolution. Substituting the macroevolutionary process of adaptive radiation for the microevolutionary process of adaptive diversification, and emphasizing the perpetuation of genetic material above the survival of the organism, sibling personality variation is herein explained as a hedge against lineage extinction. Unable either to predict environmental challenges or create pluripotent offspring, parents diversify their brood and thereby diversify their risk. As discussed, sibling personality variation as an ontogenetic process of adaptation remains relevant, but only in so far as it augments a primary genetic process of evolution.

Abstract licence: CC BY

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