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Mechanisms of heterochronic change and stasis for clutch size in swifts (Apodiformes)

Haywood, Sacha
Biological journal of the Linnean Society 2014 v.113 no.4 pp. 1067-1079
Apodidae, clutch size, females, latitude, models, natural selection
Since the mid‐1800s, explanations in evolution have focused on adaptivity and natural selection, largely at the expense of factors internal to the organism. Today, the importance of internal factors is no longer in dispute. Progress continues to lag, however, on the design of a framework that would allow the integration of their action in evolution. Herein, as part of a wider effort aimed at testing the functionality of the general principle based upon the embryological concepts of induction, competence, and determination (dubbed developmental determination), I present a model for the evolution of clutch size in swifts. It is structured around the physiological mechanism controlling the trait. In indeterminate laying species, it involves an exogenous signal to the female, or induction, and the acquired ability to respond to this signal, or competence. The model explains, with the action of natural selection, how a 2‐day shift in the onset of competence induces change in clutch size among species. Through the origin of induction, it predicts the evolutionary transition from indeterminate to determinate laying. Finally, in some species that are indifferent to the general increase in clutch size with latitude, it reveals how both types of developmental transformation may operate to create stasis.