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SnowbirdIntegrative Biology and Evolutionary Diversity in the Junco$
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Ellen D. "Ketterson and Jonathan W. Atwell

Print publication date: 2016

Print ISBN-13: 9780226330778

Published to Chicago Scholarship Online: September 2016

DOI: 10.7208/chicago/9780226330808.001.0001

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A Physiological View of Population Divergence

A Physiological View of Population Divergence

Comparing Hormone Production and Response Mechanisms

(p.263) Chapter Eleven A Physiological View of Population Divergence

Christine M. Bergeon Burns

Kimberly A. Rosvall

University of Chicago Press

Coordinated expression of multiple traits by a hormone can promote an integrated phenotype. Such phenotypic integration can be adaptive if the traits mediated by hormones are most adaptive when co-expressed, but integration may have important consequences for the tempo or ease of evolutionary change. For example, tight integration may hinder the expression of new combinations of traits that may be favored under changing selection pressures. The degree of integration is likely to depend on lability of many links in the complex mechanistic pathway from environmental stimulus to phenotypic response in the face of selection. In this chapter, we explore divergence in testosterone (T)-mediated traits from a mechanistic perspective. We compare two phenotypically distinct dark-eyed junco subspecies, reviewing both upstream variation in T production mechanisms, and downstream variation in sensitivity to T at target tissues. Our analyses suggest that endocrine response mechanisms may be more prone to divergence than circulating hormone levels. An important pattern emerging from these studies is that existing sources of individual variation in endocrine mechanisms (upon which selection may currently act) are not always reflected in differences between populations (that may reflect past selection), posing questions about the evolutionary mechanisms driving diversification.

Keywords:   divergence, individual variation, phenotypic integration, testosterone, HPG axis, hormone receptor

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