Op-icbj160047 121..125

Hormones are agents of biological coordination that circulate systemically to signal diverse cells and tissues, thereby influencing nearly all aspects of the phenotype, including behavior, morphology, physiology, and life history. Hormonal phenotypes can be both heritable and subject to natural selection (Bonier et al. 2009; McGlothlin et al. 2010; Ouyang et al. 2011; Pavitt et al. 2014; Cox et al. 2016, this issue), yet hormones and endocrine pathways have rarely been integrated into evolutionary models and analyses. As Garland et al. (2016, this issue) note this issue, ‘‘the seminal papers in modern evolutionary physiology scarcely mentioned the endocrine system.’’ Nevertheless, over the past two decades, the field of evolutionary endocrinology (Zera et al. 2007; Nepomnaschy et al. 2009) has emerged not only as a means of understanding the evolution of the endocrine system itself (Denver et al. 2009), but also as a framework for exploring the roles of hormones in shaping other evolutionary phenomena (Ketterson and Nolan 1999; Adkins-Regan 2008; Husak et al. 2009; Williams 2012). Originally centered on classic quantitative genetic approaches to the study of hormonal phenotypes themselves (Zera and Zhang 1995; Zera and Huang 1999), this field has expanded to include new ideas about the diverse roles of hormones as mediators of a variety of fundamental evolutionary phenomena. This theme of ‘‘hormones as mediators of evolutionary phenomena’’ serves as the organizing concept for this issue and can be illustrated by several examples drawn from the papers that follow. Hormones as mediators of phenotypic and genetic integration