Read an Excerpt
Snowbird
Integrative Biology and Evolutionary Diversity in the Junco
By Ellen D. Ketterson, Jonathan W. Atwell The University of Chicago Press
Copyright © 2016 The University of Chicago
All rights reserved.
ISBN: 978-0-226-33080-8
CHAPTER 1
Introduction
Ellen D. Ketterson and Jonathan W. Atwell
Snow borne on iced winds
has the juncos dancing on
one foot then the next
— Carol Knapp
A. Introduction
The avian genus Junco has served as a favored subject of biological research for at least the past 100 years. One of three species, the dark-eyed junco is among the commonest and most familiar North American passerines. It occurs across the continent and from northern Alaska to Baja California, and by one estimate (Folkard and Smith 1995) has a total population of about 630 million (compare current US census figures at 321 million). The public knows the junco not so much for its ubiquity and abundance as for the tameness and conspicuousness of its ground-foraging winter flocks. Wintering juncos are found in suburbs (often at feeders), at edges of parks and similar landscaped areas, around farms, and along rural roadsides and stream edges. Their plumage is not typical of sparrows: white outer tail feathers that flash when they take flight and a gray or blackish "hood" (head, nape, throat) and dark back that contrast with a white breast and belly. Audubon (1831) stated that "there is not an individual in the Union who does not know the little Snow-bird," and to many people "snowbird" is the junco's name today. The other three species in the genus Junco, the yellow-eyed junco, the Guadalupe junco, and the volcano junco, are birds of Middle and Central America and are far less well known, a gap in knowledge this book will help to fill.
Books about a single species or group of species, called monographs, are often single authored works that provide a comprehensive treatment of a taxonomic group. In the world of ornithology, monographs that focus on the behavior and ecology of a bird in nature, as opposed to its evolutionary relationships, are called life histories. The junco has already been the subject of a remarkable taxonomic monograph, Speciation in the Avian Genus Junco, by Alden Miller (1941) (see chapter 2, this volume, for more about Miller), and the subject of life histories including two that appeared in Birds of North America (BNA), a ten-year project including all the birds that breed in North America (Eaton 1968; Nolan, Jr. et al. 2002; Sullivan 1999) (see preface, this volume). The Cornell Laboratory of Ornithology currently hosts these BNA accounts online on a subscription basis, employing a model for dissemination that has only recently become possible. As new information on a species becomes available, authors (original or otherwise) can add to their species accounts to keep them current.
This book is not a remake of Miller's monograph or the BNA accounts; interested readers are encouraged to seek out these treatises for far more detailed information about the bird. Instead our purpose is to provide the reader with a synthesis of knowledge about a bird that has long interested biologists and to demonstrate why it is poised to serve as a model system during an era of research on organismal responses to changing environments. The authors strived to draw connections among published studies that were presented as standalone projects when first published. Many of the authors have collaborated before and share a common history. We hope that the closeness of the authors and the fact that the bird acts as a hub joining many diverse research areas will provide a sense of unity not always found in edited volumes.
B. What is This Book About?
The approach taken in this book about the junco draws on research concepts, methods, and findings from two major scientific disciplines, integrative organismal biology and evolutionary biology. From an organismal perspective, it addresses how animals match their physiology and behavior to their environment by relating endocrine and timing mechanisms to an animal's ability to know when to breed, when to migrate, and how to behave. From an evolutionary perspective, it employs the junco's challenging phylogeny as it has been interpreted over time by systematists and evolutionary biologists to address questions of how populations diverge and how species form. Knowledge of the junco's mating preferences and communication systems are included to help inform both the organismal and the evolutionary perspectives.
C. Integrative Organismal Biology in the Dark-Eyed Junco
This volume includes a synthesis of the decades of research conducted by the Ketterson-Nolan research group at Indiana University on numerous aspects of junco biology including the timing of migration, the role of sex and experience in distance migrated, courtship behavior and mate choice, parental behavior, plumage variation, hormone-mediated trade-offs in life histories, and phenotypic integration.
The key conceptual insights that have emerged from the organismal aspects of research on the junco are these: (1) Hormones are environmentally mediated molecules that act systemically and interact with target tissues to give rise to attributes referred to as traits that are referred to collectively as the phenotype; (2) coordinated trait expression mediated by a single hormone is referred to as hormonal pleiotropy; (3) if selection favors, or acts against, one or more hormonally mediated traits, other hormone-mediated traits may be dragged along, at least in the short run; (4) if coexpressed traits have multiplicative effects on fitness, then selection is referred to as correlational and can result in tight phenotypic integration; (5) dependence of traits on hormones is dynamic, so that selection can also act on tissue sensitivity to a hormone, and sensitivity to a hormone can come and go leading to less integration or phenotypic independence; (6) in the short run, tight phenotypic integration can act as a constraint on adaptive evolution; (7) phenotypic integration can also enable rapid adaptation in response to environmental change because it causes traits to co-occur in individuals when recombination might otherwise be required and take longer; (8) hormone-enabled rapid evolution can also lead to population divergence; and (9) the nature of the traits that hormones influence "makes sense" in terms of trade-offs in life histories (e.g., coordinating trait values that balance viability and sexual selection or pace of life).
We highlight key findings that (1) hormones can coordinate the expression of numerous traits (hormonal pleiotropy) (Ketterson and Nolan, Jr. 1999), potentially favoring phenotypes that are tightly integrated as a result of correlational selection (Ketterson et al. 2009; McGlothlin and Ketterson 2008); and (2) variation in hormonal pathways can predict population divergence on several time scales (Atwell et al. 2014; Bergeon Burns et al. 2013), a phenomenon with many underappreciated implications for the nature of rapid adaptation to changing environments.
D. Speciation/Population Divergence in the Dark-Eyed Junco
In the classic view, recognized groups of dark-eyed juncos diverged in geographic isolation, perhaps during glacial advances or even earlier (Klicka and Zink 1997). However, recent research from our contributors (Milá et al., chapter 8, this volume) has shown that genetic divergence in North America may have occurred as recently as the past 10,000 years (Milá et al. 2007). Not all biologists are convinced that the dark-eyed junco groups are so young or that they spent the periods of glacial advance only in Mesoamerica (Price and Hooper, chapter 9, this volume) (Klicka and Zink 1997). Regardless, whether the dark-eyed junco is one species or more, the rate of divergence has been extremely rapid, and the junco's degree of phenotypic and genetic differentiation requires explanation and bears squarely on the question of how animals respond to changing environments.
E. Merging Evolutionary and Integrative Organismal Biology
We hope this book will contribute to its objective of merging perspectives from evolution, behavior, and physiology through its focus on the importance of migratory and reproductive timing in population divergence. Traditionally, avian species were defined by morphological measures that manifested in preserved specimens (see color plate 1) (Milá et al. 2007), and modern systematists measure genetic divergence with an array of tools that have supplanted morphology. However, some of the critical traits leading to population divergence may not be gleaned from skins or sequences, and these relate to timing of population movements (i.e., migration) and reproduction (Winker 2010). In the case of the polytypic junco, some populations are sedentary, but many are migratory, exhibiting all degrees of movement from altitudinal to latitudinal, obligate to facultative, and partial to differential. The timing of migration varies geographically, and breeding dates also differ widely from north to south and along altitudinal gradients (Atwell et al. 2011).
Importantly, however, many junco groups overlap in their winter distributions, and southern populations initiate reproduction while northern populations are still present, which has a significant implication. Despite the opportunity to interbreed, timing mechanisms have apparently led to and maintain divergence. If we are to understand population divergence in this polytypic species, and other migratory species, then we need to understand not only differences in morphology and sequences but also differences in physiology. Also missing from skins and sequences is behavior, including the role that sexual selection plays in initiating and hastening divergence. Thus another goal of this book is to focus on the interaction of natural and sexual selection acting on physiology and behavior in addition to the role of hormonal pleiotropy in allowing coordinated, rapid response to environmental change.
F. New Insights Expected to Emerge from This Book
In addition to a cohesive overview of findings to date, we hope that this synthesis will extend the utility of prior findings by providing an explication that will (1) emphasize the generality of findings on the junco; (2) focus on hormones as both a proxy for genetic pleiotropy and an emergent property of pleiotropy, adding a layer of understanding to the causes and consequences of selection on correlated traits; (3) stress how hormone-mediated phenotypic plasticity, if followed by genetic assimilation, can give rise to evolutionary phenotypic divergence among populations that may — or may not — ultimately lead to speciation; and, finally, (4) explore the whole new landscape created by environmentally and hormonally driven variation in gene expression and phenotypic outcomes that relates to population divergence and will challenge and excite evolutionary and environmental biologists in the coming decade.
G. How to Use This Book
Readers will be confronted with many common and scientific names for the junco, some of which is unavoidable, and this section offers guidance. The answer to the question of how many species of juncos there are has not been resolved, in part because modern ornithologists disagree about how to define species and about the utility of the subspecies concept (Morrison 2010). Further, despite new techniques, which have generated molecular data to bear on relatedness, the interpretation of molecular data can still be a matter of opinion (Zink and Barrowclough 2008; Zink and Davis 1999). Thus, the dark-eyed junco was once five distinct species (see chapter 2, this volume): slate-colored (Junco hyemalis), white-winged (J. aikeni), Oregon (J. oreganus), Guadalupe (J. insularis), and gray-headed junco (J. caniceps), and all but J. aikeni and J. insularis had recognized subspecies with their own common names. In 1983, the American Ornithologists' Union merged these five species into one (the dark-eyed junco) and ceased to recognize subspecies (American Ornithologists' Union 1983) (see chapter 2, this volume). The yellow-eyed junco, J. phaeonotus, has a similar complex naming history (e.g., Baird's junco, Mexican junco), which will be addressed in chapters 2 and 8. The decision we made for this volume is (1) to acknowledge the primacy of the AOU when naming species, but (2) to use the common names for subspecies found in the Clements checklist (Clements et al. 2012) because they will be useful to the authors and the reader, and (3) to use the taxonomy suggested by Milá in chapter 8, this volume, because it is the most current.
Readers will also be confronted with disciplinary terminology from avian endocrinology and evolutionary biology, much of which is also unavoidable. We provide a glossary, which deliberately strips many terms to their essentials and which was borrowed partially from the Princeton Guide to Evolution (Ketterson et al. 2013) and from chapter 9 (this volume). Eight pages of color figures appear in the book's center, including (1) a photo of a "mixed" wintering flock of juncos including several different subspecies; (2) a map of the breeding distributions of junco species and subspecies; (3) a rendering of the juncos' current phylogeny; (4) and (5) "head shots" of twelve different forms of the junco; (6) a cartoon portrayal of the inputs and outputs of the hypothalamic-pituitary-gonadal axis in relation to the hormone testosterone; (7) a visual overview of a contemporary junco colonization of a novel environment in San Diego, California; and (8) a junco in flight and a close up of a feather plumage ornament (tail white). Some of these figures are also referred to in specific chapters, and others are standalone.
We hope these guides and statements of our objectives will help the reader enjoy the potential for future insights to be gleaned from study of the junco.
Acknowledgments
We thank all the colleagues, postdocs, and graduate students who have been part of the junco project over the years, as well as the numerous field assistants who provided their efforts and good will. We thank our editor Christopher Chung and three anonymous reviewers whose comments substantially improved this chapter.
CHAPTER 2
The Junco
A Common Bird and a Classic Subject for Descriptive and Experimental Studies in Evolutionary and Integrative Biology
Ellen D. Ketterson and Jonathan W. Atwell
Little mouse-bird, so soft and so gray,
You brighten our sight on a dull winter day,
So trim and so neat in your drab little coat,
So gentle and sweet is your musical note!
Junco, I'll feed you and welcome you here.
Come, stay near my home where there's nothing to fear.
— Julius King, 1934, Birds, Book 2 (Cleveland: Harter Publishing Company)
(Continues...)
Excerpted from Snowbird by Ellen D. Ketterson, Jonathan W. Atwell. Copyright © 2016 The University of Chicago. Excerpted by permission of The University of Chicago Press.
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