AN EMPHASIS in SALAMANDER EVOLUTION by JONATHAN
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ON THE ROLE OF HISTORICAL CONSTRAINT IN EVOLUTION: AN EMPHASIS IN SALAMANDER EVOLUTION By JONATHAN MICHAEL EASTMAN A dissertation submitted in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY WASHINGTON STATE UNIVERSITY School of Biological Sciences MAY 2010 To the Faculty of Washington State University: The members of the Committee appointed to examine the dissertation/thesis of JONATHAN MICHAEL EASTMAN find it satisfactory and recommend that it be accepted. ___________________________________ Andrew T. Storfer, Ph.D., Chair ___________________________________ Mark F. Dybdahl, Ph.D. ___________________________________ Eric H. Roalson, Ph.D. ___________________________________ Luke J. Harmon, Ph.D. ___________________________________ Matthew J. Parris, Ph.D. ii ACKNOWLEDGMENT Most especially, I would like to acknowledge my wife, Danielle, for her unbending love and support through what would seem to many to be intolerable. It feels like Christmas each morning. My family, both extended and immediate, has always encouraged scholarship, perseverance, and dedication. I thank them for their encouragement in this particular academic pursuit as well as for the inspiration and love they provide, always and forever. Much of what drives me is the endeavor to please my big brother, which likely began with front-yard football and ever-increasing ‘levels‘ of difficulty in our never-ending games. I thank him for the determination those scrimmages helped to instill. Many people have inspired thoughts that have directly contributed to the work contained herein and to my general worldview of evolution, to whom I am greatly indebted. Foremost in this regard is my doctoral advisor, Andrew Storfer, and dissertation committee: Mark Dybdahl, Luke Harmon, Matt Parris, and Eric Roalson. I thank Andrew for his friendship and support, and in risking the mentoring of a misdirected student with half-baked ideas. Of members of the committee, I am most appreciative of the generosity each has shown and insights provided in formal and, more often, in informal settings. Committee members are all too often the unsung and under-appreciated workhorses of academia. Many others have been influential in shaping my appreciation of science in general and its place in human culture and in guiding my understanding of processes shaping diversity in the natural world: Mike Alfaro, Edgar Boedeker, Chad Brock, Jim Demastes, Randy Mercer, ‘Captain’ Bob Nelson, Steve O’Kane, Kurt Pontasch, Jerry Soneson, Theresa Spradling, Jack Sullivan, Dave Tank, and ‘Pappy’ Paul Whitson. iii ON THE ROLE OF HISTORICAL CONSTRAINT IN EVOLUTION: AN EMPHASIS IN SALAMANDER EVOLUTION Abstract by Jonathan Michael Eastman, Ph.D. Washington State University May 2010 Chair: Andrew T. Storfer A fundamental challenge in evolutionary biology concerns estimating the extent to which ecological trade-offs impose constraints on evolution. In the first chapter, we test whether adaptive evolution of antipredator performance phenotypes in streamside salamanders was well predicted by stability in the selective regime experienced by salamander lineages. We found that the efficacy of antipredator phenotypes in salamanders is strongly related to historical duration, as well as consistency, of selection imposed by predatory fish on salamander larvae. Optimal circumstances for the evolution of effective antipredator responses in streamside salamanders seems to involve a long and consistent history of ecological contact with predatory fish. Evolutionary biologists have long debated the relative influence of species selection on evolutionary patterns. As a test in the second chapter, we assess whether variation in species’ ranges and in life-history contribute to patterns of diversification in salamanders. We find that paedotypy – wherein some organisms of a species mature in the gilled form without metamorphosing – is also associated with higher net diversification rates. Often dismissed as an insignificant process in evolution, this chapter, as well as the next, provides direct evidence for the role of species selection in lineage diversification of salamanders. iv The third chapter explores whether interspecific hybridism promotes or limits lineage diversification in salamanders. Using character-mediated diversification analyses and treating hybridism as a binary evolutionary character, results suggest that hybridism acts to swallow diversity. While evidence is provided for species selection generally disfavoring hybridism in a clade of amphibians, this chapter concludes with a discussion of circumstances where hybridism may be macroevolutionarily advantageous. The fourth chapter assesses whether coarse ecological differences among host populations are predictive of variation in forms of molecular selection in an emerging viral pathogen. We find elevated strengths of positive selection in pathogen strains associated with bait colonies of salamanders or in other captive-host environments. Given evidence of increased virulence associated with bait-associated strains of ATV, we argue that increased frequencies of tiger salamander epizootics in the western US are at least in part attributable to a relaxed virulence-transmission tradeoff in ATV-bait assemblages. v TABLE OF CONTENTS LIST OF TABLES vii LIST OF FIGURES viii INTRODUCTION 1 REFERENCES 3 CHAPTER FORMATS AND ATTRIBUTION 5 CHAPTER ONE 6 ABSTRACT 7 INTRODUCTION 8 METHODS 12 RESULTS 18 DISCUSSION 21 CONCLUSIONS 25 ACKNOWLEDGEMENTS 26 REFERENCES 26 CHAPTER TWO 32 ABSTRACT 33 INTRODUCTION 34 METHODS 37 RESULTS 46 DISCUSSION 52 CONCLUSIONS 58 ACKNOWLEDGEMENTS 60 REFERENCES 61 SUPPLEMENTARY MATERIAL 75 CHAPTER THREE 81 ABSTRACT 82 INTRODUCTION 83 METHODS 85 RESULTS 99 DISCUSSION 104 CONCLUSIONS 110 ACKNOWLEDGEMENTS 112 REFERENCES 114 SUPPLEMENTARY MATERIALS 130 CHAPTER FOUR 146 ABSTRACT 147 INTRODUCTION 148 METHODS 151 RESULTS 158 DISCUSSION 164 CONCLUSIONS 170 ACKNOWLEDGEMENTS 171 REFERENCES 172 vi LIST OF TABLES CHAPTER ONE Table 1 10 Table 2 20 CHAPTER TWO Table 1 40 Table 2 43 Table 3 47 Table 4 50 Table 5 51 Table 6 51 Supplementary Table 1 76 CHAPTER THREE Table 1 92 Table 2 99 Table 3 102 Table 4 102 Supplementary Table 1 131 CHAPTER FOUR Table 1 160 vii LIST OF FIGURES CHAPTER ONE Figure 1 9 Figure 2 17 Figure 3 19 Figure 4 20 CHAPTER TWO Figure 1 38 Figure 2 49 Figure 3 56 Supplementary Figure 1 79 Supplementary Figure 2 79 CHAPTER THREE Figure 1 88 Figure 2 89 Figure 3 97 Figure 4 100 Figure 5 108 Supplementary Figure 1 129 Supplementary Figure 2 129 Supplementary Figure 3 133 CHAPTER FOUR Figure 1 150 Figure 2 158 Figure 3 159 Figure 4 159 Figure 5 162 Figure 6 162 viii INTRODUCTION There are two senses of biological constraint, one of which has nearly been lost (Gould 2002): to constrain is either to compel or to confine. I do not regard these senses to be incongruous: to an unthinking evolutionary entity, it should not matter whether to be pushed from behind or led with a tether. Evolutionary constraint simply represents a state of affairs that permits only a narrowed set of outcomes while disallowing the remainder (see also Grantham 2004). If the recent analysis of a taxonomically broad dataset was insufficient evidence (see Estes and Arnold 2007), the very need for phylogenetic comparative methods is demonstrative of the pervasiveness of evolutionary constraint (Hansen and Houle 2004). Of particular interest to me is the nature and impact of constraint on the adaptive evolution of a lineage. Aside from fundamental inquiries into the strengths of particular constraints in lineages, I also take interest in the interrelation between constraint and novelty. How are such evolutionary constraints as stabilizing selection upset, eroded, or otherwise relaxed so that evolutionary novelty can arise? Contrary to the controversial claim of novelty arising through speciation (as proposed by Eldredge and Gould 1972), Futuyma (1987) argues and Gould (2002) concedes that the origination of novelty may often precede speciation, only that speciation solidifies the independence of the newly founded lineage. Futuyma (1987), in discussing apparent bursts of morphological evolution associated with speciation events, describes how admixture erodes any sign of genetic divergence. Only through speciation are the distinctive forms (that are already present) preserved. It may be likely that most populations maintain (or generate) sufficient variation so as to capably respond to ever changing (in both direction and strength) selective forces. Sufficient evidence of the claim is provided by the tremendous degree to which short term evolutionary rates appear to exceed those witnessed in geologic intervals (Lynch 1990; Williams 1992; Hansen and Houle 2004; Gingerich 2009). The apparent impediment to the generation of novelty would then seem to lie in something other than a lack of variation. But if sufficient variation exists and a “paradox of stasis” (Estes and Arnold 2007) permeates the fossil record (Eldredge and Gould 1972), we must ask: ‘Sufficient variation for what?’ It is interesting, and likely non-coincidental that adaptive radiation may well require the relaxation of 1 stabilizing selection (Roughgarden 1972; Lister 1976; Yoder et al., in press), an often sharp constraint. If there were no such constraints limiting the course of evolution of populations in exploring the adaptive landscape to happen upon empty peaks, we should expect few open niches. We also see tremendous rates of diversification in nearly every epoch, which seems to