Eco-evolutionary Dynamics
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Eco-evolutionary Dynamics

Andrew P. Hendry

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eBook - ePub

Eco-evolutionary Dynamics

Andrew P. Hendry

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About This Book

In recent years, scientists have realized that evolution can occur on timescales much shorter than the "long lapse of ages" emphasized by Darwin—in fact, evolutionary change is occurring all around us all the time. This book provides an authoritative and accessible introduction to eco-evolutionary dynamics, a cutting-edge new field that seeks to unify evolution and ecology into a common conceptual framework focusing on rapid and dynamic environmental and evolutionary change.Andrew Hendry covers key aspects of evolution, ecology, and their interactions. Topics range from natural selection, adaptive divergence, ecological speciation, and gene flow to population and community dynamics, ecosystem function, plasticity, and genomics. Hendry evaluates conceptual and methodological approaches, and draws on empirical data from natural populations—including those in human-disturbed environments—to tackle a number of classic and emerging research questions. He also discusses exciting new directions for future research at the intersection of ecology and evolution.An invaluable guide for students and researchers alike, Eco-evolutionary Dynamics reveals how evolution and ecology interact strongly on short timescales to shape the world we see around us.

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Chapter 1
Introduction and Conceptual Framework
Ecology and evolution1 are so closely intertwined as to be inseparable. This reality is obvious on long timescales given that different species are clearly adapted to different environments and have different effects on those environments (Darwin 1859). Yet, traditionally, evolutionary and ecological processes have been thought to play out on such different time scales that evolution could be safely ignored when considering contemporary ecological dynamics (Slobodkin 1961). However, the past few decades have seen a shift away from this “evolution as stage—ecology as play” perspective toward the realization that substantial evolutionary change can occur on very short time scales, such as only a few generations (reviews: Hendry and Kinnison 1999, Reznick and Ghalambor 2001, Carroll et al. 2007). If contemporary evolution can be this rapid, and if the traits of organisms2 influence their environment, it follows that evolution will need to be considered in the context of contemporary ecological dynamics. This point is not a new one (e.g., Chitty 1952, Levins 1968, Pimentel 1968, Antonovics 1976, Krebs 1978, Thompson 1998) but the growing realization of its importance is crystalizing into a new synthesis that seeks to integrate ecology and evolution into a single dynamic framework (Fussmann et al. 2007, Kinnison and Hairston Jr. 2007, Haloin and Strauss 2008, Hughes et al. 2008, Pelletier et al. 2009, Post and Palkovacs 2009, Schoener 2011, Genung et al. 2011, Matthews et al. 2011b, 2014, Strauss 2014, Duckworth and Aguillon 2015).
According to Web of Science and Google Scholar, the earliest use of the term “eco-evolutionary” was Kruckeberg (1969) and the first use of the term “eco-evolutionary dynamics” was Oloriz et al. (1991); yet modern usage really began with a 2007 special issue of Functional Ecology (Fussmann et al. 2007, Carroll et al. 2007, Kinnison and Hairston Jr. 2007). To illustrate, Web of Science tallies 18 articles that used “eco-evolutionary” in the title, abstract, or keywords prior to 2007 and 445 articles since that time (as of Apr. 8, 2016). Consistent with that modern usage, I here define eco-evolutionary dynamics as interactions between ecology and evolution that play out on contemporary time scales, with “contemporary” intended to encompass time scales on the order of years to centuries (or one to hundreds of generations). These interactions can work in either direction. In one, ecological changes lead to contemporary evolution (eco-to-evo), such as the ongoing adaptation of populations to changing environments. In the other direction, contemporary evolution can lead to ecological changes (evo-to-eco), such as when trait change in a focal species alters its population dynamics, influences the structure of its community, or alters processes in its ecosystem. Moreover, these interactions can feedback to influence one another: that is, ecological change can cause evolutionary change that then alters ecological change (Haloin and Strauss 2008, Strauss et al. 2008, Post and Palkovacs 2009, Genung et al. 2011, Strauss 2014). In this first chapter, I provide an overall conceptual framework for studying eco-evolutionary dynamics, and I explain how the rest of the book fits into that framework.
The style of this first chapter differs from those that follow. In this first chapter, I provide a very simple and general introduction that builds a framework on which to hang the more detailed deliberations that will follow later. I have therefore here written with a minimum of jargon, citations, and footnotes; and I have provided boxes that outline simple and clear examples. This writing style is intended to provide a stand-alone introduction accessible to all evolutionary biologists and ecologists, as opposed to only those already well versed in the topic. Rest assured, the subsequent chapters will be awash in enough jargon, citations, footnotes, and details to be of interest even to specialists.
Key elements of the book: phenotypes of real organisms in nature
When studying eco-evolutionary dynamics, one might focus on genotypes or phenotypes. My focus will be squarely on the latter: for two key reasons. First, selection acts directly on phenotypes rather than on genotypes. Genotypes are affected by selection only indirectly through their association with phenotypes that influence fitness. Understanding the role of ecology in shaping evolution therefore requires a phenotypic perspective. Second, the ecological effects of organisms are driven by their phenotypes rather than by their genotypes. Genotypes will have ecological effects only indirectly through their influence on phenotypes that have ecological effects. In some cases, eco-evolutionary dynamics might be similar at the genetic and phenotypic levels, most obviously so when a key functional trait is mainly determined by a single gene. However, this situation will be rare because most traits are polygenic and are also influenced by environmental (plastic) effects, topics considered at depth in later chapters. These two properties muddy (in interesting ways) the genotype-phenotype map and dictate that studies of eco-evolutionary dynamics should have, as their focus, organismal phenotypes. This focus does not mean that genotypes should be ignored and, indeed, genotypes are explicitly considered at many junctures in this book—but the central focus must be on phenotypes.3
Eco-evolutionary dynamics can be studied in theory or in real organisms. Theoretical studies, such as those employing analytical (symbolic) math or computer simulations, are critical for helping to delineate the various possibilities that arise from an explicit set of assumptions. Theory also can help to formalize conceptual frameworks, develop analytical tools, and evaluate predictive structures for the study of real organisms. For these reasons, theory will make frequent appearances in the book, typically as a means of setting up expectations and for helping to interpret the results of empirical studies. In the end, however, theory is only a guide to the possible—it can’t tell us what actually happens; and so an understanding of eco-evolutionary dynamics requires the study of real organisms.
Eco-evolutionary studies with real organisms could proceed in the laboratory or in nature. Advantages of the laboratory are manifold: populations can be genetically manipulated, environments can be carefully controlled, replicates and controls can be numerous, and small organisms with very short generation times (e.g., microbes) allow the long-term tracking of dynamics (Bell 2008, Kassen 2014). These properties dictate that eco-evolutionary studies in the laboratory are elegant and informative, yet only in a limited sense. That is, such studies tell us what happens when we impose a particular artificial environment on a particular artificial population and, hence, they cannot tell us what will actually happen for real populations in nature. Understanding eco-evolutionary dynamics as they play out in the natural world instead requires the study of natural populations in natural environments. I will therefore focus to the extent possible on natural contexts, although I certainly refer to laboratory studies when necessary.
The study of real populations in real environments is usually considered to be compromised in several respects. For instance, such studies have difficulty isolating a particular ecological or evolutionary effect because it might be confounded, or obscured, by all sorts of other effects that exist in the messy natural world. To me, this suggested weakness is actually a major strength because we obviously want to know the importance of a particular effect within the context of all other effects that also might be important. By contrast, it seems of limited value to isolate and evaluate a particular effect in a controlled situation if that effect is largely irrelevant in natural contexts. Moreover, elucidating causal effects and their interactions is possible even in nature through experimental manipulations (Reznick and Ghalambor 2005). However, the limitations of studying real populations and real environments are certainly real and important: replication and controls are harder to implement, experimental manipulations are less precise, and ethical and logistical concerns prevent some experiments. Yet such studies ultimately will be the key to developing a robust understanding of eco-evolutionary dynamics.
Conceptual framework and book outline
My primary goal in this first chapter is to provide a conceptual framework for eco-evolutionary dynamics. The framework will be presented in three parts. The first part (eco-to-evo) outlines how ecological change influences evolutionary change, and thereby amounts to a review and recasting of the classic field of evolutionary ecology. The second part (evo-to-eco) outlines how evolutionary change influences ecological change, and thereby amounts to the set of effects that have crystallized and driven the emergence of eco-evolutionary dynamics as a term and as a research field. The third part (underpinnings) considers the genetic and plastic basis of eco-evolutionary dynamics, which can apply with equal relevance to our understanding of both preceding parts. Within each of these parts, important components of the framework will be presented sequentially and their correspondence to the various chapters will be explained. In the current presentation, I will only rarely refer to specific empirical results because those results will be discussed in detail in the chapters that follow. Rather, I will provide a series of linked examples drawn from a single empirical system: Darwin’s finches on Galápagos (Grant 1999, Grant and Grant 2008). This choice of system doesn’t imply that Darwin’s finches provide the best illustration of every concept, but rather that they are suitable for explaining how different components of the conceptual framework fit together for a single well-known study system.
The eco-to-evo side of an eco-evolutionary framework obviously starts with ecology. By “ecology” in this context, I mean any combination of biotic or abiotic features of the environment that can impose selection on the phenotypes of some focal organism. In the context of a single population, I will generally refer to ecological change. In the context of multiple populations, I will generally refer to ecological differences. Either term might be used when generalizing to both contexts.
A single population in a stable environment should be characterized by phenotypes that are reasonably well adapted for that environment. Stated another way, the distribution of phenotypes in a population should correspond reasonably well to the phenotypes that provide high fitness (survival and reproductive success): that is, the distribution of phenotypes should be close to a fitness peak on the “adaptive landscape” (fig. 1.1). In this scenario, an obvious eco-to-evo driver is ecological change that shifts the fitness peak away from the phenotypic distribution. (A similar effect arises if the phenotypes shift away from the peak, such as through gene flow—see below.) This shift imposes selection on the population by increasing fitness variation among individuals with different phenotypes (Endler 1986, Bell 2008). If the phenotypic variation is heritable (passed on from parents to offspring), the next generation should see a phenotypic shift in the direction favored by selection: that is, toward the fitness peak. Under the right conditions, the phenotypic distribution should eventually approach the new peak and directional selection should disappear. In reality, peaks will be constantly shifting and populations might have difficulty adapting owing to genetic or other constraints as will be considered in detail later. Box 1 provides an illustrative example of directional selection and adaptation in Darwin’s finches.
Fig 1.1. Graphical representation of ecological change, directional selection, and adaptation in a single population. Before ecological change (time t), the frequency distribution of phenotypes (lower curve) in a well-adapted population is centered near the phenotypic value that maximizes fitness (optimum). This optimum corresponds to the peak of the “fitness function” (upper curve) that relates phenotypes (x-axis) to fitness (y-axis). Ecological change occurring to time t’ (upper dashed line) shifts the optimum phenotype to a new location, which imposes directional selection on the population, which should thus evolve toward the new optimum (lower dashed line). In reality, the fitness function is likely much wider than the phenotypic distribution. Also, this depiction assumes no constraints on evolution
Box 1
An example of natural selection and adaptation in Darwin’s finches on the small Galápagos island of Daphne Major (Boag and Grant 1981, Grant and Grant 1995, 2003). Conditions during 1976–1977 caused a drought that prevented reproduction by most plants. During this period, the resident population of medium ground finches (Geospiza fortis) rapidly depleted available seeds from the environment and many individuals starved to death, resulting in a population size decrease of about 85%. The depletion of seeds was nonrandom because all the finches can consume small/soft seeds whereas only finches with large beaks can consume large/hard seeds. As the drought progressed, the seed distribution therefore became increasingly biased toward larger/harder seeds, and the mortality of G. fortis became size-selective. Birds with larger beaks were more likely to survive, resulting in directional selection for larger beaks. When the rains commenced in 1978, the finches that had survived to breed were those whose beaks were larger (on average) than the population before the drought. Beak size is highly heritable (large-beaked parents produce large-beaked offspring) and, hence, the generation of birds produced after the drought had larger beak sizes than the generation of birds produced before the drought. Ecol...

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