I.9 Phenotypic Plasticity
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I.9 Phenotypic Plasticity Joseph Travis OUTLINE 1. Introduction 2. The spectrum of phenotypic plasticity 3. The evolution of adaptive plasticity 4. The ecological importance of phenotypic plasticity 5. Horizons for future ecological research on phenotypic plasticity Phenotypic plasticity is the ability of an individual to express different features under different environmental conditions. Examples of plasticity surround us: plants have broader leaves when grown in shady conditions, and animals are smaller when they develop in crowded conditions. Although some of these changes reflect unavoidable consequences of adverse conditions, many of them are the product of natural selection molding an organism’s ability to survive and reproduce in a world whose conditions vary from time to time and from place to place. Put another way, many examples of phenotypic plasticity reflect the evolution of a developmental system that attempts to produce different traits under different conditions because no single trait is best suited for all conditions. Plasticity facilitates a species’ ability to occupy a variety of habitats, persist in uncertain environments, and stabilize its interactions with other species whose incidence and numbers change over time and across space. GLOSSARY carapace. The hard outer shell surrounding the bodies of small animals such as waterfleas and larger animals such as turtles. diapause. A state of arrested development in which the animal can survive long periods of challenging conditions such as low temperatures or drought by lying dormant. ectothermic animals. Animals that use external sources of heat for metabolism and whose rates of metabolism are closely linked to external temperatures, such as invertebrates, fish, amphibians, and reptiles. fitness. The number of offspring an individual leaves behind for the next generation; fitness has two major components, survival (or length of life) and reproductive rate. numerical stability. A steady-state equilibrium in population size, that is, numbers of individuals, to which a system will return if it is perturbed; stability in predator–prey systems refers to the numerical stability of both predator and prey that allows them to coexist indefinitely. phenotypic plasticity. The ability of an individual to express different features under different environmental conditions. 1. INTRODUCTION Phenotypic plasticity is the ability of an individual to express different features under different environmental conditions. This ‘‘adaptive plasticity’’ is one of the most remarkable products of Darwinian evolution. For adaptive plasticity to emerge, the developmental machinery to build different traits must be integrated with a sensory system that detects reliable cues about the prevailing environmental condition so that suitable traits are expressed in a timely manner. Adaptive plasticity is an interesting topic for evolutionary biology, but it is also an important topic in ecology. One reason is that plasticity can enable a species to cope with highly seasonal environments or occupy diverse habitats . But more subtly, plasticity can have a substantial effect on a variety of ecological processes and thereby act as an important influence on which species we see where and at what population sizes. 2. THE SPECTRUM OF PHENOTYPIC PLASTICITY Phenotypic plasticity can be either reversible or irreversible . The most obvious examples of reversible changes are behavioral responses to environmental conditions. For example, tadpoles change their foraging patterns in response to the presence of predators. When predators are removed, the tadpoles adjust accordingly. Other well-known reversible responses include physiological changes such as the increase in mitochondrial density in terrestrial vertebrates in response to experiencing lower oxygen levels and the changes in specific fatty acids incorporated into animal cell membranes in response to changing thermal conditions. Morphological changes can also be reversible: the gills of aquatic salamanders increase or decrease in response to oxygen levels in the water, and vertebrate muscles change in form and density in response to the amount of use they receive. As one might expect, reversible plasticity appears when environmental conditions change, often within an individual’s lifetime. In most cases, individuals retain the ability to change their features for most of their lives. The exception to this rule is diapause in insects and other arthropods. Diapause is a state of arrested development in which the animal can survive long periods of challenging conditions such as low temperatures or drought by lying dormant. When conditions improve, the animal breaks diapause and resumes normal activity and development. A species can enter diapause in only one stage, for example, eggs in crickets and larvae in beetles, and once broken, diapause cannot be reentered. Irreversible changes occur trivially when an organism adjusts the timing of a life history transition...


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