Experimental Evolution: Concepts, Methods, and Applications of Selection Experiments

9. Fitness, Demography, and Population Dynamics in Laboratory Experiments

197 9 FITNESS, DEMOGRAPHY, AND POPULATION DYNAMICS IN LABORATORY EXPERIMENTS Laurence D. Mueller EARLY FOUNDATIONS OF POPULATION BIOLOGY EMPIRICAL MEASURES OF FITNESS ECOLOGICAL AND EVOLUTIONARY STUDIES OF POPULATION DYNAMICS Experimental Systems Ecological Models Early Theories of Density Dependence in Evolutionary Ecology Measurements of the Density Dependence of Fitness Evolution of Growth Rates Evolution of Population Stability DISCUSSION Experimental Evolution: Concepts, Methods, and Applications of Selection Experiments, edited by Theodore Garland, Jr., and Michael R. Rose. Copyright © by the Regents of the University of California. All rights of reproduction in any form reserved. EARLY FOUNDATIONS OF POPULATION BIOLOGY An important theme in this review will be the importance of a close balance between theory and experiments. Theory has as a goal the creation of simple and general principles. Understandably , therefore, theoreticians are often loath to spend too much time and energy worrying about the details of any specific organism’s biology. However, experimental population biologists must determine if the assumptions of the specific theories that they wish to test are biologically unrealistic for the particular experimental organism that they wish to study. It is perilous for the experimental biologist to rely on the theoretician to determine if a particular organism or experimental design is appropriate for a test of the theory that they are both interested in. Another important aspect of the work reviewed here is the interweaving of problems in ecology and evolution. Early in the history of these fields, topics like population dynamics were relegated to the realm of ecology, while the fields of evolution and population genetics considered problems like fitness components and methods for their estimation , ignoring ecological context. But over the course of the twentieth century, it became clear that population dynamics are affected by the organisms involved within what had been considered an “ecological” time frame. This idea was first treated in detail in the theoretical work of MacArthur (1962), but later the evolution of population growth rates became the focus of experimental studies (Mueller and Ayala 1981c). In a similar vein, the impact of life cycles and population regulation on fitness measurements and evolution was first clearly demonstrated by the work of Prout (1965, 1971a, 1971b, 1980). While this commingling of ecological and evolutionary research is still developing, there has already been significant progress over the last thirty years (see also Irschick and Reznick this volume). Some of the earliest work in experimental population biology was carried out early in the twentieth century by Raymond Pearl using laboratory populations of Drosophila. His choice of fruit flies as a model system was in part an accident. A fire destroyed Pearl’s laboratory and his mouse colony, forcing him to reconsider his plan to pursue questions in population biology using mice. On the advice of T. H. Morgan, he decided to study fruit flies. By 1919, Pearl had his experimental fly populations established, and experimental ecology was born. The most enduring legacy of Pearl’s work is not his experimental findings; in fact, his experimental methods are by today’s standards unacceptably imprecise. Pearl’s legacy was actually his focus on the interaction between experiment and theory. Pearl had interests in human population growth and so always viewed his fly experiments as models for other organisms. Pearl was also interested in using simple models to describe his experimental results. Because these theoretical models, like logistic population growth, were offered as general principles of population dynamics by Pearl, they caught the attention and interest of many people. Ultimately, they led others to consider more seriously the techniques used for determining population growth rates experimentally. 198 • L E V E L S O F O B S E R V A T I O N Pearl was interested in human population growth, but he thought that there must be a universal law of population growth that would apply to all organisms, including humans and fruit flies. He began his experimental research with D. melanogaster. His experimental results apparently closely followed the simple logistic equation (figure 9.1). The techniques used to maintain Pearl’s flies were somewhat haphazard, however. He would supply food to the flies at irregular intervals in varying amounts whenever it seemed as if food was needed. Pearl described his procedure this way: “The second type of experiment is one in which an attempt is made to add food as the supply is used up. The technical difficulties of doing this satisfactorily with...