The theory of life history evolution, which began to develop in the 1930's and matured through the 1980's, represents a generalization of the concept of fitness. Fitness is now defined with complete variables, such as the ability of a genotype to increase in population size. Under this theory, natural selection can do seemingly counter-intuitive things. For example, it is possible for natural selection to favor a reduction in lifespan or a reduction in the number of offspring produced, provided that there are other compensations. The goal of this resolves is to test life history theory in natural populations of guppies from the island of Trinidad. "Life history" refers to features of animals that directly influence the production of offspring, such as when animals begin to reproduce, how often they reproduce, how many offspring they have, and the resources devoted to each offspring. Most natural populations are exposed to different patterns of mortality, such as different rates of mortality in adults versus juveniles. Theoreticians have considered how this aspect of natural selection will select for changes in features of the life history. Different populations of guppies in Trinidad appear to experience such differences in age-specific mortality because of their predators. Some populations have predators that prey predominantly on large, sexually mature size classes of guppies while others have predators that feed primarily on small, immature size classes. Size-specific predation appears to cause differences in age-specific mortality rates. One prediction is that higher adult mortality rates will favor those individuals that begin to reproduce earlier and devote more of their resources to reproduction and less to their own growth and maintenance. These predictions are being evaluated by; 1) quantifying the differences in life history patterns of guppies from these two types of localities, 2) quantifying the mortality rates of guppies in natural populations, and 3) changing the distribution of guppies and their predators in natural streams to drive the evolution of life history patterns. This project has already revealed the process of evolutionary change in natural populations and will ultimately make it possible to evaluate the mechanism of this change. At the same time, it evaluates a branch of evolutionary theory which incorporates a more general definition of fitness.
