Many features of an organism that are genetically based are also subject to environmental influences. In humans, for example, taller parents tend to have taller children; yet if a child of tall parents is raised with poor nutrition, it will not grow to its full potential. Why some traits can be altered by environmental conditions (i.e. are plastic) while others are invariant is a central problem in evolutionary biology. One hypothesis is that plasticity is adaptive because the expression of the trait shifts to appropriately match the environmental conditions. However, this adaptive plasticity is only expected if a population has evolved under a range of environmental conditions. Alternatively, if a population has evolved under a particular environmental condition, it is not expected that it will respond adaptively to range of environments it has never experienced. Guppies (Poecilia reticulata) give birth to live young. The size of these offspring at birth shifts plastically in response to the environmental conditions faced by the mother. In addition, offspring size differs genetically across populations that experience different environmental conditions. Using a series of laboratory and field experiments, we will test whether these genetic and plastic shifts in guppy offspring size are adaptive. By examining the effect of offspring size on predation risk, growth rate and offsprings reproductive rate, we will determine to what extent offspring size variation is suited to a particular environment or range of environments. By furthering our understanding of how both the plastic and fixed (genetic) features of a trait evolve, this study will enhance our ability to predict how organisms will respond to both short- and long-term environmental changes.
