Aspects of life history patterns which may influence species choice include developmental rate, age at first reproduction, frequency and timing of reproduction, gestation length, litter size and life span. These parameters can be important in and of themselves, e.g., species with relatively short life spans are sometimes the most useful choice for models of the long term effects of early ontogenetic factors. Life history parameters may also combine with other species characteristics to influence the appropriateness of model choice, e.g., litter size is an important consideration when studying a naturally occurring genetic condition since a large litter will increase the likelihood that some off-spring will carry the trait.