When two populations experience different selective environments they are expected to diverge in life history strategy to optimize individual fitness. Environmental temperature can have a profound impact on aquatic ectotherm life histories, in particular. Growth rates, overwinter mortality and length of breeding season are all characteristics that are influenced by local temperature environments. Two closely related, livebearing mosquitofishes, Gambusia affinis and G. nobilis have allopatrically diverged in two very different temperature environments. Gambusia affinis is typically found in habitats with high daily and seasonal fluctuations in temperature and is considered highly invasive. Gambusia nobilis, an endangered species, is found in spring-fed habitats that are very stable in temperature throughout the year. These two species present an interesting model system in which to examine how adaptation to divergent local temperature environments may have driven divergence in life history strategy. We predicted that the nature of the divergent aquatic habitats, eurythermal and stenothermal, has given rise to divergent life history strategies that more close resemble a classic R and K strategy, respectively. To determine differences in life history strategy we examined the characteristics of both species in a field study, a laboratory breeding study and dissection of museum specimens.
Data were collected on traits including egg size, brood size, maternal investment, age and size distributions, and sex ratio. Both species have equal maternal investment per brood and appear to have similar investment over a lifetime. The investment strategy, however, is very different. Gambusia affinis is an annual species. Females have a gestation period of approximately one month and have 3-5 broods in a given year. Embryo size is at least half that of G. nobilis’ embryos. Brood size is large although it decreases throughout a given breeding season. Growth rates are higher for G. affinis and sexual maturity is typically attained at smaller body size. Gambusia nobilis females appear to live for multiple years. They exhibit slow growth rates and larger size at sexual maturity. Their embryos and offspring are larger than those of G. affinis but brood size is much smaller and decreases in a breeding season. Gambusia nobilis exhibits great variability in embryo size but G. affinis embryo size is highly constrained. These two life history strategies appear to reflect differences in environmental temperature regimes and may help reinforce unique species’ identities in secondary contact zones.