| Life histories lie at the heart of biology, because the tradeoffs each organism faces concerning the distribution of limited resources into either reproduction or maintenance and growth determine that organism's fitness. Generally speaking, an organism can choose to either invest in traits reducing age-specific mortality or to invest in traits increasing age-specific fecundity. Each decision in this regard leads to tradeoffs between current versus future reproduction, somatic maintenance or growth versus reproduction, or number, size and sex of offspring. Because of this, basic life history data provides the backbone for any organism-oriented research, effectively linking behavior, ecology, morphology, and physiology together. Not surprisingly, life-history evolution has therefore been an integral part of evolutionary research over the past decades.One family of teleosts fishes, the livebearing fishes (Poeciliidae), have proven to be excellent models for studying life history adaptations, and probably the best-studied poeciliid is the guppy, Poecilia reticulata. Originally, the more than 200 species and 22-29 genera of poeciliids were endemic to the Americas, but due to human introductions they are now found worldwide. Within Poeciliidae, at least three characters evolved to give rise to the present diversity within the family: (1) internal fertilization using a transformed anal fin, referred to as the gonopodium, for sperm transfer, (2) livebearing, and (3) different degrees of maternal provisioning for the developing young. With the sole exception of Tomeurus gracilis, which is characterized by oviparous egg retention, internal fertilization and viviparity are found in all species of poeciliids.Over the years, certain aspects of poeciliid life-history evolution have received particular attention. First and foremost, different predatory regimes have been demonstrated to drive rapid microevolutionary change in guppies, although evidence for macroevolutionary processes is as yet missing. Second, placental evolution and maternal provisioning strategies have been extensively studied, with the main focus of these studies on interspecific rather than intraspecific differences.The research detailed in my dissertation investigates several specific aspects of life-history evolution in livebearing fishes (Poeciliidae) that have so far been largely neglected. The first three chapters broaden our understanding of the importance of life-history evolution in the colonization of and ecological speciation in extreme habitats. Taken together, they highlight the relevance of life-history evolution in maintaining and even driving ecological speciation processes. The fourth chapter emphasizes the significance of environmental affects and intraspecific population differences on maternal provisioning strategies in livebearing fishes. Thus, cautioning against the traditional approach of investigating few ecologically similar populations and ascribing the identified patterns as rigid strategies for that particular species across its natural distribution. Finally, my last chapter opens the door for research on life histories as regulatory mechanisms for stability in the unisexual/bisexual mating system of Poecilia formosa, the Amazon molly. |
