Reproductive modes and strategies in vertebrate evolution

Vertebrate reproductive modes, which are characterized by egg and development type, have been compared and, in some cases, re-defined. Reproductive strategies regulate reproductive effort. Reproductive modes and strategies have been selected in species history: more anciently and with more conservative characters the former; more recently and with some flexibility in response to environmental changes the latter. Semelparity is the expression of entire reproductive effort in a single event in the life cycle; iteroparity, in several events. In iteroparous species living in uniform, aseasonal environments, continuous reproduction can be also found. However, in fluctuating environments, seasonality concentrates reproduction only in the most favourable period of the year (seasonal reproduction). Several mechanisms (i.e. refractoriness) can block reproductive processes in unfavourable periods, avoiding energy waste. Different strategies such as male sperm storage, delayed fertilization, embryonic diapause, have been selected in seasonal breeders, optimizing reproductive effort. In all vertebrate classes, except birds (all oviparous), oviparity and viviparity may co-exist in a genus or a species. Viviparity has been attained many times independently in evolutionary history of vertebrates, as an adaptive response to particular environments (i.e. acquatic environment, cold climate, deserts). Egg retention and ovoviviparity (aplacental viviparity) are considered transitional to placental viviparity; internal fertilization, modifications in oviducal structures and secretions, evolution of a corpus luteum secreting progesterone, are prerequisite for evolution to viviparity. Iteroparity, seasonal reproduction and viviparity have allowed species continuity and individual survival to be combined in reproductive effort.