Live birth is associated with increased diversification rates in Cyprinodontiformes - an order of approximately 1,250 ray-finned fish, including killifish, mollies and guppies - finds a study in Nature Communications this week. The article describes the evolutionary history of this group of fish and examines traits that are associated with diversification.
Andrew Helmstetter and Vincent Savolainen produce a molecular-based evolutionary tree and use it to study the reproductive life-history strategies of the fish over about 70 million years of evolutionary history. In particular, they investigate the evolution of live birth (viviparity) and of one-year life cycles (annualism), traits hypothesized to enable diversification into new habitats. Although the authors find that each trait has arisen independently five times in the Cyprinodontiformes, only viviparity is associated with increased diversification rates.
These results contrast with previous studies in reptiles, which found that viviparity increased both speciation and extinction rates, with no net effect on diversification. Additionally, although the reverse transition from viviparity to egg laying has occurred during the evolution of reptiles, no such transitions from either viviparity to egg laying or from annualism to multi-year life cycles were found to occur in the fish.