Maternal investment evolves with larger body size and higher diversification rate in sharks and rays

Across vertebrates, live bearing evolved at least 150 times from ancestral egg laying into diverse forms and degrees of prepartum maternal investment.^1,2 A key question is how reproductive diversity arose and whether reproductive diversification underlies species diversification.^{3,4,5,6,7,8,9,10,11} To test this, we evaluate the most basal jawed vertebrates: the sharks, rays, and chimaeras, which have one of the greatest ranges of reproductive and ecological diversity among vertebrates.^2,12 We reconstruct the sequence of reproductive mode evolution across a phylogeny of 610 chondrichthyans.¹³ We reveal egg laying as ancestral, with live bearing evolving at least seven times. Matrotrophy evolved at least 15 times, with evidence of one reversal. In sharks, transitions to live bearing and matrotrophy are more prevalent in larger-bodied tropical species. Further, the evolution of live bearing is associated with a near doubling of the diversification rate, but there is only a small increase associated with the appearance of matrotrophy. Although pre-copulatory sexual selection is associated with increased rates of speciation in teleosts,³ sexual size dimorphism in chondrichthyans does not appear to be related to sexual selection,^14,15 and instead we find increased rates of speciation associated with the colonization of novel habitats. This highlights a potential key difference between chondrichthyans and other fishes, specifically a slower rate of evolution of reproductive isolation following speciation, suggesting different rate-limiting mechanisms for diversification between these clades.¹⁶ The chondrichthyan diversification and radiation, particularly throughout shallow tropical shelf seas and oceanic pelagic habitats, appear to be associated with the evolution of live bearing and proliferation of a wide range of maternal investment in developing offspring.