Sarah A Smith
Smith, S.A. (1996). The evolution of viviparity in the Australian scincid lizard, Saiphos equalis. Bachelor of Science honours thesis, University of Sydney, NSW, Australia.
Viviparity has evolved more frequently in squamate reptiles than in any other vertebrate lineage. Several reptile species have been reported to contain both oviparous and viviparous populations, and there are some reptilian species with reproductive modes that are intermediate between "normal" oviparity and viviparity. Both of these situations offer exception opportunities to test hypotheses concerning the evolution of viviparity in reptiles, and hence, the evolution of reproductive mode in vertebrates in general. However, both reproductively intermediate and bimodal species are rare. Saiphos equalis is an Australian semi-fossorial skink that had been reported to display different modes of reproduction in different populations. Early reports suggest that one of these modes may be intermediate between oviparity and viviparity. I have investigated the reproductive mode of Saiphos equalis and clarified the phylogenetic relationships of populations within Saiphos equalis. I have also investigated the relationships of Saiphos equalis and other Sphenomorphine skink species. These data enable me to test some of the predictions and assumptions from published hypotheses on the evolution of viviparity in reptiles.
My research has confirmed previous reports of bimodal reproduction in Saiphos equalis. Of the two populations I examined in detail, lizards from one population (Riamukka, in the northern highlands of NSW) were viviparous (i.e., produced fully formed young enclosed in membranes), whereas lizards from another population (Sydney) had an "intermediate" reproductive mode (i.e., produced less developed embryos in partially calcified shells). Experimental manipulation of the proximal environment of gravid females suggests that reproductive mode in this species is not a plastic response to short-term environmental conditions. Despite this variation in reproductive mode, Saiphos equalis appears to be a single species: individuals are morphologically homogeneous throughout their geographic range, although allozyme electrophoresis showed significant genetic divergence among populations. This genetic divergence was not correlated with reproductive mode or the geographic location of the populations. Phylogenetic analyses of the combined electrophoretic and morphological data suggests that there has been a single shift in reproductive mode within this species. Contrary to previous hypotheses for the relationships of the Sphenomorphine skinks, I have found that Saiphos belongs to a lineage of fossorial, oviparous skinks. Thus, the evolutionary change in reproductive mode within Saiphos is most likely to have involved the origin of viviparity (in the northern highlands of NSW), rather than a transition from viviparity to oviparity (in costal populations). In fact, my phylogenetic analyses suggest that there have been at least seven independent origins of viviparity within the Sphenomorphine skinks of southern Australia. This group includes Lerista bougainvillii, a second reproductively bimodal species. Thus, two of the three know reproductively bimodal species of reptile are burrowing Sphenomorphine skinks from south-eastern NSW (the third is the European Lacertid Lacerta vivipara). There are also a number of Sphenomorphines from which reproductive mode is unknown, suggesting future research may uncover more origins of viviparity within this group.
The geographic distribution of reproductive modes within these scincid lizards supports the major perdition of the "cold climate" hypothesis for the evolution of viviparity. Viviparous taxa occur in colder areas than their closest oviparous relatives, a pattern evident in the comparison between oviparous and viviparous populations of Saiphos, and between i and its closest relatives. A similar pattern of the relative distribution of oviparous and viviparous species exists for three other evolutionary transitions to viviparity, in the Sphenomorphus genera, Anomalopus, Glaphyromorphus and Lerista.