Examine the numbers of amphibians and reptiles in various clades and a great disparity is readily apparent. The Lepidosauria (tuataras + (snakes+lizard)) is an excellent example, there are two species of living tuataras, but their sister group the Squamata (lizards+snakes) contains about 9000 species. This is not only true for snakes, salamander species number about 600, while frog species number more than 6000. Ideas to explain these kinds patterns have not been lacking. Species richness has usually been attributed to either the age of the clade (older clades have fewer species due to extinctions) or the rate of diversification (evolution rate of a particular clade) has been constrained by ecological factors, like the number of niches available.
In a forthcoming paper R. Alexander Pyron and Frank Burbrink examine global snake diversity and find it varies by two orders of magnitude in living lineages. Many older lineages contain only one or two species while a few younger clades may contain more than 700 species. They suggest that the patterns cannot be explained by background rates of speciation and extinction. Instead most of the diversity appears to derive from a radiation within the superfamily Colubroidea. After the colubroids evolved they invaded new geography and they evolution advanced venom-delivery systems. Pyron and Burbrink also found negative relationships between clade age, clade size, and the diversification rate suggesting the potential for possible bias in estimated diversification rates. This has been interpreted by some authors as support for ecologically mediated limits on diversity. However, evidence from the fossil record suggests that numerous clades were much more diverse in the past, and that extinction has been an important factor on the diversity patterns of living snakes. Thus, failure to adequately account for extinction appears to prevent both rate- and diversity-limited models from fully characterizing richness dynamics in snakes. The authors suggest that clade-level extinction may provide a key mechanism for explaining negative or hump-shaped relationships between clade age and diversity, and the prevalence of ancient, species-poor lineages in numerous groups.
Pyron, R. A. and Burbrink, F. T. (2011), Extinction, Ecological Opportunity, and the Orgins of Global Snake Diversity. Evolution doi: 10.1111/j.1558-5646.2011.01437.x