A skull built for digging must remain within certain limits of shape and size, and these limitations also restrict the prey of fossorial reptiles to soft-bodied prey. Burrowing reptiles tend to be very small, and have highly specialized skulls for burrowing into the soil. But if a trait results in specialization at many levels, could it increase the chances of extinction? Adapting to life underground (fossoriality) imposes specialization along multiple axes. These apparently contradictory results could be because species can be specialists in one aspect while being a generalist in another aspect. However, there appears to be mixed support for this hypothesis: some studies suggest that specialization increases diversification or that it has no effect. In such cases, specialists can have a higher chance of going extinct than generalists. But such specializations can become problematic when new environmental challenges emerge. When a species is a specialist, it can use specific resources and survive better in a specific environment than a so-called generalist species. Ecological specializations are often considered evolutionary dead-ends. Some traits also hinder diversification by increasing extinction rates, and these are termed evolutionary dead-ends. Variations in diversification rates across lineages are generally associated with the colonization of new geographic areas or the evolution of some trait or key innovation that allows some species to diversify rapidly. Differences in diversification rates - the overall rate at which new species accumulate (speciation rate) and disappear (extinction rate) - often explain such huge variations in the number of species in different groups. Why is there variation in the number of species between different groups of animals and plants? Why do some groups seem to be more "evolutionarily successful" with a higher number of species than other? For instance, there are over 10,000 species of birds but just one species of tuatara (a lizard-like reptile from New Zealand) even though both groups originated around the same time.
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