Conclusion
Combing distinct genetic clades from different geographic areas with correlative niche models and morphological evolution models, we have shown considerable variations in the degree of realized niche expansion and unfilling across the clades within S. boulengeri toads in the Qinghai-Tibet Plateau region. In the case of S. boulengeri toads, niche divergence occurs accompanied by niche expansion rather than unfilling, niche expansion are more prevalent than niche unfilling in E. A, E. B and E. C clades, while niche unfilling presents just in W. b clade.
Meanwhile, niche divergence caused by niche expansion indeed accompanies key morphological innovations of preadaption in novel climates than niche unfilling and stability. Factors such as enlarged body size and enhanced locomotor performance have been shown to increase expansion success by helping toads to cope with novel conditions.
Recognizing true niche shifts accompanied by key morphological innovations do exist, further assessments should seek to understand molecular mechanisms of key morphological innovations or related life strategies combined that have allowed these particular clades to expand their niches dramatically. It would be particularly interesting to use the same framework to test whether the same pattern is found in other organisms, especially in such widespread species or clades in future.