These simulations assumed that all speciation events followed the "budding cladogenesis model" as referred to above and thus maximized the difference between ages of species.

Figure 1: The relationship between morphological variance and number of species for 95 passerine clades. The environmental template undoubtedly is clade-specific and difficult to evaluate ...

Rather, the cladogenesis events we observed required that changes in resource utilization incurred a tradeoff in competitive ability within the parental niche (MacLean and Bell, 2003).