Understanding and describing niche evolution is central to evolutionary ecology.   The widespread availability of species-level phylogenies and world-wide environmental climate layers has made possible meta-analyses of climate-niche evolution.  In an early application of such methods, Peterson et al. (1999) examined allopatric sister taxa of mammals, birds, and butterflies separated by the Isthmus of Tehuantepec in Mexico.  They built environmental niche models for each species and asked whether the models predicted the geographic ranges of sister species better than a null prediction based only on predicted range size.  Given that this null hypothesis was typically rejected for sister species, but not for confamilial non-sister taxa, Peterson et al. (1999) claimed that niches were generally conserved over millions of years.  We present new methods for analyzing niche differentiation, and illustrate these methods by reevaluating niche conservatism in these Mexican fauna.