The geographic variation of taxonomic richness may be directly determined by climate through contemporaneous/ecological processes, versus other (e.g., historical/evolutionary processes) that happen to be collinear with contemporaneous climate. In Chapter 1 I evaluated hypotheses from both groups of explanations in North America. If contemporaneous climate controls patterns of richness, then richness should vary with climate through time in the same way that richness varies with current climate through space. Over the last ca. 11,000 yr, richness-temperature relationships remained reasonably constant. Between 12,000 and 14,000 yr BP, when climate fluctuated rapidly, richness gradients as a function of temperature were significantly shallower. If historical climate over the last 21,000 years determines patterns of richness, then historical climate should be a better predictor of richness than contemporaneous climate. I rejected historical-climate as a better predictor of richness. Contemporaneous climate stands as the most plausible explanation for contemporaneous patterns of richness, at least over the last 11,000 yr. In Chapter two, I tested the prediction that richness of most taxa should increase with temperature in all but the warmest and driest areas. Climate warming during Pleistocene-Holocene transition led richness increases in wet areas, but richness declines in dry regions, as expected from current richness-climate relationships. A decline in small mammal species richness in Northern California since the late Pleistocene was expected from the current richness-climate relationship for this group in North America. These results contest the view that future global warming may lead to species extinction rates that would qualify as the sixth mass extinction in the history of the earth. In chapter three, I first tested the hypothesis that richness gradients mainly reflect the sum of individual species climatic tolerances. I tested this hypothesis for birds, mammals and trees native to eastern North America (ENA, where there are no major barriers to dispersal). The number of species present in any given area in ENA is usually much smaller than the number of species in the continental pool that tolerate the climatic conditions in that area. Second, I tested several explanations for patterns of unfilled potential richness. Unfilled potential richness is inconsistent with postglacial dispersal lags, climatic variability since the Last Glacial Maximum, or with biotic interactions. In contrast, unfilled richness is highly consistent with a probabilistic model of species climate occupancy. Individual species climatic tolerances is not the process generating the main current patterns of richness, nor are post-glacial dispersal lags, climatic variability since the LGM or biotic interactions. This thesis is consistent with the hypothesis that contemporaneous climate directly controls spatial patterns of richness. Generally, there seems to be little need to invoke historical processes as determinants of current gradients of richness.