Several promising hypotheses have the potential to explain the dramatic increase in diversity towards. We tested the Evolutionary Speed Hypothesis, which proposes that species richness increases towards tropics because of temperature-induced increases in rates speciation. To test it we used E. coli in a long-term stationary phase. E. coli cultures were grown at different temperatures, 25°, 30°, 34°, 37°, and 39° C in a complex standard medium. We tracked phenotypic diversity associated with rpoS, an important sigma factor, using colony color assays that reveal the degree of mutation in rpoS. Consistent with the evolutionary speed hypothesis, warmer populations diversified more rapidly. Enhanced evolutionary speed may result from any of three mechanisms: shorter generation times, higher mutation rates, and accelerated selection. Our results could not discriminate among these. Nonetheless, our results were consistent with the basic prediction that warmer temperatures alone, in the absence of spatial heterogeneity or top down trophic interactions, enhanced diversification rates.