Environmental niche models (ENMs), generated by combining species occurrence data with environmental GIS data, are increasingly used to study niche evolution, speciation, and ecological diversity. The question of whether environmental niches are conserved over evolutionary time has generated conflicting conclusions, often due to differences in how niche similarity is measured and the null hypotheses tested. This study develops new methods to quantify niche overlap using traditional ecological measures and statistical metrics. It reexamines a classic study of niche conservatism between sister species in Mexican animals and addresses alternative definitions of niche conservatism within a single framework. Results show that sister species have more similar environmental niches than expected under three null hypotheses, but rarely identical. The study also demonstrates how these measures can be used in phylogenetic comparative analyses, showing that environmental niche overlap is closely tied to geographic overlap but not phylogenetic distance, suggesting niche conservatism does not constrain local communities to closely related species. The study proposes randomization tests for use in ecology and evolutionary biology. It also introduces new metrics for quantifying niche similarity, including Schoener's statistic and Hellinger distance, and applies them to test niche equivalency and similarity. The study finds that niche models are more similar than expected by chance, but not identical, and that niche conservatism does not strongly constrain community composition. The results suggest that niche conservation should be treated as a continuum, as with phenotypic evolution or range overlap. The study also highlights the importance of considering environmental variables in niche evolution and the sensitivity of conclusions to the choice of variables. The robustness of results to ENM construction methods is discussed, emphasizing the need for careful consideration of methodological choices in ENM development.Environmental niche models (ENMs), generated by combining species occurrence data with environmental GIS data, are increasingly used to study niche evolution, speciation, and ecological diversity. The question of whether environmental niches are conserved over evolutionary time has generated conflicting conclusions, often due to differences in how niche similarity is measured and the null hypotheses tested. This study develops new methods to quantify niche overlap using traditional ecological measures and statistical metrics. It reexamines a classic study of niche conservatism between sister species in Mexican animals and addresses alternative definitions of niche conservatism within a single framework. Results show that sister species have more similar environmental niches than expected under three null hypotheses, but rarely identical. The study also demonstrates how these measures can be used in phylogenetic comparative analyses, showing that environmental niche overlap is closely tied to geographic overlap but not phylogenetic distance, suggesting niche conservatism does not constrain local communities to closely related species. The study proposes randomization tests for use in ecology and evolutionary biology. It also introduces new metrics for quantifying niche similarity, including Schoener's statistic and Hellinger distance, and applies them to test niche equivalency and similarity. The study finds that niche models are more similar than expected by chance, but not identical, and that niche conservatism does not strongly constrain community composition. The results suggest that niche conservation should be treated as a continuum, as with phenotypic evolution or range overlap. The study also highlights the importance of considering environmental variables in niche evolution and the sensitivity of conclusions to the choice of variables. The robustness of results to ENM construction methods is discussed, emphasizing the need for careful consideration of methodological choices in ENM development.