pattern is reversed: 54% of adult toads are males, and only 19% are females. Regardless of which hypothesis is correct, their diurnal habits make color and pattern subject to increased selection. Lineage diversification and fusion Karlstrom was the first to hypothesize, based on the distribution of A. canorus in Yosemite National Park, that glaciers had helped the species differentiate from A. boreas. The idea was that repeated glacial action had helped isolate the species in montane areas, and gradual adaptation there had helped it competitively exclude A. boreas from high elevation terrain. Recent work has showed this is likely true, and part of a larger pattern of glacial action bifurcating the species into new lineages. The species likely originated in the early Pleistocene, and many of the glacial cycles since then have isolated toads into western and eastern refugia; here they have adapted to different climatic conditions, reinforcing the formation of new lineages. Where the lineages came back into secondary contact, they occasionally fused into additional hybrid lineages, in cases where pre-post-zygotic barriers were low enough to allow hybridization, but high enough to allow hybrids to become distinct from parental types. Four distinct lineages were discovered in Yosemite National Park, in addition to three fused (hybrid) lineages; two lineages were also found in Kings Canyon National Park. Conservation Yosemite toads are reported to have declined substantially in distribution and abundance across their restricted range. Although threats to persistence of the species are less understood than in other California amphibians such as the Sierra Nevada yellow-legged frog, the two most significant appear to be drought (increasing with climate change) and disease (primarily chytridiomycosis). Interactions between the two are not well understood. The species is listed as "Vulnerable" by the International Union for Conservation of Nature (IUCN), "Threatened" by the US Fish & Wildlife Service (USFWS) under the ESA, and "California species of special concern" by the California Department of Fish & Wildlife (CDFW). Systematics References External links California Herps Guide AmphibiaWeb Page Paul Maier Research on Yosemite toads US Fish & Wildlife Page Center for Biological Diversity Page Sierra Forest Legacy Page Anaxyrus Amphibians described in 1916 Amphibians of the United States Endemic fauna of California Fauna of the Sierra Nevada (United States) Yosemite National Park Taxonomy articles created by Polbot ESA threatened species