Conifers arose in the Carboniferous, when Pangaea was still forming, and fossil evidence supports their once widespread distribution. As tectonic activity continued throughout the Mesozoic and Cenozoic, California conifers migrated and diverged from their Eurasian sister taxa in the late Cretaceous and Quaternary. Conifer forests of western North America were affected by Quaternary glaciations, which resulted in much fragmentation, refugial populations, and secondary contact. The Sherwin glacial maximum of 1 Ma influenced the divergence of many high elevation species at the ends of their ranges. Subsequent climatic fluctuations have resulted in the environmental stress and loss of populations intermediate to the northern and southern extremes of range, in the north in the Cascades and Klamath-Siskiyou region and in the south in the southern Sierra Nevada.
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