Asa Gray disjunction

The Asa Gray disjunction, also known as the Eastern Asian–Eastern North American disjunction, refers to a biogeographical pattern or biogeographical phenomenon in which closely related plant, fungi and animal species are found in temperate forests of eastern Asia and eastern North America, but not in Western North America or Europe. This striking distribution pattern was first systematically documented by American botanist Asa Gray in the mid-19th century, who noted the remarkable floristic similarities between the two distant regions. Gray’s observations were foundational in both plant and animal geography and evolutionary theory, providing early support for Charles Darwin's ideas of common descent and speciation. The phenomenon includes over 65 flowering plant genera alone.

Today, most scientists consider the Eastern Asian–Eastern North American disjunction to be best explained by the Tertiary relict hypothesis—the idea that many of the disjunct taxa are surviving remnants of a once widespread temperate forest flora that extended across the Northern Hemisphere during the Paleogene and early Neogene periods. Fossil evidence from Europe, Asia, and western North America confirms the historical presence of numerous genera now exhibiting disjunct distributions, including Magnolia, Liriodendron, and Cotinus, among others. Notably, fossil sites such as the Latah Formation and other deposits within the Columbia River Basalt Group demonstrate that many of the taxa now restricted to eastern Asia and eastern North America were present in western North America as recently as the Neogene, approximately 17 to 4 million years ago. The western fossils of these plants that today experience the disjunction are considered part of the Arcto-Tertiary Geoflora. This floristic kingdom primarily existed during the Paleocene–Eocene Thermal Maximum. Climatic shifts at the end of the Neogene caused fragmentation in the populations of many organisms, ultimately leading to the modern disjunction.