Biogeography, Diversification, and Domestication in the Coca Family (Erythroxylaceae)
White, Dawson M
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The Coca family (Erythroxylaceae) is most infamously known as the natural source of cocaine, isolated from four South American taxa called coca, but it also comprises ca. 283 more species of trees and shrubs distributed in tropical habitats throughout the world. This study has two major foci: First, a population-level analysis of diversity and ancestry of the cultivated cocas and their closest relatives has revealed a hypothesis that coca has been domesticated from the wild species, Erythroxylum gracilipes, two or three times. This supports a paradigm that different Holocene peoples were able to breed the same natural resource into domestication to serve their needs; in this case, a mild workaday stimulant and medicine. The second focus describes the biogeography and patterns of diversification of the Coca family as it evolved and migrated around the tropical regions of the world. The study finds that the clade originated in Africa in the late Cretaceous before migrating into the Indo-Pacific region as well as the Americas. The timing of this migration out of Africa appears to have occurred ~50 Ma and dispersal into the Americas might have been facilitated by a north Atlantic land bridge in combination with the warm climates of the early Eocene climatic optimum. We also find evidence for a diversification rate increase at the origin of the Erythroxylum lineage concurrent with its global migration, followed by a universal slowdown. However, we point to the influence of fossil calibrations and possibly sequencing and alignment artifacts in shaping the tree and thus dictating this chronology. The thesis also describes patterns of biome evolution during the diversification of the Coca family; highlighting that the rainforest biome is home to about half of all Erythroxylaceae species and that rainforest lineages have frequently transitioned into other biomes (dry forest and savanna/grassland). Finally, the thesis includes the description of a new variety of Erythroxylum collected by the Field Museum of Natural History’s Rapid Inventory team from the Sierra Escalera of Peru.