Plant Dispersal between the American Continents and the Special Case of the Popcorn Flowers

For over a hundred years, botanists and biogeographers have noticed an interesting pattern: the occurrence of the same plant species or very close relatives on either side of the tropics in North and South America. This biogeographic pattern, known as American amphitropical disjunction or AAD, represents the repeated formation of North America–South America sister lineages through dispersal and subsequent diversification on separate continents.
 
Subtribe Amsinckiinae (Boraginaceae) — which includes our speciose California genera Amsinckia, Cryptantha, Pectocarya, and Plagiobothrys — is one of the richest groups of plants displaying the AAD pattern. Michael Simpson has been studying that plant group for years and is therefore in a prime position to relay new and emerging data. The field of systematics now has the phylogenetic, biogeographic, and dating tools to assess the evolutionary and biogeographic history of AAD organisms and to begin relating these with common mechanisms of dispersal, climatic shifts, and geologic events.
 
Michael G. Simpson is Professor Emeritus at the Department of Biology, San Diego State University and Curator of the SDSU Herbarium. He continues to actively study the subtribe Amsinckiinae (Boraginaceae), this year alone describing two new California species of Cryptantha. He also works on floristic studies, having been involved with voucher and iNaturalist-based checklists (see https://cch2.org/portal/checklists) in Orange County (Caspers Wilderness Park, Whiting Ranch Wilderness Park) and in San Diego County (Mission Trails Regional Park, Steele/Burnand Anza-Borrego Desert Research Center).