Across the globe are many plant and animal populations that have become biogeographically isolated from the populations from which they arose. At least some of those populations have evolved into new genotypes and are now genetically unique from their ancestors. Some have evolved into new races, species, and even genera and families. Although now genetically and morphologically unique from their ancestral stock, these closely related pairs are referred to as vicariads, or vicarious populations.

Catastrophic natural phenomena are responsible for triggering the vast majority of these biogeographic isolations. Among the most significant of these have been the drifting of continents and uplifting of mountain ranges through the forces of plate tectonics, the formation of volcanic islands, and disruption and creation of refugia due to advancing and receding continental ice sheets. To a lesser degree, other natural phenomena, including hurricanes and typhoons, both of which can disperse or displace plant parts, or even relocate whole organisms into virgin territories, are also responsible for biogeographic isolations. Once established and gene flow with the ancestral stock ceases, these separate populations can then evolve in different directions in response to the unique environments in which they occur.

Intercontinentally, the key factor responsible for most vicariance is attributable to a single phenomenon explained by plate tectonics: the splitting of a single massive supercontinent called Pangaea, into what we recognize today as the five major continents. The present-day continents sit on seven to fifteen major tectonic plates, which continue to move in various directions around the globe. Classic among the tectonic plate splits is the movement of continents away from each other, such as South America away from Africa; India away from Australia and Antarctica; and North America away from Eurasia and northwest Africa. The results of these separations have led to catastrophic shifts in biodiversity at the biome level. The existence of thousands of vicarious pairs of species, genera and entire families, provide evidence supporting this theory, with many vicariads seen around the globe. As these tectonic plates continue to separate, albeit extremely slowly, slower in some cases than the rate of growth of our toenails, given hundreds of millions of years of continual movement, it is conceivable that they could separate or collide again, thereby resulting in further isolation and mixing.

In North America, the two major historical events most responsible for vicariance in vascular plants involved the upthrusting of major mountain ranges, (Rockies, Appalachians, and Sierras), and the influence of continental ice sheets, which at one time blanketed much of the northern half of the continent. Both events resulted in significant biotic barriers that first divided plant populations, then prevented gene flow between them. Based upon the phytogeographic data compiled by the BONAP staff, we have been able to produce the following maps that depict some of the more dramatic examples of vicariance of vascular plant species across North America. -JTK