Part III
COMPARATIVE PHYLOGEOGRAPHY IN A TAXONOMIC SENSE
Because nucleic acids are the universal hereditary material of life on Earth, molecular approaches to phylogeography can in principle be applied to creatures ranging from bacteria to whales and humans. The chapters in this section will further illustrate the catholic nature of comparative phylogeography by describing a few chosen examples spanning a wide array of disparate taxonomic groups.
Although microbes have not been a traditional focus of phylogeographic research (mostly for logistical or technical reasons), Michaeline Nelson and colleagues (Chapter 10) show how a trait-based approach can enlighten our understanding of microbial biogeography on a global scale. The genetic traits they surveyed involve different pathways for nitrogen cycling in soil microbes sampled from around the world. By analyzing the spatial and taxonomic distributions of such microbial traits, the authors develop an argument for how comparative phylogeographic methods could be highly relevant to predicting the responses of biogeochemical cycles to future climatic and other environmental changes.
Kelly Zamudio and colleagues (Chapter 11) make a similar case for vertebrate animals by emphasizing the need to integrate trait (phenotypic) data with molecular genetic data in future phylogeographic appraisals. The authors argue that because phenotypes are the actual targets of selection, they should be assessed in conjunction with other phylogeographic data to illuminate the ecological and evolutionary forces shaping biodiversity. Ever since the time of Alfred Russel Wallace (the founder of biogeography in the mid-1800s), biogeographic appraisals have relied
primarily on the spatial distributions of phenotypic attributes. Thus, if the integrative prescriptions of Zamudio et al. are to be followed, the field of comparative phylogeography may in effect partially return to its intellectual biogeographic roots even while simultaneously expanding the latter in new genealogical directions. Bowen and colleagues (Chapter 1) and Papadopoulou and Knowles (Chapter 8) likewise endorsed the links between biogeography defined with organismal traits and phylogeography founded on gene genealogies.
In Chapter 12, Anne Yoder and colleagues focus on comparative genetic patterns in one particular taxonomic assemblage: congeneric mouse lemurs on the island of Madagascar. Apart from demonstrating the profound phylogeographic ramifications of altered landscapes across recent geological and human timescales, this survey is instructive because it exemplifies the types of comparative phylogeographic analyses that have been applied to many other animal taxa in various regions around the globe, and because it involves a single large island (as opposed for example to an oceanic, continental, or archipelago setting).
In a phylogeographic study of North American bison, Peter Heintzman and colleagues (Chapter 13) advocate for a greater emphasis on paleoecology as a major driving force shaping contemporary phylogeographic patterns. Their example involves the purported role of an “Ice-Free Corridor” as a plausible route for dispersal (of humans and other animals) between Beringia and more southerly areas of North America. Without knowledge of such paleoecological conditions in times past, the authors argue that phylogeographers will remain handicapped in their attempts to interpret the spatial genetic architecture of extant taxa.
Whereas mitochondrial (mt) DNA has been the traditional molecular workhorse of phylogeographic appraisals in animals, chloroplast (cp) DNA has performed the corresponding role in plant studies. In Chapter 14, Victoria Sork and coauthors summarize evolutionary lessons learned from comparative analyses of phylogeographic patterns displayed by diverse plant taxa in California. Most plants have several biological features that differ from those of most (not all) animals and that presumably have major phylogeographic and demographic relevance: a sessile growth form; high reproductive output; leptokurtic dispersal through pollen and seeds; intimate interactions with local environmental conditions; and the potential for individual longevity. Sork et al. provide empirical examples of how these botanical features apparently have impacted phylogeographic patterns in “plants” as diverse as oak trees and lichens.
Unsurprisingly, perhaps no single species has received greater phylogeographic attention than Homo sapiens. In the final chapter of this section of the book, Alexander Harcourt (Chapter 15) provides a panoramic overview of the phylogeography of our species, beginning with origins
on the African continent some 200,000 years ago and continuing with the diaspora(s) of our ancestors from that landmass to eventually occupy the entire world. Included in this chapter are the deduced evolutionary dates and specific migration routes employed by our ancestors to “people the planet,” as well as discussions of the patterns of cultural diversity associated with this phylogeographic history. Harcourt’s concise summary of a vast literature on human origins should be of considerable interest to anthropologists and general audiences alike.
