Phylogeography: retrospect and prospect

Phylogeography is a relatively new discipline that deals with the spatial arrangements of genetic lineages, especially within and among closely related species. The word phylogeography was coined in 1987 (Avise et al., 1987). About a decade earlier, scientists had begun using mitochondrial (mt) DNA to address precisely how conspecific individuals are genealogically linked through shared ancestors (the mitochondrial genome is well suited to this purpose, for reasons that this paper will highlight). In genetic surveys of various species, striking patterns were being uncovered in the spatial arrangements of mtDNA lineages. In other words, genealogy and geography seemed to be connected. After 1987, ‘phylogeography’ became the convenient shorthand for referring to explicit genealogical inquiries (including those for nuclear genes) into the spatial and temporal dimensions of microevolution. In physics, Albert Einstein’s Theory of General Relativity formalized the notion that time and space are interrelated and that the curvature of space–time predictably affects the paths of free particles. In phylogeography, no comparable mathematical laws govern precisely how genetic lineages are spatially configured (evolutionary processes are much too varied and idiosyncratic for that). However, comparative phylogeographic assessments of many species have revealed a great deal about the demographic and historical nature of intraspecific evolution. I begin this overview with some by-now-standard background on cytoplasmic genomic data and on coalescent theory (Hudson, 1990) which are often used in phylogeographic appraisals. Then I will illustrate, by empirical examples, some key findings and emerging principles of phylogeography, before concluding with some thoughts about future directions for this burgeoning discipline. This paper offers an introductory synopsis of the field; for a more in-depth treatment and extensive citations to the primary phylogeographic literature, see Avise (2000).