The other revolution of the late 1980s took full advantage of the potential of molecular phylogenetics and population genetics, but differed from the traditional realm of phylogenetic systematic in a very important way. While the vast majority of phylogenetically based studies in biogeography had been conducted among taxa at or above the species level, the new discipline of phylogeography was defined explicitly as "a field of study concerned with the principles and processes governing the geographic distributions of genealogical lineages, especially those within and among closely related species" (Avise 2000). In other words, whereas biogeography has typically concentrated on species as the most fundamental units of analysis, and phylogenetic trees were built among species or higher taxa, the newly developing molecular approaches were providing a basis for mapping the spread of lineages (gene trees) during their development (Figure 11.10)