Part I
COMPARATIVE PHYLOGEOGRAPHY IN A SPATIAL SENSE
Phylogeographers have roamed the planet in their quest to identify patterns that might typify organisms occupying diverse environmental regimes. Understanding the biotic response to past challenges provides a compass for contemporary and future challenges. Brian Bowen and colleagues (Chapter 1) summarize a vast literature on phylogeographic findings for the world’s oceans. At face value, the oceanic realm might seem to be featureless and relatively free of impediments to gene flow. Nevertheless, many migrant species in the sea (such as marine turtles, and some cetaceans and pelagic fishes) display phylogeographic patterns implicating physical barriers as promoters of both intraspecific genetic breaks and subsequent allopatric speciation events (properly viewed as stages along a temporal continuum). For marine taxa more generally, a wide variety of phylogeographic outcomes implies the operation of other evolutionary processes as well, such as sympatric or ecological speciation in some cases, environmental effects from historically shifting climates, and the important role of organismal behavior and natural history in shaping current phylogeographic motifs. Empirical phylogeographic patterns in the sea have also illuminated the evolutionary role of biodiversity hotspots and the historical dispersal routes that enhance global biodiversity.
Brett Riddle (Chapter 2) then summarizes an equally large scientific literature for species that occupy terrestrial or semiterrestrial regimes on continental landmasses. The author identifies phylogeographic hotspots, usually ecological transition zones that are focal points for study. Previ-
ously, these hotspots were primarily in the Northern Hemisphere and proximal to well-equipped labs, but success and improved technology have inspired a diaspora to geographic settings around the world. Here, as might generally be expected, range contraction-expansion episodes and dispersal from and into Neogene and Quaternary refugia played major roles in shaping the current-day arrangements of phylogeographic lineages for many continental taxa. The classical vicariance-dispersal paradigm, which generated heated debate and much research in the previous century, is transforming into a more nuanced and multifactorial perspective on why lineages are distributed as we observe them today in extant faunas.
In Chapter 3, Ivan Prates and colleagues use a case-history approach to reveal how historical climatic and habitat changes affected three broadly codistributed lizard species in Amazonia and the Atlantic Forest of the Neotropics. This study finds partial congruence in phylogeographic patterns that can be attributable to shared historical influences plus similarities in life history and environmental needs. However, the authors then go much further by embracing the close link between phylogeographic pattern and historical population demography to deduce that these taxa had species-specific population demographies in their recent past. These demographic parameters (in conjunction with ecological niche models related to climate change) forecast very distinct population trajectories over the next 60 years for these three environmentally sensitive lizard species.
Remote oceanic archipelagos offer especially intriguing evolutionary studios for studying phylogeographic processes on isolated chains of islands that originated (typically in sequential temporal order) via hotspot volcanism. These mid-oceanic islands offer a proverbial tabula rasa in which every species has origins elsewhere, and community composition is stilted by the filter of dispersal ability. Several such oceanic archipelagos are scattered around the world’s oceans, in most cases with the islands being displaced unidirectionally by plate tectonic movements, as if on an evolutionary conveyor belt. In Chapter 4, Kerry Shaw and Rosemary Gillespie review comparative phylogeographic findings for diverse taxa on six such oceanic archipelagos. They focus especially on the “progression rule,” which predicts that older genealogical lineages should map to older islands within an archipelago, with progressively younger lineages mapping to progressively younger islands within each such island chain. Shaw and Gillespie find support for the progression rule in some cases (notably in the Hawaiian archipelago), but less so on some others. They also highlight the precedence effect, whereby the success of initial colonizers makes it more difficult for subsequent colonizers to establish. Both the
general trend and particular exceptions to it are highly informative about dispersal and speciational processes in these distinctive environmental settings. As noted here (and by Riddle in Chapter 2), the simplified evolutionary arena of oceanic islands can reveal phylogeographic patterns that are obscured in a more complex continental setting.
