Kemp’s ridley sea turtle nesting. Image courtesy of the National Park Service.
The Kemp’s ridley sea turtle, a small, crab-eating, day-nesting turtle, is currently listed as endangered throughout its range in the western Gulf of Mexico. Its sister species, the olive ridley, nests around the globe, from Brazil to Australia to Baja California, and is the most abundant sea turtle species worldwide. Before its listing, some turtle biologists speculated that the Kemp’s ridley were simply a closely related outpost of the olive ridley and not worth the trouble and expense of an Endangered Species Act listing. When determining how much time and expense to dedicate to conservation of the Kemp’s ridley, the US Congress considered genetic data suggesting the species had diverged far earlier than anyone thought. Now the United States and Mexico are setting aside a critical habitat to ease the turtle’s recovery.
Such is the power of phylogeography, a field that sits square between macro- and microevolutionary perspectives. The term was coined in 1987 by John Avise, then at the University of Georgia. Avise sought to wed molecular systematics, the study of how species split along the tree of life, to traditional biogeography, the study of regional differences among living things. Avise’s central insight was to see that genetic differences could reveal more than that one species differs from another. It could measure the depth of that difference as well, as a measurement of time and percentage of sequence divergence.
Phylogeographic work in the late 1980s and early 1990s primarily analyzed mitochondrial DNA sequences. Now, the field uses several types of genetic variation, each suited to a different time resolution. Noncoding regions of mtDNA reveal divergences dating from thousands to tens of thousands of years. Coding regions of mtDNA and nuclear introns reveal differences several million years old. Ribosomal DNA changes very slowly and can show differences of tens of millions of years, back when the continents were in radically different places.
The field has refocused from studying genetic lineage branching itself to examining certain populations of interest. The attention shift aids conservation work, enabling phylogeographers to highlight biodiversity hotspots and species that are conservation priorities. Phylogeography has also begun incorporating coalescent theory, a hind-looking analysis that traces the source of modern alleles for a particular gene to the most recent common ancestor. This approach helps biologists study the effects of historical expansion, migration, or bottlenecks on modern genetic diversity.
As in many other fields, researchers are quantifying their study subjects to ever-finer resolution. Using geo-referenced species data to map range and location helps test divergence scenarios. Modeling environmental niches with satellite-based climate data makes it possible to predict the historical, current, and future distributions. Phylogeographic software models are growing more elaborate. Such models have critics, however. Not all feel these models are reliable or valuable. There are still arguments over how demographic scenarios lead to genetic patterns, the relative merits of analytic methods, and how to weave information from different genes into a single historical scenario for a population.
In the last 5 years a new undercurrent has begun creating waves. Classically, speciation was the assumed result of populations separating physically, such as on an island or on different sides of a mountain range, for example. Intriguingly, speciation also occurs where there are few barriers, such as in the ocean. The view that ecological, or sympatric, speciation might be common and important is, in the words of Brian Bowen, a phylogeographer at the University of Hawaii, “the best controversy of the field right now.”