Although many classic radiations on islands are thought to be the result of repeated lineage splitting, the role of past fusion is rarely known because during these events, purebreds are rapidly replaced by a swarm of admixed individuals. Here, we capture lineage fusion in action in a Galápagos giant tortoise species, Chelonoidis becki, from Wolf Volcano (Isabela Island). The long generation time of Galápagos tortoises and dense sampling (841 individuals) of genetic and demographic data were integral in detecting and characterizing this phenomenon. In C. becki, we identified two genetically distinct, morphologically cryptic lineages. Historical reconstructions show that they colonized Wolf Volcano from Santiago Island in two temporally separated events, the first estimated to have occurred ~199 000 years ago. Following arrival of the second wave of colonists, both lineages coexisted for approximately ~53 000 years. Within that time, they began fusing back together, as microsatellite data reveal widespread introgressive hybridization. Interestingly, greater mate selectivity seems to be exhibited by purebred females of one of the lineages. Forward-in-time simulations predict rapid extinction of the early arriving lineage. This study provides a rare example of reticulate evolution in action and underscores the power of population genetics for understanding the past, present and future consequences of evolutionary phenomena associated with lineage fusion.

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In this study, we used a recently developed supertrees method to test for shared phylogeographical signal in partially overlapping geographical ranges of lizards of the genus Liolaemus from the Andean Range in south-central Chile. We reconstruct mtDNA gene trees for three partially codistributed species (Liolaemus tenuis, L. lemniscatus and L. pictus), and our sampling effort is sufficient to allow statistical tests of shared signal between the combinations L. tenuis-L. pictus, and L. tenuis-L. lemniscatus. For both combinations, standardized maximum agreement subtrees scores showed statistically significant signal for shared pattern in regions of overlap, as evaluated by randomization tests (P < 0.001 and < 0.05, respectively). The matrix representation with parsimony tree obtained from the combination of the three different gene trees revealed concordant phylogeographical associations of all species, and was consistent with the geographical association of intraspecific haploclades with three Chilean bioclimatic zones. A multidimensional scaling analysis of several climate variables showed highly significant differences among these zones, which further suggests that they may have contributed to similar patterns of intraspecific divergence across all three species. In the mesomorphic zone in Central Chile, the species L. tenuis and L. lemniscatus may have codiverged in response to shared orogenic vicariant events, which likely predominated over climatic events associated with cycles of glacial advance and retreat. In the hygromorphic zone in southern Chile, however, glacial cycles likely predominated in structuring the phylogeographical histories of L. tenuis and L. pictus, although important ecological differences between these two caution against broad generalizations at this point.

The "lava lizards" (Microlophus) are distributed throughout the Galápagos Archipelago, and consist of radiations derived from two independent colonizations. The "Eastern Radiation" includes M. bivittatus and M. habeli endemic to San Cristobal and Marchena Islands. The "Western Radiation" includes five to seven historically recognized species distributed across almost the entire Archipelago. We combine dense geographic sampling and multilocus sequence data to estimate a phylogenetic hypothesis for the Western Radiation, to delimit species boundaries in this radiation, and to estimate a time frame for colonization events.Our phylogenetic hypothesis rejects two earlier topologies for the Western Radiation and paraphyly of M. albemarlensis, while providing strong support for single colonizations on each island. The colonization history implied by our phylogeny is consistent with general expectations of an east-to-west route predicted by the putative age of island groups, and prevailing ocean currents in the Archipelago. Additionally, combined evidence suggests that M. indefatigabilis from Santa Fe should be recognized as a full species. Finally, molecular divergence estimates suggest that the two colonization events likely occurred on the oldest existing islands, and the Western Radiation represents a recent radiation that, in most cases, has produced species that are considerably younger than the islands they inhabit. K E Y W O R D S :Galápagos, lizards, mitochondrial DNA, molecular timing of colonization, nuclear DNA, oceanic islands, phylogeny.Oceanic islands have been model systems in evolutionary studies for well over a century (Emerson 2002;Whittaker et al. 2008), and the Galápagos Archipelago, located about 960 km west from the coast of Ecuador, has figured prominently among them. Galápagos is one of the most recent oceanic island formations (Christie et al. 1992), and a consensus biogeographic history (reviewed in Grehan 2001) has favored an over-water colonization model for the origin of its many endemic radiations. Most of these studies have emphasized early models of Galápagos colonization events that were initially constrained to a 4 to 5 million year time frame set by the estimated ages of the oldest current islands (Cox 1983). The subsequent discovery of underwater seamounts representing former Galápagos islands to the east of the current archipelago on the east-shifting Nazca Plate, extended the 1 6 0 6

a b s t r a c tThe death of Lonesome George, the last known purebred individual of Chelonoidis abingdoni native to Pinta Island, marked the extinction of one of 10 surviving giant tortoise species from the Galápagos Archipelago. Using a DNA reference dataset including historical C. abingdoni and >1600 living Volcano Wolf tortoise samples, a site on Isabela Island known to harbor hybrid tortoises, we discovered 17 individuals with ancestry in C. abingdoni. These animals belong to various hybrid categories, including possible first generation hybrids, and represent multiple, unrelated individuals. Their ages and relative abundance suggest that additional hybrids and conceivably purebred C. abingdoni individuals still occur on Volcano Wolf. Spatial analyses suggest locations where additional individuals with C. abingdoni ancestry are most likely to be recovered, consistent with historical records of human movement of tortoises. These results provide an opportunity for species recovery of Pinta Island tortoises using individuals with C. abingdoni ancestry.