A Comparative Study in Ancestral Range Reconstruction Methods: Retracing the Uncertain Histories of Insular Lineages

Abstract

Island systems have long been useful models for understanding lineage diversification in a geographic context, especially pertaining to the importance of dispersal in the origin of new clades. Here we use a well-resolved phylogeny of the flowering plant genus Cyrtandra (Gesneriaceae) from the Pacific Islands to compare four methods of inferring ancestral geographic ranges in islands: two developed for character-state reconstruction that allow only single-island ranges and do not explicitly associate speciation with range evolution (Fitch parsimony [FP; parsimony-based] and stochastic mapping [SM; likelihood-based]) and two methods developed specifically for ancestral range reconstruction, in which widespread ranges (spanning islands) are integral to inferences about speciation scenarios (dispersal-vicariance analysis [DIVA; parsimony-based] and dispersal-extinction-cladogenesis [DEC; likelihood-based]). The methods yield conflicting results, which we interpret in light of their respective assumptions. FP exhibits the least power to unequivocally reconstruct ranges, likely due to a combination of having flat (uninformative) transition costs and not using branch length information. SM reconstructions generally agree with a prior hypothesis about dispersal-driven speciation across the Pacific, despite the conceptual mismatch between its character-based model and this mode of range evolution. In contrast with narrow extant ranges for species of Cyrtandra, DIVA reconstructs broad ancestral ranges at many nodes. DIVA results also conflict with geological information on island ages; we attribute these conflicts to the parsimony criterion not considering branch lengths or time, as well as vicariance being the sole means of divergence for widespread ancestors. DEC analyses incorporated geological information on island ages and allowed prior hypotheses about range size and dispersal rates to be evaluated in a likelihood framework and gave more nuanced inferences about range evolution and the geography of speciation than other methods tested. However, ancestral ranges at several nodes could not be conclusively resolved, due possibly to uncertainty in the phylogeny or the relative complexity of the underlying model. Of the methods tested, SM and DEC both converge on plausible hypotheses for area range histories in Cyrtandra, due in part to the consideration of branch lengths and/or timing of events. We suggest that DEC model-based methods for ancestral range inference could be improved by adopting a Bayesian SM approach, in which stochastic sampling of complete geographic histories could be integrated over alternative phylogenetic topologies. Likelihood-based estimates of ancestral ranges for Cyrtandra suggest a major dispersal route into the Pacific through the islands of Fiji and Samoa, motivating future biogeographic investigation of this poorly known region.

Patterns of diversification and ancestral range reconstruction in the southeast Asian–Pacific angiosperm lineage Cyrtandra (Gesneriaceae)

“…Because of the uniformity in habitat across a broad and diverse region, it does not appear that Pacific Cyrtandra radiated in response to ecological pressures (Cronk et al, 2005). Rather, the genus more likely diverged under a classic dispersal-mediated allopatry model (sensu Clark et al, 2008). Most species within Cyrtandra are narrowly distributed endemics occupying no more than a single archipelago, a single island or even a single valley, further supporting this hypothesis.…”

“…Previous divergence dating analysis has indicated that a large number of species have arisen over a relatively short time (Clark et al, 2008); a rapid species divergence resulting in potentially homoplastic character suites between lineages could be contributing to taxonomic issues in Cyrtandra.…”

“…Sampling builds on the previous work of Clark et al (2008) and includes Bornean, Philippine and Taiwanese specimens for a more diverse and representative southeast Asian-Malesian grade. Sampled Pacific taxa correspond to lineages present on most high islands and all attempts were made to include at least one specimen from principal lineages as defined by Gillett (1973) and Wagner et al (1990).…”

“…For a distribution map of the genus Cyrtandra along with approximate species numbers for major regions discussed in this paper, see Fig. 1 in Clark et al (2008).…”

New Guinea highland origin of a widespread arthropod supertramp

“…Dransfield 1981). Genera such as Cyrtandra (Gesneriaceae) colonized Oceania in a single event out of southeast Asia via Malesia, radiating extensively to produce numerous island endemics (Cronk et al 2005;Clark et al 2008). Similarly, drynaroid ferns possibly originated in southeast Asia and West Malesia, and then dispersed across Indonesia and the Philippines to New Guinea (Schneider et al 2008).…”