Evolutionary islands in the Andes: persistence and isolation explain high endemism in Andean dry tropical forests

Abstract

Aim: The tropical Andes are a world biodiversity hotspot. With diverse biomes and dramatic, geologically recent mountain uplift, they offer a system to study the relative contributions of geological and biome history to species richness. There are preliminary indications that historical species assembly in the Andes has been influenced by physiographical heterogeneity and that distinct biomes have evolved in relative isolation despite physical proximity. Here we test this ‘Andean biotic separation hypothesis’ by focusing on the low-elevation, seasonally dry tropical forest (SDTF) biome to determine whether patterns of plant diversification within the SDTF differ from those in mid- and high-elevation biomes.

Location: Tropical Andes, South America.

Methods: Densely sampled time-calibrated phylogenies for five legume genera (Amicia, Coursetia, Cyathostegia, Mimosa and Poissonia) containing species endemic to the Andean SDTF biome were used to investigate divergence times and levels of geographical structure. Geographical structure was measured using isolation-by-distance methods. Meta-analysis of time-calibrated phylogenies of Andean plant groups was used to compare the pattern and tempo of endemic species diversification between the major Andean biomes.

Results: Long-term persistence of SDTF in the Andes is suggested by old stem ages (5–27 Ma) of endemic genera/clades within genera, and deep divergences coupled with strong geographical structure among and within species. Comparison of species diversification patterns among different biomes shows that the relatively old, geographically confined pattern of species diversification in SDTF contrasts with the high-elevation grasslands that show rapid and recent radiations driven by ecological opportunities.

Main conclusions: The SDTF biome has a long history in the Andes. We suggest that the diverse SDTF flora has been assembled gradually over the past c. 19 Ma from lineages exhibiting strong phylogenetic niche conservatism. These patterns suggest that Andean SDTFs have formed stable and strongly isolated ‘islands’ despite the upheavals of Andean uplift. Indeed, the Andean SDTFs may represent some of the most isolated and evolutionarily persistent continental plant communities, similar in many respects to floras of remote oceanic islands.