It is thought a lot of speciation has occured in the Andes due to elevation gradients and seperation by vallays and such. However, the effect of such processes is not very well understood and there are competing models. Vicariance, in which adaptive differentiation is the lynchpin, is a well regarded hypothesis telling of a more gradual change. Another model the ecotone model, exists relating to recently formed areas like the Andes in which new habitats are formed with little competition and as such diversification is rapid.In the Guarnizo study, the hyptheses were tested by analyzing nuclear and mitochondrial DNA sequences across 24 different species. Correlations were found between elevation and phylogenetic variation. Two highly divergent clades were found, partitioned into northern and southern populations. Data from two mitochondrial fragments and an independent nuclear gene suggest that the clades were formed by reproductive isolation. The hypothesis for the mechanism creating different clades and speciation in the Andes relates to the temperature changes as the mountains rose. Before the Andes rose high, it would have been about 10 degrees celcius warmer at the mountain tops, presumably a common ancestor to the northern clade was isolated at the peaks of mountains where it was cold enough for its survival. After the rather fast elevation of the Andes, and expansion of cold area, these populations then had a lot more habitat and different gradients within which speciation could occur.
Hence, the paper seems to find that both mechanisms highlighted earlier played a role in the diversification of the Andes. If we can take these frogs as an example, we see that vicariance would have expected as usual, but when the mountains rose ecotone speciation would have occured rapidly. Ancient isolation between populations on mountain peaks acting effectively as islands would have also led to deep deivergences by seperation of gene flow.This study was just a beginning, and other Andean species with wide alititudinal ranges must be analysed. It is a high priority to understand the genetic and geographic natural history, especially with amphibian isolations occuring at such a rapid rate in the region.
By Lachlan Noble (41773071)
References:
Biodiversity Hotspots: Tropical Andes, Conservation International, 2009. Retreieved March 29, 2009, from http://www.biodiversityhotspots.org/xp/Hotspots/andes/Pages/default.aspx
Junin Andes Frog, Arkive, 2009. Retrieved March 29, 2009, from http://www.arkive.org/junin-andes-frog/phrynopus-kempffi/
Carlos E. Guarnizo a,*, Adolfo Amézquita b, Eldredge Bermingham c. The relative roles of vicariance versus elevational gradients in the genetic
differentiation of the high Andean tree frog, Dendropsophus labialis. Molecular Phylogenetics and Evolution, 50, (2009) 84–92.
