A general macroecological pattern that natural historians have observed and quantified is that tropical regions harbor more biodiversity than temperate regions. This pattern holds across all taxonomic groups. This difference in levels of biodiversity has fascinated biologists for hundreds of years. Despite this widespread interest, biologists still debate why there are so many species in the tropics and why most tropical groups of organisms rarely diversify into temperate areas of the planet. Two of the leading hypotheses for the striking difference in biodiversity between tropical and temperate habitats is that tropical climates have been stable for a long time (~30-40 million years), allowing sufficient time for speciation and more species to accumulate, and that the evolution of cold tolerance is inherently difficult and has happened relatively rarely. The latter idea is known as the tropical niche conservatism hypothesis. Even though the idea of niche conservatism makes intuitive sense, the hypothesis has little direct support. This research will provide an explicit test of the factors underlying differences in biodiversity between tropical and temperate regions. This is an important and timely test because it will help us understand how biodiversity evolves, and how we can best conserve that biodiversity. In addition, multiple students at all levels (undergraduate, graduate, and postdoctoral) will be trained in evolutionary biology, and the work will support a beginning Latino investigator.
Tests of the topical niche conservatism hypothesis have been only correlative in nature. Unfortunately, evolutionary processes other than niche conservatism can also explain these patterns. This research will integrate phenotypic, phylogenetic, genetic, and genomic data to provide the most comprehensive test of the role of niche conservatism, and the ability of organisms to adapt to novel thermal environments, in generating the latitudinal species gradient. The researchers will leverage the species rich, and globally distributed genus Drosophila, to conduct these tests. The project involves (1) the generation of a new genus-wide phylogeny of Drosophila, (2) the analysis of the evolution of thermal performance traits for hundreds of species across the genus, (3) experimental tests of the evolvabilty of the thermal niche in both tropical and temperate Drosophila clades, and (4) the identification and testing of the genes controlling thermal niche evolution. Ultimately, the integration of these data will allow the researchers to make the strongest possible inferences about whether and how tropical niche conservatism is responsible for global diversity gradients, and how species evolve under changing thermal environments.
