A central goal of biology is to uncover mechanisms generating patterns of biodiversity, including large disparities in species richness across geographic regions and across groups of organisms. The Neotropics harbor the earth’s most species-rich flora, which underlies high diversity across all other taxonomic groups, and many tropical plant lineages have diversified rapidly. Two evolutionary hypotheses for high Neotropical diversification rates have been proposed: 1) climatic stability leads to the evolution of narrow physiological tolerances which limit range expansion and dispersal, and 2) spatial variation in biotic interactions with diverse mutualists and antagonists drives divergent adaptation and reproductive isolation. Yet integrative studies that test these hypotheses have thus far been unachievable.

Here we propose to use spiral gingers in the monocot genus Costus to investigate biotic and abiotic drivers of speciation. This exceptionally tractable clade of Neotropical herbs occupies a wide range of habitats, extending from lowlands to montane forests and from dense understory to gaps and forest edges. Shifts from bee to hummingbird pollination have evolved independently multiple times, as has the relationship between defending ants and extrafloral nectaries. Moreover, there is emerging evidence that the group is intimately associated with diverse and highly specialized herbivorous beetles, providing new opportunities to study the role of plant-herbivore interactions in plant adaptation and speciation. With decades of foundational research and recent advances, the extraordinary variation in abiotic and biotic factors in a single clade of closely-related, rapidly evolving species, provides a unique opportunity to address long-standing hypotheses about the main drivers of adaptation and speciation in this Neotropical plant clade. We will integrate phylogenetic studies, broad scale observational approaches, focused field experiments (reciprocal transplants and direct manipulations of biotic associates and abiotic factors), and genetic mapping to 1) determine how plants interact with, and adapt to, mutualists, antagonists, and abiotic conditions across their geographic ranges, 2) evaluate whether adaptive tradeoffs at the genotypic level underlie these interactions, and 3) assess the consequences of these interactions for plant reproductive isolation and speciation. Moreover, the research fills a gap in our understanding of tropical systems, and is thus an important step towards building a synthesis through comparison to the wealth of excellent temperate studies. Finally, our work will greatly increase basic knowledge of plant ecophysiological limits and interactions between plants and their pollinators and herbivores in ecosystems that are rapidly changing due to anthropogenic influences.