Replicate radiations provide powerful comparative systems to address questions about the interplay between opportunity and innovation in driving episodes of diversification and the factors limiting their subsequent progression. However, such systems have been rarely documented at intercontinental scales. Here, we evaluate the hypothesis of multiple radiations in the genus Lupinus (Leguminosae), which exhibits some of the highest known rates of net diversification in plants. Given that incomplete taxon sampling, background extinction, and lineage-specific variation in diversification rates can confound macroevolutionary inferences regarding the timing and mechanisms of cladogenesis, we used Bayesian relaxed clock phylogenetic analyses as well as MEDUSA and BiSSE birth-death likelihood models of diversification, to evaluate the evolutionary patterns of lineage accumulation in Lupinus. We identified 3 significant shifts to increased rates of net diversification (r) relative to background levels in the genus (r = 0.18-0.48 lineages/myr). The primary shift occurred approximately 4.6 Ma (r = 0.48-1.76) in the montane regions of western North America, followed by a secondary shift approximately 2.7 Ma (r = 0.89-3.33) associated with range expansion and diversification of allopatrically distributed sister clades in the Mexican highlands and Andes. We also recovered evidence for a third independent shift approximately 6.5 Ma at the base of a lower elevation eastern South American grassland and campo rupestre clade (r = 0.36-1.33). Bayesian ancestral state reconstructions and BiSSE likelihood analyses of correlated diversification indicated that increased rates of speciation are strongly associated with the derived evolution of perennial life history and invasion of montane ecosystems. Although we currently lack hard evidence for "replicate adaptive radiations" in the sense of convergent morphological and ecological trajectories among species in different clades, these results are consistent with the hypothesis that iteroparity functioned as an adaptive key innovation, providing a mechanism for range expansion and rapid divergence in upper elevation regions across much of the New World.

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Coevolution between flowering plants and their pollinators is thought to have generated much of the diversity of life on Earth, but the population processes that may have produced these macroevolutionary patterns remain unclear. Mathematical models of coevolution in obligate pollination mutualisms suggest that phenotype matching between plants and their pollinators can generate reproductive isolation. Here, we test this hypothesis using a natural experiment that examines the role of natural selection on phenotype matching between yuccas and yucca moths (Tegeticula spp.) in mediating reproductive isolation between two varieties of Joshua tree (Yucca brevifolia var. brevifolia and Y. brevifolia var. jaegeriana). Using passive monitoring techniques, DNA barcoding, microsatellite DNA genotyping, and sibship reconstruction, we track host specificity and the fitness consequences of host choice in a zone of sympatry. We show that the two moth species differ in their degree of host specificity and that oviposition on a foreign host plant results in the production of fewer offspring. This difference in host specificity between the two moth species mirrors patterns of chloroplast introgression from west to east between host varieties, suggesting that natural selection acting on pollinator phenotypes mediates gene flow and reproductive isolation between Joshua-tree varieties.

Lupinus microcarpus is a self-compatible annual plant that forms a species complex of morphologically variable but indeterminate varieties. In order to examine the hypothesis that varieties of L. microcarpus comprise genetically differentiated and reproductively isolated species, populations of L. microcarpus var. horizontalis and var. densiflorus were sampled from an area of sympatry in central California and genotyped using six microsatellite loci. Bayesian clustering divided the total sample into two groups corresponding to the named varieties with extremely low levels of inferred coancestry. Similarly, maximum likelihood and distance methods for genetic assignment placed individuals in two nonoverlapping groups. Evidence for isolation by distance (IBD) within each variety was found at shorter distance classes, but varieties remained differentiated in sympatry. Furthermore, coalescent estimates of divergence time indicate separation within the past 950-5050 generations, with minimal gene flow after divergence. A four-level hierarchical analysis of molecular variance (amova) found significant levels of genetic differentiation among varieties (theta(P) = 0.292), populations within varieties (theta(S) = 0.449), subpopulations within populations (theta(SS) = 0.623), and individuals within subpopulations (f = 0.421); but the greatest degree of differentiation was at the subpopulation level. Although it is sometimes assumed that the magnitude of genetic differences (e.g. F(ST)) should be greater between species than among populations or subpopulations of the same species, shared ancestral polymorphism may lead to relatively low levels of differentiation at the species level, even as the stochastic effects of genetic drift generate higher levels of differentiation at lower hierarchical levels. These results suggest that L. microcarpus var. horizontalis and var. densiflorus are recently diverged yet reproductively isolated species, with high levels of inbreeding resulting from the combined effects of limited gene flow, demographic bottlenecks, and partial selfing in finite, geographically structured populations.

In animal-pollinated plants, local adaptation to pollinator behaviour or morphology can restrict gene flow among plant populations; but gene flow may also prevent divergent adaptation. Here, we examine possible effects of gene flow on plant-pollinator trait matching in two varieties of Joshua tree (Agavaceae: Yucca brevifolia). The two varieties differ in strikingly in floral morphology, which matches differences in the morphology of their pollinators. However, this codivergence is not present at a smaller scale: within the two varieties of Joshua tree, variation in floral morphology between demes is not correlated with differences in moth morphology. We use population genetic data for Joshua tree and its pollinators to test the hypotheses that gene flow between Joshua tree populations is structured by pollinator specificity, and that gene flow within the divergent plant-pollinator associations 'swamps' fine-scale coadaptation. Our data show that Joshua tree populations are structured by pollinator association, but the two tree varieties are only weakly isolated -meaning that their phenotypic differences are maintained in the face of significant gene flow. Coalescent analysis of gene flow between the two Joshua tree types suggests that it may be shaped by asymmetric pollinator specificity, which has been observed in a narrow zone of sympatry. Finally, we find evidence suggesting that gene flow among Joshua tree sites may shape floral morphology within one plant-pollinator association, but not the other.