Evidence for Isolation-by-Habitat among Populations of an Epiphytic Orchid Species on a Small Oceanic Island

Abstract

Identifying factors that promote population differentiation is of interest for understanding the early stages of speciation. Gene flow among populations inhabiting different environments can be reduced by geographical distance (isolation-by-distance) or by divergent selection resulting from local adaptation (isolation-by-ecology). Few studies have investigated the influence of these factors in small oceanic islands where the influence of geographic distance is expected to be null but where habitat diversity could have a strong effect on population differentiation. In this study, we tested for the spatial divergence of phenotypes (floral morphology and floral scent) and genotypes (microsatellites) among ten populations of Jumellea rossii, an epiphytic orchid endemic to Réunion growing in three different habitats. We found a significant genetic differentiation between populations that is structured by habitat heterogeneity rather than by geographic distance between populations. These results suggest that ecological factors might reduce gene flow among populations located in different habitats. This pattern of isolation-by-habitat may be the result of both isolation-by-ecology by habitat filtering and asynchrony in flowering phenology. Furthermore, data on floral morphology match these findings, with multivariate analysis grouping populations by habitat type but could be only due to phenotypic plasticity. Indeed floral scent compounds were not significantly different between populations indicating that specific plant-pollinator mutualism does not seem to play a major role in the population differentiation of J. rossii. In conclusion, the results from our study emphasize the importance of habitat diversity of small oceanic islands as a factor of population differentiation.

Evolution of population structure in an estuarine‐dependent marine fish

“…Basic habitat differences, initiated by glacial retreat and perpetuated by contemporary oceanic and atmospheric forces interacting with the geomorphology of the northeastern Gulf basin, followed by selection, appear to have led to reduced gene flow across the region, reinforcing accrued differences and resulting in continued genomewide divergence. This process, termed isolation by adaptation or IBA (Nosil, Egan, & Funk, ), has been observed in animal and plant species (Van Bocxlaer, ; Funk, Egan, & Nosil, ; Mallet, Martos, Blambert, Martos, Blambert, Pailler, & Humeau, ) and, potentially, is a harbinger of incipient speciation (Schemske, ). The same dynamic may pertain to other coastal or nearshore fishes (18 species in 14 families) where genetically or morphologically defined sister taxa (species, subspecies, populations) occur in the three regions (Supplemental Table ; Portnoy & Gold, ).…”

Climate vs. topography – spatial patterns of plant species diversity and endemism on a high‐elevation island

“…In addition, high intra‐island precipitation variability increases the potential for adaptive evolutionary divergence within and among species, leading to adaptations to specific conditions within the island in focus (Mallet et al . ; Harter et al . in press).…”

Genomic footprints of adaptation in a cooperatively breeding tropical bird across a vegetation gradient

“…This study allowed us to identify footprints of IBE in the presence of gene flow. Despite the controversies about IBE, recent studies suggest that it is rather common in nature (Dennenmoser et al., ; Lonsinger et al., ; Mallet et al., ; Manthey & Moyle, ; Mendez et al., ) and that selection can be detected in the presence of (and even promoted by) high gene flow (Nielsen et al., ; Saint‐Laurent et al., ; Schweizer et al., ). Such conditions may well apply to the Yucatan jay, as the dynamics of the cooperative breeding social system can be altered by environmental gradients.…”

“…This effect is, however, controversial because gene flow is thought to stall local adaptation (Bridle, Polechová, Kawata, & Butlin, ; Haldane, ; Wright, ), implying that IBD patterns may be more common in nature (Meirmans, ; Slatkin, ; Wang, Glor, & Losos, ). However, IBE has become a likely explanation for population genetic differences in many taxa and environmental frameworks, including some orchids (Mallet, Martos, Blambert, Pailler, & Humeau, ), fish (Dennenmoser et al., ), birds (Manthey & Moyle, ) and mammals (Lonsinger, Schweizer, Pollinger, Wayne, & Roemer, ; Mendez et al., ), which suggests that IBE is also frequently found in nature. Disentangling the relative effects of IBD and IBE is thus essential to understanding the ecology of local adaptation and phenotypic evolution (Bradburd et al., ; Sexton et al., ; Wang & Bradburd, ; Wang et al., ).…”