Genetic and morphological differentiation in Populus nigra L.:isolation by colonization or isolation by adaptation?

Identifying processes underlying the genetic and morphological differences among populations is a central question of evolutionary biology. Forest trees typically contain high levels of neutral genetic variation, and genetic differences are often correlated with geographic distance between populations [isolation by distance (IBD)] or are due to historic vicariance events [isolation by colonization (IBC)]. In contrast, morphological differences are largely due to local adaptation. Here, we examined genetic (microsatellite) and morphological (from a common garden experiment) variation in Populus nigra L., European black poplar, collected from 13 sites across western Europe and grown in a common garden in Belgium. Significant genetic differentiation was observed, with populations from France displaying greater admixture than the distinct Spanish and central European gene pools, consistent with previously described glacial refugia (IBC). Many quantitative traits displayed a bimodal distribution, approximately corresponding to small-leaf and large-leaf ecotypes. Examination of nine climatic variables revealed the sampling locations to have diverse climates, and although the correlation between morphological and climatic differences was significant, the pattern was not consistent with strict local adaptation. Partial Mantel tests based on multivariate summary statistics identified significant residual correlation in comparisons of small-leaf to large-leaf ecotypes, and within the small-leaf samples, but not within large-leaf ecotypes, indicating that variation within the small-leaf morphotype in particular may be adaptive. Some small-leaf populations experience climates very similar to those in large-leaf sites. We conclude that adaptive differentiation and persistent IBC acted in combination to produce the genetic and morphological patterns observed in P. nigra

Is Sexual Monomorphism a Predictor of Polygynandry? Evidence from a Social Mammal, the Collared Peccary

Sexual dimorphism is common in polygynous species, and there is clear evidence that both intra-sexual competition and female preferences can drive the evolution of large body size in males. In contrast, sexual monomorphism is often argued to reflect a relaxation of male mate competition or an intensification of resource competition among females. Alternatively, it might imply opportunities for females to circumvent or counteract male mate competition in a polygynandrous mating system. We test the prediction that sexual monorphism is associated with polygynandry in the collared peccary (Pecari tajacu, Tayassuidae), a social ungulate closely related to the old-world suids. The genetic mating system in the Tayassuidae is unknown, but its sexual monomorphism presents a striking contrast to the strong size dimorphism found in most Suidae, so that a departure from the polygynous system common in Suidae would be noteworthy. We characterized genetic relationships among adults within herds in three geographically distinct populations, assigned parents to 75 offspring, and tested for skew in individual reproductive success. Parentage assignment data indicated that multiple males sire offspring within a herd, and in the population for which genetic data were most complete, 19% of parentage assignments were potentially sired by extra-herd males. Some litters have multiple sires, and neither males nor females monopolized reproduction, even in small herds. This result supports our prediction and suggests that sexual monomorphism may either select for or be an evolutionary consequence of a promiscuous mating system

Evolutionary and genetic basis of morphological variation in Populus nigra (European black poplar)

Changes in precipitation over the next century may impact the distribution of species, particularly in southern Europe, where droughts are predicted to increase in frequency. In forest trees, intraspecific variation in leaf size, branching architecture, and growth rate among populations are considered adaptive and likely related to climatic differences between sites. A previous common garden study of Populus nigra L. showed morphological variation to be highly heritable and significantly differentiated among populations, indicating phenotypic differences may be adaptive.This project studied the evolutionary processes that have contributed to the morphological differentiation observed in P. nigra. Examining scales ranging from landscape-level patterns of variation to cellular differences within developing leaves identified historic and developmental processes contributing to the phenotypic differences in this species. Both isolation by distance, where migrants do not move equally across the landscape, and isolation by adaptation, where genetically divergence varies with morphological differences, have influenced differentiation among populations of P. nigra in western Europe. These patterns broadly correspond to the recolonization routes following the most recent glacial event, indicating that historic vicariance and not just adaptive divergence influence phenotypic variation. Identification of quantitative trait loci (QTL) for insect herbivory in a common garden study of hybrid poplar indicated that leaf morphology might also be influenced by insect preference.Among individuals, differences in leaf size corresponded to variation in cell number and not cell size, indicating natural selection may have influenced the regulation of cell division. Further, variation in gene regulation across the developing leaf identified differences across the leaf lamina. Finally, simulations of demographic, genetic, and adaptive processes among populations revealed that a lack of correspondence between the optimal phenotype of colonists and the optimal phenotype and newly colonized populations significantly affects levels of phenotypic differentiation among populations. In addition, changes in phenotypic optima, as may occur due to climate change, impacted the level of genetic variance, and thus the future adaptive potential of populations. Together, these results provide insight into the evolution of phenotype in P. nigra, and contribute information for management efforts in the context of a changing climate.<br/