I examined mitochondrial ND2 sequences in species of tits (genus Parus) to determine whether substitutions were neutral or under selection. Haplotype trees in these species are shallow and geographically unstructured, which could be a signature of recent demographic expansion or purifying selection. McDonald-Kreitman tests revealed a pattern of excess replacement polymorphisms for closely related taxa. Replacement substitutions tended to be less common in the basal parts of haplotype trees, suggesting that they are selected against over evolutionary time. Thus, the pattern of selection is consistent with a model of mildly deleterious haplotypes. To investigate the significance of this selection, the ND2 gene was subdivided into surface and transmembrane portions, and synonymous-non-synonymous sites, and F_ST-values (the amount of variance distributed among populations) computed for each. Among sampling localities in P.major, both partitions gave similar estimates of F_ST, which is consistent with demographic expansion as the cause of the shallow trees. In P.montanus, two localities contained high percentages of individuals that had a single replacement substitution in the transmembrane portion of the gene, which inflated F_ST-values relative to the surface sites. Thus, the interpretation of the genetic differentiation of these two sites could be either geographical isolation or selection. Given that the sole substitution causing the high F_ST-values was a replacement one, selection is implicated. However, this substitution occurred in only 2 of 12 localities, revealing that overall the phylogeographic pattern was not biased by selection, once the cause of the two high pairwise F_ST-comparisons was recognized. Investigators should consider the source of significant genetic heterogeneity within species, to determine if the variation is likely caused by demographic isolation or selection. However, it seems unlikely that most phylogeographic analyses are invalidated by the effects of natural selection.

Because invasive species threaten the integrity of natural ecosystems, a major goal in ecology is to develop predictive models to determine which species may become widespread and where they may invade. Indeed, considerable progress has been made in understanding the factors that influence the local pattern of spread for specific invaders and the factors that are correlated with the number of introduced species that have become established in a given region. However, few studies have examined the relative importance of multiple drivers of invasion success for widespread species at global scales. Here, we use a dataset of >5,000 presence/absence records to examine the interplay between climatic suitability, biotic resistance by native taxa, human-aided dispersal, and human modification of habitats, in shaping the distribution of one of the world's most notorious invasive species, the Argentine ant ( Linepithema humile ). Climatic suitability and the extent of human modification of habitats are primarily responsible for the distribution of this global invader. However, we also found some evidence for biotic resistance by native communities. Somewhat surprisingly, and despite the often cited importance of propagule pressure as a crucial driver of invasions, metrics of the magnitude of international traded commodities among countries were not related to global distribution patterns. Together, our analyses on the global-scale distribution of this invasive species provide strong evidence for the interplay of biotic and abiotic determinants of spread and also highlight the challenges of limiting the spread and subsequent impact of highly invasive species.

Background: Reproductive division of labor in eusocial insects is a striking example of a shared genetic background giving rise to alternative phenotypes, namely queen and worker castes. Queen and worker phenotypes play major roles in the evolution of eusocial insects. Their behavior, morphology and physiology underpin many ecologically relevant colony-level traits, which evolved in parallel in multiple species. Results: Using queen and worker transcriptomic data from 16 ant species we tested the hypothesis that conserved sets of genes are involved in ant reproductive division of labor. We further hypothesized that such sets of genes should also be involved in the parallel evolution of other key traits. We applied weighted gene co-expression network analysis, which clusters co-expressed genes into modules, whose expression levels can be summarized by their ‘eigengenes’. Eigengenes of most modules were correlated with phenotypic differentiation between queens and workers. Furthermore, eigengenes of some modules were correlated with repeated evolution of key phenotypes such as complete worker sterility, the number of queens per colony, and even invasiveness. Finally, connectivity and expression levels of genes within the co-expressed network were strongly associated with the strength of selection. Although caste-associated sets of genes evolve faster than non-caste-associated, we found no evidence for queen- or worker-associated co-expressed genes evolving faster than one another. Conclusions: These results identify conserved functionally important genomic units that likely serve as building blocks of phenotypic innovation, and allow the remarkable breadth of parallel evolution seen in ants, and possibly other eusocial insects as well.