Adaptive evolution of a candidate gene for aging in Drosophila

Examination of the phenotypic effects of specific mutations has been extensively used to identify candidate genes affecting traits of interest. However, such analyses do not reveal anything about the evolutionary forces acting at these loci, or whether standing allelic variation contributes to phenotypic variance in natural populations. The Drosophila gene methuselah (mth) has been proposed as having major effects on organismal stress response and longevity phenotype. Here, we examine patterns of polymorphism and divergence at mth in population level samples of Drosophila melanogaster, D. simulans, and D. yakuba. Mth has experienced an unusually high level of adaptive amino acid divergence concentrated in the intra- and extracellular loop domains of the receptor protein, suggesting the historical action of positive selection on those regions of the molecule that modulate signal transduction. Further analysis of single nucleotide polymorphisms (SNPs) in D. melanogaster provided evidence for contemporary and spatially variable selection at the mth locus. In ten surveyed populations, the most common mth haplotype exhibited a 40% cline in frequency that coincided with population level differences in multiple life-history traits including lifespan. This clinal pattern was not associated with any particular SNP in the coding region, indicating that selection is operating at a closely linked site that may be involved in gene expression. Together, these consistently nonneutral patterns of inter- and intraspecific variation suggest adaptive evolution of a signal transduction pathway that may modulate lifespan in nature.

Hybridization as a stimulus for the evolution of invasiveness in plants?

Invasive species are of great interest to evolutionary biologists and ecologists because they represent historical examples of dramatic evolutionary and ecological change. Likewise, they are increasingly important economically and environmentally as pests. Obtaining generalizations about the tiny fraction of immigrant taxa that become successful invaders has been frustrated by two enigmatic phenomena. Many of those species that become successful only do so (i) after an unusually long lag time after initial arrival, and/or (ii) after multiple introductions. We propose an evolutionary mechanism that may account for these observations. Hybridization between species or between disparate source populations may serve as a stimulus for the evolution of invasiveness. We present and review a remarkable number of cases in which hybridization preceded the emergence of successful invasive populations. Progeny with a history of hybridization may enjoy one or more potential genetic benefits relative to their progenitors. The observed lag times and multiple introductions that seem a prerequisite for certain species to evolve invasiveness may be a correlate of the time necessary for previously isolated populations to come into contact and for hybridization to occur. Our examples demonstrate that invasiveness can evolve. Our model does not represent the only evolutionary pathway to invasiveness, but is clearly an underappreciated mechanism worthy of more consideration in explaining the evolution of invasiveness in plants.

Positive selection and rates of evolution in immunodeficiency viruses from humans and chimpanzees.

Evolutionary theory predicts the recent spread of primate immunodeficiency viruses (PIVs) to new human populations to be accompanied by positive selection in response to new host environments and/or by random genetic drift. I assess evidence for positive selection in human and chimpanzee PIVs type I (PIV1s), using ratios of synonymous to nonsynonymous nucleotide change based on branch lengths and outgroup rooting. Ratios are smaller for PIV1s from humans than for PIV1 from a chimpanzee for the pol, gag, and env glycoprotein 120 (gp120) regions, indicating greater effects of positive selection in PIV1s from humans. Parsimony-based relative rate tests for amino acid changes showed significant differences between PIV1s from humans and chimpanzees in 18 of 48 pairwise comparisons, with all 18 showing faster rates of change in PIV1s from humans. This study indicates that in some instances, the recent evolution of human PIV1s follows a speciational pattern, in which increased diversification of taxa is correlated with greater amounts of character change appearing and being maintained through time. This extends the generality of the speciational pattern to a group of organisms (viruses) having the fastest known rates of anagenetic change for nucleotide characters and indicates that comprehensive understanding of PIV1 evolution requires consideration of both anagenetic change within viral lineages and the relative historical success of different viral clades. Phylogenetic analyses show that neither PIV1s infecting humans nor those infecting chimpanzees represent monophyletic groups and suggest multiple host-species shifts for PIV1s.