The West European house mouse, Mus musculus domesticus, is a particularly suitable model to investigate the role of chromosomal rearrangements in reproductive isolation. In fact, it exhibits a broad range of chromosomal polymorphism due to Robertsonian Chromosomal rearrangements (e.g., deletions, inversions, centromeric fusion or fissions) are extensively studied events known to result in various karyotypes in natural populations of plants and animals (White 1978;Rieseberg and Willis 2007).

Two Cordulegaster dragonflies present in Italy, the Palaearctic and northern distributed Cordulegaster boltonii and the endemic to the south of the peninsula Cordulegaster trinacriae, meet in central Italy and give rise to individuals of intermediate morphology. By means of mitochondrial and nuclear markers and of Geometric Morphometrics applied to sexual appendages, we defined i) the geographical boundaries between the two species in Italy and ii) we determined the presence, the extent, and the genetic characteristics of the hybridization. Genetic data evidenced asymmetric hybridization with the males of C. trinacriae able to mate both interspecifically and intraspecifically. The results contrast with expectations under neutral gene introgression and sexual selection. This data, along with the morphological evidence of significant differences in size and shape of sexual appendages between the males of the two species, seem indicative of the role of mechanical constraints in intraspecific matings. The origin of the two species is dated about to 1.32 Mya and the hybridization resulted related to range expansion of the two species after Last Glacial Maximum and this led to the secondary contact between the two taxa in central Italy. At last, our results indicate that the range of C. trinacriae, a threatened and protected species, has been moving northward probably driven by climate changes. As a result, the latter species is currently intruding into the range of C. boltonii. The hybrid area is quite extended and the hybrids seem well adapted to the environment. From a conservation point of view, even if C. trinacriae has a strong genetic identity, the discovery of hybridization between the two species should be considered in a future species management.

A selective sweep is the result of strong positive selection driving newly occurring or standing genetic variants to fixation, and can dramatically alter the pattern and distribution of allelic diversity in a population. Population-level sequencing data have enabled discoveries of selective sweeps associated with genes involved in recent adaptations in many species. In contrast, much debate but little evidence addresses whether "selfish" genes are capable of fixation-thereby leaving signatures identical to classical selective sweeps-despite being neutral or deleterious to organismal fitness. We previously described R2d2, a large copy-number variant that causes nonrandom segregation of mouse Chromosome 2 in females due to meiotic drive. Here we show population-genetic data consistent with a selfish sweep driven by alleles of R2d2 with high copy number (R2d2 HC ) in natural populations. We replicate this finding in multiple closed breeding populations from six outbred backgrounds segregating for R2d2 alleles. We find that R2d2 HC rapidly increases in frequency, and in most cases becomes fixed in significantly fewer generations than can be explained by genetic drift. R2d2HC is also associated with significantly reduced litter sizes in heterozygous mothers, making it a true selfish allele. Our data provide direct evidence of populations actively undergoing selfish sweeps, and demonstrate that meiotic drive can rapidly alter the genomic landscape in favor of mutations with neutral or even negative effects on overall Darwinian fitness. Further study will reveal the incidence of selfish sweeps, and will elucidate the relative contributions of selfish genes, adaptation and genetic drift to evolution.