The west European subspecies of house mouse (Mus musculus domesticus) has gained much of its current widespread distribution through commensalism with humans. This means that the phylogeography ofM. m. domesticusshould reflect patterns of human movements. We studied restriction fragment length polymorphism (RFLP) and DNA sequence variations in mouse mitochondrial (mt) DNA throughout the British Isles (328 mice from 105 localities, including previously published data). There is a major mtDNA lineage revealed by both RFLP and sequence analyses, which is restricted to the northern and western peripheries of the British Isles, and also occurs in Norway. This distribution of the ‘Orkney’ lineage fits well with the sphere of influence of the Norwegian Vikings and was probably generated through inadvertent transport by them. To form viable populations, house mice would have required large human settlements such as the Norwegian Vikings founded. The other parts of the British Isles (essentially most of mainland Britain) are characterized by house mice with different mtDNA sequences, some of which are also found in Germany, and which probably reflect both Iron Age movements of people and mice and earlier development of large human settlements. MtDNA studies on house mice have the potential to reveal novel aspects of human history.

Molecular markers and morphological characters can help infer the colonization history of organisms. A combination of mitochondrial (mt)d-loop DNA sequences, nuclear DNA data, external measurements and skull characteristics shows that house mice (Mus musculus) in New Zealand and its outlying islands are descended from very diverse sources. The predominant genome isMus musculus domesticus(from western Europe), butMus musculus musculus(from central Europe) andMus musculus castaneus(from southern Asia) are also represented genetically. These subspecies have hybridized to produce combinations ofmusculusanddomesticusnuclear DNA coupled withdomesticusmtDNA, andcastaneusormusculusmtDNA withdomesticusnuclear DNA. The majority of the mice withdomesticusmtDNA that we sampled hadd-loop sequences identical to two haplotypes common in Britain. This is consistent with long-term British–New Zealand cultural linkages. The origins of thecastaneusmtDNA sequences widespread in New Zealand are less easy to identify.

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. R2d2^HC 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.

In contrast to a substantial inventory of mitochondrial D-loop sequences from the West European part of the range of the western house mouse Mus musc