Increasing evidence suggests that size-selective mortality imposed by commercial fishing results in directional changes in life histories of exploited species, including effects on maturation age, growth rate and body size. Whether these changes are the result of fisheries-induced evolution or other selective pressures in the natural environment and/ or phenotypic plasticity continues to be the subject of much debate. Currently, molecular genetic data revealing fisheries-induced shifts at candidate loci are lacking. Here, the hypothesis that harvesting can induce genetic change over few generations was tested directly by subjecting laboratory-reared offspring of wild-caught Trinidadian guppies, Poecilia reticulata, to divergent, size-specific selection over three generations. The smallest/ largest/ random twenty percent of males was selected each generation and changes in standard length, as well as in the frequencies of alleles at neutral microsatellite loci and putative candidate genes for selection were recorded.
Significant divergence between differently selected lines was observed for male standard length (± 7%), size (± 8-12%) and age (± 4-6%) at maturation, compared to only 1% change in standard length over generations in the random breeding control line. Significant drift between lines, but no genetic erosion or inbreeding, was apparent over generations at microsatellite markers. Signatures of selection and significant genetic divergence between selected lines were detected at five out of 17 putative candidate loci (Pr39, M9, M30, M987 and prolactin) which confirmed strong Y-linkage of genes underlying male body size in guppies, as indicated by the phenotypic data. Additionally, significant genotype-phenotype associations were obtained for twelve of the candidate genes. For two of these loci (M30 and M1046) an association between the same single nucleotide polymorphism and a QTL for standard length had been observed previously. To our knowledge, this is the first study where selection on body size with known intensity and direction has been compared directly with both a phenotypic response and changes at individual genetic marker loci. Hereby, this study forms one of the first pieces of molecular genetic evidence for fisheries-induced evolution: by demonstrating that phenotypic shifts in body size resulting from size-selective harvesting, comparable to commercial fisheries in principle, are underlain by quantifiable genetic change.