The biological reality of species: gene flow, selection, and collective evolution

Some authors have claimed that species are passive end products of evolution and thus not substantially different than higher taxa. This claim is based on reports that (1) levels of intraspecific gene flow may be too low to account for species' integration, and (2) populations are likely to diverge rather than evolve in parallel when exposed to uniform selection pressures. These conclusions are premature. A review of the plant literature reveals that there is sufficient gene flow to enable the efficient spread of strongly favourable alleles (s > 0.05), the most likely agents of collective evolution. Moreover, estimates of s for major quantitative trait loci (QTLs) are sufficiently large to enable their spread across the range of a species, although minor QTLs seem more likely to evolve locally. In addition, evidence that intraspecific variation in genetic background affects the response of alleles to selection is rare, but examples of parallel genotypic evolution are becoming increasingly common. We conclude that, as traditionally believed, species are the most inclusive entities that directly participate in evolutionary processes. However, we also note that the traditional role of gene flow as a force that constrains differentiation due to genetic drift or local adaptation has been over-emphasised relative to its creative role as a mechanism for the spread of advantageous mutations.