RIESEBERG, LOREN H.* and JOHN BURKE. Biology Department, Indiana University, Bloomington, IN 47405. - 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
favorable alleles (s greater than 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-emphasized relative to its creative
role as a mechanism for the spread of advantageous mutations.
Key words: advantageous mutations, gene flow, genetic background, QTLs, selection, species