Neural basis of time changes during saccades

because newly arising variation modifies existing organismal blueprints, large differences between taxa imply differences in the kinds and amounts of new variation that can arise. The new variation immediately available to a metazoan population, for example, is obviously different from that immediately available to a single-celled eukaryote population. It follows that the evolvabilities of metazoans and single-celled eukaryotes are probably different at present, at least in the short term. It would be far more interesting, though, to know whether differences in evolvability explain in the first place why some single-celled lineages became metazoans whereas others remained single-celled, and this is a much more difficult problem. Invoking variability as a retrospective explanation for why one clade has diversified or changed more than another does not rule out the possibility that the clades evolved differently for reasons unrelated to variability. And finding isolated examples of evolutionary novelties related to distinctive variability mechanisms — for example, mutations of major phenotypic effect caused by transposable elements — provides only anecdotal evidence for the importance of such variability mechanisms in evolution. As other commentators on evolvability have noted, there is a need for quantitative, testable predictions concerning evolvability rather than retrospective and anecdotal arguments. Approaches such as computer simulation and long-term experimental evolution may yield some progress in this direction because they allow direct manipulation and assessment of the effects of variability differences on evolution, but even these kinds of approaches may not provide dependable insights into whether and how variability differences have actually affected the evolution of natural populations. Conclusion Our knowledge of molecular mechanisms that affect the origin of variation in populations has grown very rapidly in recent decades; in contrast, our fundamental genetic understanding of natural selection developed before 1950 and has not changed in major ways since then. To some, this historical disjunction suggests that evolutionary theory cannot account for the origin and maintenance of mechanisms affecting variability and is overdue for major revision. It is indeed attractive to suppose that the most important evolutionary feature of organisms — their very capacity to evolve and adapt — is itself an adaptation, but this is probably only true in highly restricted circumstances. Instead, variability is probably most often a byproduct of the messy and intricate ways in which genomes have evolved. And the possibility that incidental differences in variability between populations have caused differences in evolvability with profound consequences for evolutionary history …