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sudden discontinuities in the lineages imagined to be most evolvable (animals and land plants) (Lynch, 2006, 2007). Such variation appears to be a simple by-product of alterations in chromosome lengths and numbers of germ-line cell divisions: because chromosome number is independent of genome size, and there is typically one meiotic cross-over event per chromosome, a doubling in genome size generally results in a 50% reduction in the recombination rate per physical distance; and because a large fraction of mutations are generated during replication, a doubling in the number of germ-line cell divisions doubles the per-generation mutation rate.

Fourth, comparative genomics provides no support for the idea that genome architectural changes have been promoted in multicellular lineages so as to enhance their ability to evolve (Lynch, 2007). Indeed, other than the appearance of spliceosomal introns, some forms of mobile elements, and organelles in the stem eukaryote, there are no discontinuities in the basic features of genomes across the entire domain of cellular life. Moreover, as noted above, the additional genomic complexities of multicellular eukaryotes appear not to have arisen by positive selection but instead to have emerged passively in population-genetic environments where the efficiency of selection is relaxed, quite contrary to the view espoused by the evolvability school. Many unicellular species are excluded from certain evolutionary pathways that are open to multicellular species, and vice versa, but this is simply an indirect consequence of the altered power of nonadaptive evolutionary forces in these different contexts, not a direct outcome of natural selection for the ability to engage in particular evolutionary pursuits.

CLOSING COMMENTS

Because it deals with observations on historical outcomes, frequently in the face of incomplete information, the field of evolution attracts significantly more speculation than the average area of science. Nevertheless, a substantial body of well tested theory provides the basis for understanding the pathways that are open to evolutionary exploration in various population-genetic contexts. Four of the major buzzwords in biology today are complexity, modularity, evolvability, and robustness, and it is often claimed that ill-defined mechanisms not previously appreciated by evolutionary biologists must be invoked to explain the existence of emergent properties that putatively enhance the long-term success of extant taxa. This stance is not very different from the intelligent-design philosophy of invoking unknown mechanisms to explain biodiversity. Although those who promote the concept of the adaptive evolution of the above features are by no means intelligent-design advocates, the burden of evidence for invoking an all-powerful guiding hand of natural selection



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