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Pages 83-104

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From page 83... ... These issues are examined in the context of the view that the ori gins of many aspects of biological diversity, from gene-structural embellishments to novelties at the phenotypic level, have roots in nonadaptive processes, with the population-genetic environment imposing strong directionality on the paths that are open to evo lutionary exploitation. A lthough biologists have always been concerned with complex phenotypes, the matter has recently become the subject of height ened speculation, as a broad array of academics, from nearly every branch of science other than evolutionary biology itself, claim to be in Department of Biology, indiana University, Bloomington, in 47405. Read the entire page →
From page 84... ... For example, Kirschner and Gerhart (2005) argue that evolutionary biology has been ‘‘woefully inadequate'' with respect to understanding the origins of complexity and promise ‘‘an original solution to the long-standing puzzle of how small random genetic change can be converted into complex, useful innovations.'' however, this book and many others like it (e.g., Depew and Weber, 1985; Kauffman, 1993; Carroll, 2005a; Davidson, 2006) Read the entire page →
From page 85... ... 5. natural selection is the Both mutation and gene conversion are nonrandom only force capable of processes that can drive the patterning of genomic promoting directional evolution in populations with sufficiently small evolution. Read the entire page →
From page 86... ... 9. natural selection There is no evidence that phylogenetic variation in promotes the ability to the pathways open to evolutionary exploration is evolve. Read the entire page →
From page 87... ... second, all four major forces play a substantial role in genomic evolution. it is impossible to understand evolution purely in terms of natural selection, and many aspects of genomic, cellular, and developmental evo lution can only be understood by invoking a negligible level of adaptive involvement (Kimura, 1983; lynch, 2007) Read the entire page →
From page 88... ... To quote Carroll (2005a) again, ‘‘simplification may indeed be necessary for news articles, but it can distort the more complex and subtle realities of evolutionary patterns and mechanisms.'' INTERNAL VERSUS EXTERNAL EVOLUTIONARY FORCES The literature is permeated with dogmatic statements that natural selection is the only guiding force of evolution, with mutation creating variation but never controlling the ultimate direction of evolutionary change (for a review, see stoltzfus, 2006a) Read the entire page →
From page 89... ... The Frailty of Adaptive Hypotheses for the Origins of Organismal Complexity / hand, standard theory (Kimura, 1983) shows that the fixation probability of a mutation to A is eS times that for a mutation to a, where S = 2 Ngs is twice the ratio of the power of selection to the power of random genetic drift (1/Ng) Read the entire page →
From page 90... ... The genomes of multicellular eukaryotes are invariably packed with mobile elements, and individual genes are generally subdivided by multiple introns, harbor multiple transcription-factor binding sites, and are transcribed into units containing substantial untranslated flanking sequences. in contrast, prokaryotic genomes are usually nearly completely devoid of mobile elements and introns and have genes with very simple regulatory structures, often transcribed into polycistronic units (operons) Read the entire page →
From page 91... ... To become established, the modifications that led to such mutational hazards must have either had a substantial immediate selective advantage or arisen in populations with effective sizes sufficiently small to render them immune to selection. letting s = nu, the latter condition requires that 2Ngnu < 1, and because the mutational cost of individual modifications is small, generally <30u, it is difficult to reject the hypothesis that incremental expansions of eukaryotic gene complexity were largely driven by nonadaptive processes. Read the entire page →
From page 92... ... however, simply making the counterclaim that natural selection is all powerful (without any direct evidence) is not much different from invoking an intelligent designer (without any direct evidence) Read the entire page →
From page 93... ... , it need not follow that natural selection is a sufficient force for the exit from the unicellular world. Many developmental genes previously thought to have originated in the vertebrate lineage, owing to their absence in arthropods and nematodes, are now known to be present in basal lineages of animals Read the entire page →
From page 94... ... . Could nonadaptive processes have played a role in the evolution of something as intricate as cell architecture or developmental complexity? Read the entire page →
From page 95... ... in summary, the near-complete absence of the concept of nonadaptive processes from the lexicon of those concerned with cellular and devel opmental evolution does not reflect any formal demonstration of the negligible contribution of such mechanisms, and indeed, there is no fundamental reason why development should be uniquely immune to nonadaptive evolutionary forces. one could even argue that the stringency of natural selection is reduced in complex organisms with behavioral and/or growth-form flexibilities that allow individuals to match their phenotypic capabilities to the local environment. Read the entire page →
From page 96... ... For this particular gene, within their respective tissues, the hatched and crosshatched transcription factors are redundant with respect to the white factor, but the additional black factors are essential for com plete expression. Three hypothetical phases of gene architectural modification Read the entire page →
From page 97... ... (Top) Accretion and degeneration of transcription-factor binding sites. Read the entire page →
From page 98... ... Under this view, the reductions in Ng that likely accompanied both the origin of eukaryotes and the emergence of the animal and land-plant lineages may have played pivotal roles in the origin of modular gene architectures on which further developmental complexity was built. Despite the initial invariance of phenotypic expression patterns during this type of gene-architectural repatterning, the emergence of independently mutable subfunctions in modularized alleles can contribute to adaptive evolution in significant ways. Read the entire page →
From page 99... ... in principle, this process could be repeated anew as B acquires sensitivity to a further upstream gene C and loses the ability to constitutively express. Mutational C o n v e r s io n vulnerability rate A S e l f - s u f f i ci e n t ul ug ul B A 0 Redundantly regulated ul ug B A ul Upstream dependent FiGUre 5.3 A series of allelic states for locus A, defined by the ability to selfexpress and/or be activated by an upstream transcription factor B Read the entire page →
From page 100... ... EVOLVABILITY All replicating populations are capable of evolution, but it has recently been argued that some species are better at it than others, with natural selection directly advancing features of genomic architecture, genetic networks, and developmental pathways to promote the future ability of a species to adaptively evolve. such speculation, which is almost entirely restricted to molecular and cell biologists and those who study digital organisms (e.g., Gerhart and Kirschner, 1997; Kirschner and Gerhart, 1998, 2005; rutherford and lindquist, 1998; True and lindquist, 2000; Caporale, 2003; earl and Deem, 2004; Bloom et al., 2006; Federici and Downing, 2006) Read the entire page →
From page 101... ... , but aside from the matter of kin selection in behavioral evolution (hamilton, 1964a,b; Wilson, 1975) , the evidence for the operation of group selection is weak, although some investigators remain more optimistic than others (Coyne et al., 2000; Goodnight and Wade, 2000) Read the entire page →
From page 102... ... 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. Read the entire page →
From page 103... ... The field of population genetics is technically demanding, and it is well known that most biologists abhor all things mathematical. however, the details do matter in the field of evolutionary biology. Read the entire page →

From page 83...

... These issues are examined in the context of the view that the ori gins of many aspects of biological diversity, from gene-structural embellishments to novelties at the phenotypic level, have roots in nonadaptive processes, with the population-genetic environment imposing strong directionality on the paths that are open to evo lutionary exploitation. A lthough biologists have always been concerned with complex phenotypes, the matter has recently become the subject of height ened speculation, as a broad array of academics, from nearly every branch of science other than evolutionary biology itself, claim to be in Department of Biology, indiana University, Bloomington, in 47405.