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TABLE 5.2 Aspects of Gene and Genomic Architectural Evolution That Appear to Be Explainable Only After Accounting for Variation in the Relative Power of Nonadaptive Evolutionary Forces

Genomic streamlining in microbial species versus genome bloating in multicellular lineages.

Nucleotide composition variation within and among genomes: genomewide A/T composition, strand asymmetry, isochores, and codon-usage bias in unicellular species.

Differential proliferation of mobile elements in unicellular versus multicellular species.

Gene number: preservation of duplicate genes by degenerative mutations (subfunctionalization).

Origin of the spliceosome by subfunctionalization and proliferation of introns in lineages of multicellular species.

Expansion of UTRs of the messenger RNAs of eukaryotes.

Origin of modular regulatory regions in eukaryotic genes.

Demise of operons in eukaryotes.

Variation in organelle genome architecture: lean in animals; bloated in land plants.

Messenger RNA editing in plant organelle genomes.

Restriction of sex chromosomes to multicellular lineages.

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). If a successful adaptive counterargument is to be mounted, simpler nonadaptive models must be shown to be inadequate, and to accomplish that, something must be known about the expected pattern of evolution in the absence of selection. If nothing else, the ideas presented above provide the basis for a null model for genomic evolution. Certainly, many of the above-mentioned embellishments of eukaryotic genes have adaptive functions in today’s multicellular species, but observations on current deployment may have little bearing on matters of initial origins. Most of the repatterning of the genomic real estate of eukaryotes occurred before the evolution of multicellularity (Lynch, 2007).



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