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ABSTRACT: Molecular genetics teaches three lessons relevant to the nature of genetic change during evolution: (1) Genomes are organized as hierarchies of composite systems (multidomain protein-coding sequences; functional loci made up of regulatory, coding, processing, and intervening sequences; and multilocus regulons and replicons) interconnected and organized into specific “system architectures” by repetitive DNA elements. (2) Genetic change often occurs via natural genetic engineering systems (cellular biochemical functions, such as recombination complexes, topoisomerases, and mobile elements, capable of altering DNA sequence information and joining together different genomic components). (3) The activity of natural genetic systems is regulated by cellular control circuits with respect to the timing, activity levels, and specificities of DNA rearrangements (e.g., adaptive mutation, Ty element mobility, and P factor insertions). These three lessons provide plausible molecular explanations for the episodic, multiple, nonrandom DNA rearrangements needed to account for the evolution of novel genomic system architectures and complex multilocus adaptations. This molecular genetic perspective places evolutionary change in the biologically responsive context of cellular biochemistry.