Phil. Trans. R. Soc. B 372: 20150480.
Genome or genes sometimes get duplicated just partially within a single cell even when it is not dividing. The gene duplication and divergence provide a critical source of genetic novelty during evolution. This review paper discusses the fates of duplicated homeobox genes, focusing on asymmetric divergence after gene duplication. Duplicated genes differentiate independently (or asymmetrically) to different degrees. They often become key genes of new functions.
One of the copied genes can mutate and sometimes acquires a new function, while the others may lose their function. Overall number of genes changes as a result of total duplication-modification and deletions. Repetition of this process over millions of years result in a whole family of genes in a single genome. Duplicate copies often change asymmetrically to have different functions. Their expression becomes different in time and loci. This is how orthologs and paralogs were made.
This review provides examples are Hox genes in Lepidoptera, TALE-class genes in molluscs, extra PRD-class genes in placental mammas. Tapeworms have lost around one-third of all homeobox genes generally present in bilaterian animals. The evolution of the globulin gene family shows how DNA duplications contribute to the evolution of organisms (Molecular Biology of the Cell, pp461).
Production of redundant copies and repurposing them for other functions sounds similar to the neural circuit evolution. It seems like evolution is all about repurposing of given resources to create new functions.
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