FIGURE 1.

Retroposition mediates the expansion of gene families responsive to stress. Genes highly expressed by certain environmental conditions (gene B) have higher chances to be retroposed, so the increased expression level is fixed in the genome through an increased retrogene copy number. In contrast, gene families not stimulated by environmental stress (gene A) remain constant during evolution. The different rate of retroposition of these two classes of genes will thus allow accumulation of genes responsive to environmental changes.