Transposable elements were first discovered by Barbara McClintock in kernels of corn, where certain mutations caused loss and reinstatement of purple pigment (due to gain and loss of an insertion element that activated pigment genes). The human genome contains ~300,000 DNA transposons, which have extensively accelerated evolution due to the modularity of exons and regulatory regions. Transposition is one avenue for exon shuffling to occur, whereby an exon and two flanking transposons are all excised and reinserted elsewhere as a single element (potentially adding a new exon to a gene).
There are conservative (cut + paste) and replicative (copy + paste) mechanisms for transposition. However, transposition is potentially mutagenic and over-transposition is very deleterious; thus, it remains a rare event that occurs in about 1:105 or 1:107 cells per generation. Transposable elements occur in both eukaryotes (transposons; retrotransposons) and prokaryotes (insertion elements).