"Hopeful monsters," transposons, and Metazoan radiation.

The appearance of many novel morphologies, frequently expressed taxonomically as new phyla, classes, or orders, occurs with such rapidity in evolutionary time that microevolutionary substitutions involving structural genes seem an implausible mechanism. It has been suggested that such novelties are produced by changes in developmental and regulatory structures and patterns rather than by an accumulation of single structural gene changes. The horizontal transmission of genetic material via RNA-based viruses between members of a population may rapidly create intrafertile sub-populations that differ markedly from their parents and form the basis of new morphological types, avoiding the usual fitness problems associated with "hopeful monsters."