Old and new views of the evolution of genes. (A) Many 20th Century biologists viewed genes as traits of species, exquisitely tuned to current utility. This resulted in the assumption that each species should, for the most part, possess different genes. Gene duplication was recognized, but was implicitly assumed to have occurred recently. (B) Many biologists now assume that most genes have their origins in gene duplication events, which happen throughout evolutionary history. As a result, many genes form families that have persisted for hundreds of millions of years.