This paper applies principles from evolutionary biology to the study of unions. We show that unions that implement the preferred wage and organizing policies of workers will be displaced in evolutionary competition by unions that either extract less from firms, allowing them to live longer, or spend more on union organizing, or both. This implies that unions with constitutional incumbency advantages that allow leaders to depart from members' preferences may have a selective advantage, allowing them to grow at the expense of unions lacking such provisions. Evidence from the history of American unions supports these predictions. (JEL A12, J51)