Alternative reproductive tactics and life history phenotypes

Alternative reproductive tactics (ARTs) coexisting within a population are found in many organisms. Their existence has been an enduring puzzle in evolutionary biology. Why should selection produce distinctly different alternatives to reach the same goal? How can such alternative solutions coexist in a population? What determines their evolutionary stability? Here we outline ultimate and proximate mechanisms responsible for the origin, coexistence and stability of ARTs. We argue that behavioural and reproductive polymorphisms often reflect different allocation decisions in response to trade-offs in reproduction or life-history optima that may involve heritable threshold responses to environmental variation. Alternative tactics may either be fixed for life or plastic, with simultaneous or sequential switches between tactics. General principles include disruptive selection, negative frequency dependence, density dependence, and an interaction between genetic and environmental components to generate alternative tactics. ARTs are found often where individuals invest heavily in reproduction in a way that can be circumvented and exploited by competitors, which reflects disruptive selection on reproductive investment. This often coincides with consistent size variation between individuals pursuing bourgeois and parasitic tactics.