Longevity is a life-history trait that is shaped by natural selection. An unexplored consequence is how selection on this trait affects diversity and diversification in species assemblages. Motivated by the diverse rockfish (Sebastes) assemblage in the North Pacific, the effects of trade-offs in longevity against competitive ability are explored. A competition model is developed and used to explore the potential for species diversification and coexistence. Invasion analyses highlight that life-history trait trade-offs in longevity can mitigate the effects of competitive ability and favour the coexistence of a finite number of species. Our results have implications for niche differentiation, limiting similarity and assembly dynamics in multispecies interactions.