Genetic mixture promotes invasive range expansion via a catapult effect of heterosis
Biological invasions often stem from multiple introductions from genetically distinct source populations. Multiple introductions can modify the genetic composition of invasive populations, but whether and how they promote invasiveness remain poorly resolved. Using a laboratory model (the beetle Callosobruchus maculatus spreading through artificial landscapes), we quantified the velocity of range expansion for invasions initiated with 1, 2, 4, or 6 genetically differentiated beetle populations. Our experiment was designed to quantify the roles of competing mechanisms for effects of genetic mixture, including increased evolutionary potential, fitness benefits of outcrossing (heterosis), and “ecological” effects of genetic diversity such as sampling effects.
Over six generations, invasions with any amount of genetic mixture spread farther and faster than single-source invasions. Our data indicate that benefits of heterosis provided a “catapult effect” that left a lasting signature on range expansion even though the benefits of out-crossing were transient. We show that multiple introductions can modify the spatio-temporal trajectories of range expansion – the phase of invasion associated with the greatest ecological and economic impacts – through a mechanism that can easily go undetected in field settings.