The genetic Allee effect: A unified framework for the genetics and demography of small populations
The Allee effect is a theoretical model that predicts the low growth rate and possible extinction of small populations. Because of its historical focus on demography, the Allee effect generally ignores underlying processes other than the direct effect of population density on fitness components. However, despite heated debates on the relative roles of demographic and genetic processes, the importance of genetic factors in driving small populations to extinction is now clearly established. Unfortunately, population dynamics and population genetics still rely on distinct concepts and terminology, which impede the development of a unified theory.
Our aim is to bridge genetic and demographic approaches in a common conceptual framework that is both novel and non-overlapping with the concepts from each separate field. For this, we demonstrate that the Allee effect can be extended to various genetic processes that are at the hinge between population size and fitness. We call this extension a “genetic Allee effect”. Specifically, we clearly define the “genetic Allee effect” as a two-step process characterized by (1) a change in genetic structure caused by a decrease in population size and (2) a consequent decrease in individual fitness. We describe potential underpinning processes, and review evidence for such an original type of Allee effect using examples from both plant and animal ecology literature.
Envisaging demogenetic feedbacks in small populations through the scope of genetic Allee effects should constitute a significant step ahead for the population biology of small populations, catalyzed by the very dynamic research front developing from the Allee effect and uniting two scientific communities working on two faces of the same object: populations genetics and dynamics. We therefore expect the concept of genetic Allee effects to strongly impact the fields of theoretical evolutionary and population ecology, as well as more applied perspectives such as invasion biology and conservation biology.