Experimental evidence of rapid heritable adaptation in the absence of initial standing genetic variation

Abstract

Abstract The success of genetically depauperate populations in the face of environmental change is contrary to the expectation that high genetic diversity is required for rapid adaptation. Alternative pathways such as environmentally induced genetic modifications and non‐genetic heritable phenotypes have been proposed mechanisms for heritable adaptation within an ecologically relevant time frame. However, experimental evidence is currently lacking to establish if, and to what extent, these sources of phenotypic variation can produce a response. To test if adaptation can rapidly occur in the absence of initial standing genetic variation and recombination in small populations, we (a) exposed replicate monoclonal populations of the microzooplankton Brachionus calyciflorus to a culturing regime that selected for phenotypic variants with elevated population growth with either high or low phosphorus food for a period of 55 days and (b) examined population level response in two fully factorial common garden experiments at day 15 and 35 of the exposure experiment. Within six generations, we observed heritable local adaptation to nutrient limitation. More specifically, populations with a history of exposure to low P food exhibited higher population growth rates under low P food conditions than populations with a high P exposure history. However, the capacity for such a response was found to vary among clones. Our study finds that although standing genetic variation is considered essential for rapid heritable adaptation, the rapid emergence of de novo genetic variation or alternative sources of phenotypic variation could aid in the establishment and persistence of low‐diversity populations. A free Plain Language Summary can be found within the Supporting Information of this article.