Intraspecific variation is central to our understanding of evolution and ecology, but these fields generally consider either the mean trait value or its variance. Alternatively, the keystone individual concept from behavioral ecology posits that a single individual with an extreme phenotype can have disproportionate and irreplaceable effects on group dynamics. Here, I generalize this concept to include non-behavioral traits and broader ecological and evolutionary dynamics. I test for the effects of individuals with extreme phenotypes on the ecology and evolution or a gall-forming fly and its natural enemies that select for opposite gall sizes. Specifically I introduce a putatively keystone predator-attracting individual gall-maker, hypothesizing that the presence of such an individual should (i) increase gall-maker population-level mortality, (ii) cause consumer communities to be dominated by species that are most attracted to the keystone individual, (iii) increase selection for traits conferring defense against the most common consumer, and (iv) weaken patterns of stabilizing selection. I find support for both the ecological and evolutionary consequences of single individuals with extreme phenotypes, suggesting that they can be considered keystone individuals. I discuss the generality of the keystone individual concept, suggesting likely consequences for ecology and evolution.