Many studies of evolution focus on the benefits to the individual of competing successfully – those who survive produce the most offspring, in Darwin's classic 'survival of the fittest'. But how does this translate to the evolution of species? A new paper, published in this week's issue of PLoS Biology, studies an aspect of the natural world that, like survival of the fittest individual, is explained by natural selection: namely, mutualism – an interaction between species that has benefits for both. The work shows that some species of butterfly that live alongside one another have evolved in ways that, surprisingly, benefit both species.