Exploring negative frequency-dependent selection across levels: from genetics to ecology and back again

Populations harbour enormous diversity in ecologically important traits and genes that underlie those traits, and we still do not know why. Recent genome sequence analysis suggests that ‘balancing selection’ plays a major role in maintaining genetic diversity. Of the different types of balancing selection, negative frequency-dependent selection (NFDS) might be the most prevalent. NFDS occurs when fitness of a particular genetic type increases as its frequency within a population decreases. The importance of NFDS was explored in a Philosophical Transactions of the Royal Society special issue published in 1988, which concluded that NFDS played only a limited role in natural systems. Here, we review progress in the field since that time and introduce a new special issue that re-examines the significance of NFDS. This issue brings together contributions that: (i) investigate the organismal and ecological causes of NFDS; (ii) links NFDS to genomic signatures and the development of genome-scan methods tailored specifically to detecting NFDS; and (iii) explores long-term ecological consequences of NFDS. Collectively, these studies demonstrate that NFDS is a more widespread and important evolutionary force than previously thought. Finally, we highlight emerging questions that we hope will shape future research on NFDS. This article is part of the theme issue ‘Exploring negative frequency dependent selection across levels: from genetics to ecology and back again’.