Individual identity signatures: roles of negative frequency-dependent selection and cognition in shaping and maintaining individuality and trait diversity

Recognition is critical for a wide range of biological interactions from the immune system at the cellular level to individual recognition mediating complex animal social structures. Identity signatures are common features of many animal traits and facilitate self/non-self and individual recognition across diverse taxa. Inherently, identity signatures show high levels of intra-population phenotypic diversity. While the existence of individually distinctive traits has attracted much attention, the developmental mechanisms and evolutionary pressures shaping identity signature diversity have been less well studied. Here, we review the evidence that selection shapes the phenotypic diversity of individual identity signals used in a range of social and sexual contexts. Evidence from comparative, developmental and behavioural studies shows that selection often shapes phenotypes to be signals rather than cues of identity. Population genomic evidence implicates negative frequency-dependent selection maintaining identity signalling diversity in humans and mice. Like the extreme diversity seen at allorecognition loci, such as major histocompatibility complex (MHC) in animals and self-incompatibility in plants, social processes in animals are drivers of phenotypic and genetic diversity. However, the quantitative traits contributing to identity signals in animals are more complex than allorecognition loci and thus the consequences of selection for individual identity signals probably differ in multiple ways in terms of the maintenance of allelic diversity. The role of cognition in shaping identity signalling traits and long-term patterns of phenotypic and allelic variation and divergence across populations and species is discussed. This article is part of the Theme Issue 'Exploring negative frequency dependent selection across levels: from genetics to ecology and back again'.