Michael Sheehan Morphological and population genomic evidence of selection for individual identity signaling in human faces
There's no abstract, but one area where I suspect selection of this sort may turn out to be relevant (at least more relevant than Peter Frost-style sexual selection) is in explaining European hair and eye color variation.
"Traits signaling identity should be highly variable, often display polymodal distributions, not be condition dependent (i.e., be cheap to produce and/or maintain), not be associated with fitness differences, exhibit independent assortment of component characters, and often occur as fixed phenotypes with a high degree of genetic determination."
"Is human facial distinctiveness an adaptive signal of individual identity? From a sociobiological perspective, humans seem to have the ‘perfect storm’ of selection pressures that might favor recognizability. We are extremely social, interacting repeatedly with large numbers of individuals, each with varying roles in our lives. We are extremely cooperative, and we make complex decisions about whether and how much to cooperate based on kinship, friendship and social reputation [39,78]."
Signaling Individual Identity versus Quality: A Model and Case Studies with Ruffs, Queleas, and House Finches (pdf)
We develop an evolutionary model that predicts that characters selected to signal individual identity will have properties differing from those expected for indicator signals of quality. Traits signaling identity should be highly variable, often display polymodal distributions, not be condition dependent (i.e., be cheap to produce and/or maintain), not be associated with fitness differences, exhibit independent assortment of component characters, and often occur as fixed phenotypes with a high degree of genetic determination. We illustrate the existence of traits with precisely these attributes in the ornamental, conspicuously variable, and sexually dimorphic breeding plumages of ruff sandpipers Philomachus pugnax and red-billed que- leas Quelea quelea. Although ruffs lek and queleas are monogamous, both species breed in high-density aggregations with high rates of social interactions (e.g., aggression and territory defense). Under these socioecological conditions, individual recognition based on vi- sual cues may be unusually important. In contrast to these species, we also review plumage characteristics in house finches Carpodacus mexicanus, a nonterritorial, dispersed-breeding species in which plumage ornamentation is thought to signal quality. In keeping with expectations for quality signals, house finch plumage is relatively less variable, unimodally distributed, condition dependent, correlated with fitness measures, has positively correlated component charac- ters, and is a plastic, environmentally determined trait. We briefly discuss signals of identity in other animals. [. . .]
Variance in human facial appearance provides another interesting polymorphism that may have been shaped by selection for recognizability. The diversity in human faces offers a rich source of information that is regularly used for identifying individuals. Identity signals in our species could be adaptive for a variety of reasons, such as large group sizes (most human groups include 150 people or more; Ridley 1998) coupled with the importance of status hierarchies, reputations, and widespread delayed reciprocal altruism. If human facial characteristics are identity sig- nals, then they should be composed of genetically deter- mined subcomponents that assort independently and dis- play complex distributions with high variance.
Individual recognition: it is good to be different (pdf)
Individual recognition (IR) behavior has been widely studied, uncovering spectacular recognition abilities across a range of taxa and modalities. Most studies of IR focus on the recognizer (receiver). These studies typi- cally explore whether a species is capable of IR, the cues that are used for recognition and the specializations that receivers use to facilitate recognition. However, rela- tively little research has explored the other half of the communication equation: the individual being recog- nized (signaler). Provided there is a benefit to being accurately identified, signalers are expected to actively broadcast their identity with distinctive cues. Consider- ing the prevalence of IR, there are probably widespread benefits associated with distinctiveness. As a result, selection for traits that reveal individual identity might represent an important and underappreciated selective force contributing to the evolution and maintenance of genetic polymorphisms.
Individual recognition as communication
Recognition is required for almost all social behavior. Recognition ranges across a wide spectrum, including self, kin, mate, gender, neighbor, rival, friend, species, predator and prey . Individual recognition (IR) refers to a subset of recognition that occurs when one organism identifies another according to its individually distinctive character- istics . Although IR is the most precise form of recog- nition, it is always associated with some other form of recognition. Depending on the context in which an indi- vidual’s identity is learned, IR can be used to discriminate a mate, offspring, sibling, friend or rival. During IR, the signaler is recognized by unique recognition cues, and the receiver learns the cues and uses them to identify the signaler during future interactions. [. . .]
Receiver specialization for recognition
When the ability to recognize individuals is strongly favored by selection, receivers might evolve specializations that help them identify individuals more easily. For example, humans (Homo sapiens) [58,59], sheep  and macaques (Macaca mulatta)  have neural specialization for facial IR. Faces are processed in a specific area of the brain and the brain treats faces differently from other objects. A striking con- sequence of human neural specialization for face recognition is that some humans experience a condition called ‘face blindness’ during which they cannot recognize individual faces but can still recognize objects . Neural specializ- ations for IR are probably not universal, but they might evolve when there is strong selection for quick and easy identification of many individuals. [. . .]
Identity signaling in humans
Our faces provide powerful images that are full of multiple messages. Our expressions provide information about cur- rent motivational state. Our male-like or female-like facial proportions provide information about gender and hor- mone-exposure [75,76]. High symmetry and youthfulness signal characteristics associated with attractiveness . In addition, there are countless subtle differences that collectively contribute to our overall distinctiveness (Figure 1): a key aspect of being human.
Is human facial distinctiveness an adaptive signal of individual identity? From a sociobiological perspective, humans seem to have the ‘perfect storm’ of selection press- ures that might favor recognizability. We are extremely social, interacting repeatedly with large numbers of indi- viduals, each with varying roles in our lives. We are extremely cooperative, and we make complex decisions about whether and how much to cooperate based on kin- ship, friendship and social reputation [39,78]. These beha- viors require accurate IR and the cognitive ability to associate complex information with each individual’s iden- tity. If human facial variability has evolved to signal individual identity, the properties of human facial vari- ation are expected to be consistent with those expected for identity signals  (Box 3). Targeted research is needed to evaluate how well human faces fit the general model. If human faces are identity signals, humans who are difficult to individually distinguish are expected to suffer costs. For example, perhaps career success in the entertainment industry is determined not only by attractiveness and talent, but also by a particularly distinctive appearance?