, 2007). Negative frequency-dependent selection (NFDS), in which a rare morph has a fitness advantage over common morphs, can account for the existence of different morphs at stable frequencies in a population (Clarke & O’Donald, 1964; Ayala & Campbell, 1974), and has been proposed to explain

polymorphisms in a number of contexts (Hori, 1993; Fincke, 2004; Sinervo & Calsbeek, 2006; McKillup & McKillup, 2008; Hampton, Hughes & Houde, 2009; Koskella & Lively, 2009). Evidence of NFDS has been observed both in laboratory (Kojima & Tobari, AP24534 clinical trial 1969; Maskell, Parkin & Verspoor, 1977; Anderson & Brown, 1984; Gigord, Macnair & Smithson, 2001; Fitzpatrick et al., 2007; Koskella & Lively, 2009) Talazoparib supplier and natural conditions (Reid, 1987; Hori, 1993; Svensson, Abbott & Hardling, 2005; Olendorf et al., 2006; Bleay, Comendant & Sinervo, 2007; McKillup & McKillup, 2008; Takahashi & Watanabe, 2010). Nevertheless, considerable uncertainty exists about the relative importance of this and other mechanisms in the maintenance of genetic and

phenotypic diversity in real populations. There are some genetic polymorphisms that do not affect phenotypic traits. They occur in non-coding areas of the genome, and have been used as markers for studies in population genetics, evolution and medicine (Hacia et al., 1999; Jorde et al., 2000; Syvänen, 2001; Williamson et al., 2007). Polymorphisms that do affect phenotypic traits are not always apparent to the observer, such as some of those involving why behaviour and resistance to parasites or diseases (Thornhill, 1979; Field & Keller, 1993; Kirkup & Riley, 2004; Duncan & Little, 2007; Laine & Tellier, 2008). In contrast, conspicuous polymorphisms, particularly those involving colouration, are easy to score, and their study has been central in attempts to understand the mechanisms that could be maintaining genetic and phenotypic variation in populations. Colouration is known to serve an adaptive function in processes such as thermoregulation (Quartau & Borges, 1997; Phifer-Rixey

et al., 2008), attraction of mates (Nielsen & Watt, 2000), avoidance of predators (Hoese et al., 2006) and attraction of prey (Hauber, 2002; Heiling et al., 2005; Bush, Yu & Herberstein, 2008). This strongly suggests that the maintenance of conspicuous colour polymorphisms is influenced by selection, and NFDS in particular has often been assumed to play a key role. Many species of insect, mollusc, arachnid and crustacean display conspicuous and easily measured polymorphic colour traits. Such invertebrates are typically easier to manipulate than vertebrates, both in the field and in the laboratory, and it is relatively easy to get large sample sizes. As a result, many of the most detailed case studies of the potential influence of NFDS on traits come from the study of colour-polymorphic invertebrates.