Understanding altruistic behaviours, those actions that increase another individual's fitness at a cost to your own, is one of the greatest challenges to evolutionary biologists, as natural selection appears to favour selfish and uncooperative individuals (Hamilton 1963
). Despite this, there are many examples in the animal kingdom where this form of cooperation has been successfully demonstrated. However, it is only recently that social behaviour in micro-organisms has been studied with respect to evolutionary theory (Crespi 2001
), and so there is a strong potential to develop complementary research in this area from both molecular and adaptive (Darwinian) perspectives (West et al. 2006
Bacteria exhibit remarkable social behaviours, which some workers have suggested are similar to those performed by insects, vertebrates and humans (). For example, Myxococcus xanthus
cells exhibit socially dependent swarming across surfaces (Velicer & Yu 2003
), which allows the population to seek out bacterial prey in a manner that has been likened to hunting wolf packs (Dworkin 1996
; Crespi 2001
). Biofilms are a collection of bacterial cells (both single and mixed species) enclosed in a polysaccharide matrix and have been likened in nature to ant nests or beehives (Crespi 2001
; Webb et al. 2003
; Parsek & Greenberg 2005
). Biofilms, for example those found on the human teeth, can contain up to 500 species of bacteria (Kolenbrander et al. 2002
), providing an environment that is ripe for social interactions both within and between species.
Social traits exhibited by bacteria compared with examples from vertebrates and invertebrates.
Perhaps the paradigm for bacterial cooperation and social behaviour can be seen in the diverse quorum-sensing (QS) systems found in both Gram-negative and Gram-positive bacteria (Swift et al. 2001
). It is generally assumed that QS represents both intra- and interspecies signalling and that QS cooperation is for the benefit of the local group or population as a whole. However, as previously mentioned, cooperative communication can require specific conditions for it to evolve. Therefore, this raises the question as to whether QS in microbes is truly cooperative behaviour (Redfield 2002
; Keller & Surette 2006
This review aims to promote awareness of some of the evolutionary problems provided by QS. A key issue in research on communication is the careful and consistent use of terminology (Maynard Smith & Harper 2003
). We (i) provide the standard definitions for terms such as signalling, (ii) discuss the conditions required for signalling to evolve, and be stable to invasion from mutants that exploit the signalling of others to their selfish benefit, and (iii) use specific microbial examples to illustrate whether the molecules produced are primarily used as signals, cues or for coercion.