||This article may be too technical for most readers to understand. (August 2012)|
The green-beard effect is a hypothesis used in evolutionary biology to explain selective altruism between designated individuals of a species. It is based on the gene-centered view of evolution, which emphasizes an interpretation of natural selection from the point of view of the gene which acts as an agent that has the metaphorical "goal" of maximizing its own propagation. A gene for (behavioral) selective altruism can be favored by (natural) selection if the altruism is primarily directed at other individuals who share the same allele.
- a perceptible trait, — the hypothetical "green beard"
- recognition of this trait in others; and
- preferential treatment to those recognized.
The carrier of the allele is essentially recognizing copies of the same allele in other individuals. Whereas most alleles that are favored by kin selection spread by individuals who show family altruism toward others who share their alleles in a non-specific way, green-beard alleles would rise in frequency by promoting altruism toward individuals who share a specific phenotypic trait (allele). This can have the effect of delineating a subset of organisms within a population that is characterized by members who show greater cooperation toward each other, this forming a "clique" that can be advantageous to its members who are not necessarily kin.
Green-beard effect could increase altruism on green-beard phenotypes and therefore its presence in a population even if alleles are assisting other genes that are not exact copies of themselves in a molecular sense; all that is required is that they produce the three phenotypic characteristics described above. Green-beard alleles are vulnerable to mutant genes arising that produce the perceptible trait without the helping behaviour.
In the last several years, evolutionary biologists have questioned the potential validity of green-beard alleles, suggesting it would be extraordinarily rare for a single allele to produce three complex phenotypic effects. This criticism has led some to believe that they simply cannot exist or that they only can be present in less complex organisms, such as microorganisms. Several discoveries within the past ten years have illuminated the validity of this critique.
The concept remained a merely theoretical possibility under Dawkins' selfish gene model until 1998, when a green-beard allele was first found in nature, in the red imported fire ant (Solenopsis invicta). Polygyne colony queens are heterozygous (Bb) at the Gp-9 gene locus. Their worker offspring can have both heterozygous (Bb) and homozygous (BB) genotypes. The investigators discovered that homozygous dominant (BB) queens, which in the wild form produce monogyne rather than polygyne colonies, are specifically killed when introduced into polygyne colonies, most often by heterozygous (Bb) and not homozygous (BB) workers. They concluded that the allele Gp-9b is linked to a greenbeard allele which induces workers bearing this allele to kill all queens that do not have it. A final conclusion notes that the workers are able to distinguish BB queens from Bb queens based on an odor cue.
The gene csA in the slime mould Dictyostelium discoideum, discovered in 2003, codes for a cell adhesion protein which binds to gp80 proteins on other cells, allowing multicellular fruiting body formation on soil. Mixtures of csA knockout cells with wild-type cells yield spores, "born" from the fruiting bodies, which are 82% wild-type (WT). This is because the wild-type cells are better at adhering and more effectively combine into aggregates; knockout (KO) cells are left behind. On more adhesive but less natural substances, KO cells can adhere; WT cells, still better at adhering, sort preferentially into the stalk.
In 2006, green beard-like recognition was seen in the cooperative behavior among color morphs in side-blotched lizards, although the traits appear to be encoded by multiple loci across the genome.
A more recent example, found in 2008, is a gene that makes brewer's yeast clump together in response to a toxin such as alcohol. By investigating flocculation, a type of self-adherence generally present in asexual aggregations, Smukalla et al. showed that S. cerevisiae is a model for cooperative behavior evolution. When this yeast expresses FLO1 in the laboratory, flocculation is restored. Flocculation is apparently protective for the FLO1+ cells, which are shielded from certain stresses (ethanol, for example). In addition FLO1+ cells preferentially adhere to each other. The authors therefore conclude that flocculation is driven by this greenbeard allele.
A mammalian example appears to be the reproductive strategy of the wood mouse, which shows cooperation among spermatozoa. Single sperms hook in each other to form sperm-trains, which are able to move faster together than single sperm would do.
- Maternal effect dominant embryonic arrest (the "Medea" gene): an example of intergenerational gene self-selection, whereby a gene present in a mother organism selectively terminates offspring that do not receive that gene.
- Hamilton WD (July 1964). "The genetical evolution of social behaviour. I". J Theor Biol 7 (1): 1–16. doi:10.1016/0022-5193(64)90038-4. PMID 5875341.
- Hamilton WD (July 1964). "The genetical evolution of social behaviour. II". J Theor Biol 7 (1): 17–52. doi:10.1016/0022-5193(64)90039-6. PMID 5875340.
- Dawkins, Richard (1976). The Selfish Gene. Oxford University Press, Oxford. ISBN 0-19-217773-7.
- Grafen, Alan (6 August 1998). "Green beard as death warrant" (PDF). Nature 394 (6693): 521–522. doi:10.1038/28948. Retrieved 29 November 2009.
- Keller, Laurent; Ross, Kenneth G (6 August 1998). "Selfish genes: a green beard in the red fire ant". Nature 394 (6693): 573–575. doi:10.1038/29064. Retrieved 29 November 2009.
- Queller, David C; Ponte, Eleonora; Bozzaro, Salvatore; Strassmann, Joan E (3 January 2003). "Single-gene greenbeard effects in the social amoeba Dictyostelium discoideum" (PDF). Science 299 (5603): 105–106. doi:10.1126/science.1077742. PMID 12511650. Retrieved 29 November 2009.
- Sinervo B, Chaine A, Clobert J, Calsbeek R, Hazard L, Lancaster L, McAdam AG, Alonzo S, Corrigan G, Hochberg ME (May 2006). "Self-recognition, color signals, and cycles of greenbeard mutualism and altruism". Proc Natl Acad Sci USA 103 (19): 7372–7377. doi:10.1073/pnas.0510260103. PMC 1564281. PMID 16651531.
- Prakash, Sheila (18 December 2008). "Yeast Gone Wild". Seed. Retrieved 29 November 2009.
- Smukalla, Scott; Caldara, Marina; Pochet, Nathalie; Beauvais, A; Guadagnini, S; Yan, C; Vinces, MD; Jansen, A; Prevost, MC (14 November 2008). "FLO1 is a variable green beard gene that drives biofilm-like cooperation in budding yeast". Cell 135 (4): 726–737. doi:10.1016/j.cell.2008.09.037. PMC 2703716. PMID 19013280. Retrieved 29 November 2009.
- Harry Moore, Katerina Dvoráková, Nicholas Jenkins, William Breed (1 March 2002), "Exceptional sperm cooperation in the wood mouse", Nature 418, 174-177, doi:10.1038/nature00832;