||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 altruism between individuals. It is linked to the gene-centered view of evolution, which suggests that natural selection will increase the frequency of those alleles whose phenotypic effects ensure their successful replication. An allele is a piece of genetic code; see its article for a more precise definition. Phenotypic effects are just effects on the nature of the organism; they are what a gene or allele does. A gene for altruism can be favoured by selection if the altruism is primarily directed at other individuals who share the same allele (kin selection).
A green-beard effect occurs when an allele, or linked alleles, produce three phenotypic effects:
- 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 directly recognizing copies of the alleles in other individuals. Whereas most alleles that are favored by kin selection spread by promoting altruism towards those likely to be carrying the same allele, green-beard alleles would rise to frequency by promoting altruism toward individuals certain to be carrying the same allele.
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.
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