Green-beard effect
The gene-centered view of evolution
Gene-centered view of evolution
The gene-centered view of evolution, gene selection theory or selfish gene theory holds that evolution occurs through the differential survival of competing genes, increasing the frequency of those alleles whose phenotypic effects successfully promote their own propagation, with gene defined as...

 postulates that natural selection
Natural selection
Natural selection is the nonrandom process by which biologic traits become either more or less common in a population as a function of differential reproduction of their bearers. It is a key mechanism of evolution....

 will increase the frequency of those genes whose phenotypic
A phenotype is an organism's observable characteristics or traits: such as its morphology, development, biochemical or physiological properties, behavior, and products of behavior...

 effects ensure their successful replication
DNA replication
DNA replication is a biological process that occurs in all living organisms and copies their DNA; it is the basis for biological inheritance. The process starts with one double-stranded DNA molecule and produces two identical copies of the molecule...

. A gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...

 for altruism
Altruism is a concern for the welfare of others. It is a traditional virtue in many cultures, and a core aspect of various religious traditions, though the concept of 'others' toward whom concern should be directed can vary among cultures and religions. Altruism is the opposite of...

 can be favored by selection if the altruism is primarily directed at other individuals who share the same gene (kin selection
Kin selection
Kin selection refers to apparent strategies in evolution that favor the reproductive success of an organism's relatives, even at a cost to the organism's own survival and reproduction. Charles Darwin was the first to discuss the concept of group/kin selection...


A green-beard effect occurs when a gene, or linked genes, produce three phenotypic effects:
  1. a perceptible trait
    Trait (biology)
    A trait is a distinct variant of a phenotypic character of an organism that may be inherited, environmentally determined or be a combination of the two...

     — the hypothetical "green beard";
  2. recognition of this trait in others; and
  3. preferential treatment to those recognized.

So this gene is directly recognizing copies of itself, regardless of average relatedness. 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 altruism could increase the presence of green-beard phenotypes in a population even if genes 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 genes are vulnerable to mutant genes arising that produce the perceptible trait without the helping behaviour.

The idea of a green-beard gene was proposed by William D. Hamilton
W. D. Hamilton
William Donald Hamilton FRS was a British evolutionary biologist, widely recognised as one of the greatest evolutionary theorists of the 20th century....

 in his articles of 1964, and named as "Green Beard" by Richard Dawkins
Richard Dawkins
Clinton Richard Dawkins, FRS, FRSL , known as Richard Dawkins, is a British ethologist, evolutionary biologist and author...

 in The Selfish Gene
The Selfish Gene
The Selfish Gene is a book on evolution by Richard Dawkins, published in 1976. It builds upon the principal theory of George C. Williams's first book Adaptation and Natural Selection. Dawkins coined the term "selfish gene" as a way of expressing the gene-centred view of evolution as opposed to the...



In the last several years, evolutionary biologists have questioned the potential validity of greenbeard 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 gene was first found in nature, in the red imported fire ant
Red imported fire ant
See main article Fire ant.The red imported fire ant , or simply RIFA, is one of over 280 species in the widespread genus Solenopsis...

 (Solenopsis invicta). Polygyne colony queens are heterozygous (Bb) at the Gp-9 gene locus. The investigators discovered that homozygous dominant (BB) queens are specifically killed, most often by heterozygous workers (Bb) and not homozygous dominants (BB). 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 Dictyostelium discoideum
Dictyostelium discoideum
Dictyostelium discoideum is a species of soil-living amoeba belonging to the phylum Mycetozoa. D. discoideum, commonly referred to as slime mold, is a eukaryote that transitions from a collection of unicellular amoebae into a multicellular slug and then into a fruiting body within its lifetime. D...

, discovered in 2003, codes for a cell adhesion
Cell adhesion
Cellular adhesion is the binding of a cell to a surface, extracellular matrix or another cell using cell adhesion molecules such as selectins, integrins, and cadherins. Correct cellular adhesion is essential in maintaining multicellular structure...

 protein which binds to gp80 proteins on other cells, allowing multicellular fruiting body formation on soil. Mixtures of csA knockout
Gene knockout
A gene knockout is a genetic technique in which one of an organism's genes is made inoperative . Also known as knockout organisms or simply knockouts, they are used in learning about a gene that has been sequenced, but which has an unknown or incompletely known function...

 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
Flocculation, in the field of chemistry, is a process wherein colloids come out of suspension in the form of floc or flakes by the addition of a clarifying agent. The action differs from precipitation in that, prior to flocculation, colloids are merely suspended in a liquid and not actually...

, 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 gene.

A mammalian example appears to be the reproductive strategy, in particular cooperation among spermatazoa, of the wood mouse
Wood mouse
The wood mouse is a common murid rodent from Europe and northwestern Africa. It is closely related to the yellow-necked mouse but differs in that it has no band of yellow fur around the neck, has slightly smaller ears, and is usually slightly smaller overall: around 90 mm in length...


See also

  • Maternal effect dominant embryonic arrest
    Maternal effect dominant embryonic arrest
    Maternal effect dominant embryonic arrest is a selfish gene composed of a toxin and an antidote. A mother carrying Medea will express the toxin in her germline, killing her progeny. If the children also carry Medea, they produce copies of the antidote, saving their lives...

     (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.

Further reading

  • Haig, D.
    David Haig (biologist)
    David Haig, is an Australian evolutionary biologist and geneticist, professor in Harvard Department of Organismic and Evolutionary Biology. He is interested in intragenomic conflict, genomic imprinting and parent-offspring conflict, and wrote the book Genomic Imprinting and Kinship...

     (1997) The social gene. In Krebs, J. R.
    John Krebs
    John Richard Krebs, Baron Krebs FRS is a world leader in zoology and more specifically bird behaviour. He is currently the Principal of Jesus College, Oxford University...

    & Davies, N. B. (editors) Behavioural Ecology: an Evolutionary Approach, 4th ed. pp. 284–304. Blackwell Publishers, London.
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