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Bet hedging (biology)

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Variance in egg size may be an example of bet-hedging. Fitness may be maximized by producing many, small eggs. However, larger eggs may help offspring survive stressful conditions. Producing a range of egg sizes can both ensure that some offspring survive difficult conditions, and that many offspring are produced in good conditions.

Biological bet hedging, analogous to hedging in finance, occurs when organisms suffer decreased fitness in "normal" conditions in exchange for increased fitness in stressful conditions. Biological bet hedging was originally proposed to explain the observation of a seed bank, or a reservoir of ungerminated seeds in the soil.[1] For example, an annual plant's fitness is maximized for that year if all of its seeds germinate. However, if a drought occurs that kills germinated plants, but not ungerminated seeds, plants with seeds remaining in the seed bank will have a fitness advantage. Therefore, it can be advantageous for plants to "hedge their bets" in case of a drought by producing some seeds that germinate immediately and other seeds that lie dormant. Other examples of biological bet hedging include female multiple mating,[2] foraging behavior in bumble bees,[3] nutrient storage in rhizobia,[4] and bacterial persistence in the presence of antibiotics.[5]

The evolution of bet hedging

The evolution of an allele that is deleterious is the normal environment (white) but advantageous in an alternate environment (grey). The bet hedging allele arises twice due to mutation, the first occurrence is lost before the environment changes, but the second mutant persists long enough due to drift for the environment to change, at which point it reaches fixation due to selection.

Bet hedging traits are, by definition, neutral or slightly deleterious in the "normal" environment, and beneficial in a less common, often more stressful, environment or condition. Therefore, the conditions under which a bet hedging allele may be favored due to natural selection are slightly more complicated than for a globally advantageous allele. First, a bet hedging mutant must persist in the "normal" environment due to genetic drift long enough for the alternative environment, in which the bet hedger has an advantage, to occur. At that point selection may sweep the allele to fixation.[6]

References

  1. ^ Cohen, Dan (1966-09-01). "Optimizing reproduction in a randomly varying environment". Journal of Theoretical Biology. 12 (1): 119–129. doi:10.1016/0022-5193(66)90188-3.
  2. ^ Yasui, Yukio (2001-12-01). "Female multiple mating as a genetic bet-hedging strategy when mate choice criteria are unreliable". Ecological Research. 16 (4): 605–616. doi:10.1046/j.1440-1703.2001.00423.x. ISSN 1440-1703.
  3. ^ "Diversity of speed-accuracy strategies benefits social insects". {{cite journal}}: Cite journal requires |journal= (help)
  4. ^ Ratcliff, William C.; Denison, R. Ford (2010-10-12). "Individual-Level Bet Hedging in the Bacterium Sinorhizobium meliloti". Current Biology. 20 (19): 1740–1744. doi:10.1016/j.cub.2010.08.036.
  5. ^ Kussell, E. (31 January 2005). "Bacterial Persistence: A Model of Survival in Changing Environments". Genetics. 169 (4): 1807–1814. doi:10.1534/genetics.104.035352.
  6. ^ King, Oliver D.; Masel, Joanna (2007-12-01). "The evolution of bet-hedging adaptations to rare scenarios". Theoretical Population Biology. 72 (4): 560–575. doi:10.1016/j.tpb.2007.08.006. PMC 2118055. PMID 17915273.
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