Cheating (biology)

Theoretical models

Organisms communicate and cooperate to perform a wide range of behaviors. Mutualism, or mutually beneficial interactions between species, is common in ecological systems.^[5] These interactions can be thought of "biological markets" in which species offer partners goods that are relatively inexpensive for them to produce and receive goods that are more expensive or even impossible for them to produce.^[6] However, these systems provide opportunities for exploitation by individuals that can obtain resources while providing nothing in return. Exploiters can take on several forms: individuals outside a mutualistic relationship who obtain a commodity in a way that confers no benefit to either mutualist, individuals who receive benefits from a partner but have lost the ability to give any in return, or individuals who have the option of behaving mutualistically towards their partners but chose not to do so.^[5]

Cheaters, who do not cooperate but benefit from others who do cooperate, gain a competitive edge. In an evolutionary context, this competitive edge refers to a greater ability to survive or to reproduce. If individuals who cheat are able to gain survivorship and reproductive benefits while incurring no costs, natural selection should favor cheaters. What then prevents cheaters from undermining mutualistic systems? One main factor is that the advantages of cheating are often frequency-dependent. Frequency-dependent selection occurs when the fitness of a phenotype depends on its frequency relative to other phenotypes in a population. Cheater phenotypes often display negative frequency-dependent selection, where fitness increases as a phenotype becomes less common and vice versa.^[7] In other words, cheaters do best (in terms of evolutionary benefits such as increased survival and reproduction) when there are relatively few of them, but as cheaters become more abundant, they do worse.

For example, in Escherichia coli colonies, there are antibiotic-sensitive "cheaters" that persist at low numbers on antibiotic-laced mediums when in a cooperative colony. These cheaters enjoy the benefit of others producing antibiotic-resistant agents while producing none themselves. However, as numbers increase, if they persist in not producing the antibiotic agent themselves, they are more likely to be negatively impacted by the antibiotic substrate because there is less antibiotic agent to protect everyone.^[7] Thus, cheaters can persist in a population because their exploitative behavior gives them an advantage when they exist at low frequencies but these benefits are diminished when they are greater in number.

Others have proposed that cheating (exploitive behavior) can stabilize cooperation in mutualistic systems.^[4][8] In many mutualistic systems, there will be feedback benefits to those that cooperate. For instance, the fitness of both partners may be improved. If there is a high reward or many benefits for the individual that initiated the cooperative behavior, mutualism should be selected for. When researchers investigated the co-evolution of cooperation and choice in a choosy host and its symbiont (an organism that lives in a relationship that benefits all parties involved), their model indicated that although choice and cooperation may be initially selected for, this would often be unstable.^[4] In other words, one cooperative partner will choose another cooperative partner if given a choice. However, if this choice is made over and over, variation is removed and this selection can no longer be maintained. This situation is similar to the lek paradox in female choice. For example, in lek paradox, if females consistently choose for a particular male trait, genetic variance for that trait should eventually be eliminated, removing the benefits of choice. However, that choice somehow still persists.

What maintains genetic variability in the face of selection for mutualism (cooperative behavior)? One theory is that cheating maintains this genetic variation. One study shows that a small influx of immigrants with a tendency to cooperate less can generate enough genetic variability to stabilize selection for mutualism.^[4] This suggests that the presence of exploitive individuals, otherwise known as cheaters, contribute enough genetic variation to maintain mutualism itself. Both this theory and the negative frequency-dependent theory suggest that that cheating exists as part of a stable mixed evolutionary strategy with mutualism. In other words, cheating is a stable strategy used by individuals in a population where many other individuals cooperate. Another study supports that cheating can exist as a mixed strategy with mutualism using a mathematical game model.^[9] Thus, cheating can arise and be maintained in mutualistic populations.