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. 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.