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. 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. 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. 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. One theory states that cheating maintains genetic variation in the face of selection for mutualism. 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. 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