A mutualistic symbiosis occurs when organisms of different species cooperate closely for a net benefit over time. Mutualistic relationships are important for human health, food production, and ecosystem maintenance. However, they can evolve to parasitism or breakdown all together and the conditions that maintain and influence them are not completely understood. Vertical and horizontal transmission of mutualistic endosymbionts are two factors that can influence the evolution of mutualism. Using the artificial life system, Symbulation, we studied the effects of different rates of mutation during horizontal transmission on mutualistic symbiosis at different levels of vertical transmission. We propose and provide evidence for the “Dirty Transmission Hypothesis”, which states that higher rates of mutation during horizontal transmission can select for increased mutualism to avoid deleterious mutation accumulation.