Quantifying internal conflicts and their threats to organismal form and fitness

Evolutionary biologists often treat organisms as both fitness-maximizing agents and as the primary level at which adaptation manifests. Yet, genes and cells may also seek to optimize their fitness by distorting the Mendelian rules of transmission or by influencing organismal traits for their own benefit. Organismal form and fitness are therefore threatened from within by selfish genes and cells. However, to what extent such internal conflicts actually harm individual organisms and threaten our concept of the organism as the sole bearer of adaptation remains unclear. We introduce a mathematical framework to capture the threat posed by internal conflicts and develop two metrics to measure their various forms of harm. We name these metrics fitness unity and trait unity, and use them to refer to the threats posed by internal conflicts to an organism’s role as the optimizing agent and the strategy wielded to achieve that optimization, respectively. We apply our framework to two examples of internal conflicts, genomic imprinting and sex ratio distortion, to illustrate how such harms from internal conflict may be quantified. We conclude by discussing the conditions under which internal conflict becomes sufficiently disruptive to organisms that it no longer makes sense to think of them as unified fitness-maximizing agents, but instead as adaptive compromises of multiple competing sub-agents.