Results and Discussion With the standard settings (see "Online Simulation") we can observe the following effects when we vary only one of the parameters.
nr-groups: The fewer groups there are, the greater the likelihood that the entire population will die out as a result of the infections. The number of groups is therefore important for the survival of the population.
n-groupmembers: The size of the individual groups has little influence on the development of the harm values.
p-sick: With infection rates p-sick between 0.25 and 0.5, the whole population achieves average harm values less than 2 (= almost hairless) within a few infection cycles, i.e., individuals become quite hairless quite quickly.
mortality-harm-1, mortality-harm-8:
Remember, mortality-harm-1 and mortality-harm-8 specify the mortality rates between for harm values 1 and 8; the values in between are proportional. Lower harm values generally mean higher mortality, e.g., due to insufficient protection against injuries, sun, or cold provided by the hair.
In the standard setup, mortality-harm-1 (20%) is twice as high as mortality-harm-8 (10%), nevertheless, a low harm value of around two quickly evolves. But even without mortality for the shaggy (mortality-harm-8 = 0%) and 50% for the naked mortality-harm-1, a stable populations develops with harm values around 3.
fill-groups and migration: The most stable populations are obtained with fill-groups=empty, i.e., when only empty groups are filled with offspring from the other groups. However, even with random migration, stable populations with low hair density can develop. With stronger migration, i.e., from 30% onwards, the populations become increasingly unstable and perish more frequently.
set-disturber: With infection rates p-sick between 0.25 and 0.5, the whole population achieves average harm values less than 2.
Example evolve: This simulation starts with the standard setup except that p-sick=0 (no infection) and all agents in start population have harm=8. As p-sick is stepwise increased during the simulation, the harm values decrease and approach the minimal harm=1.
After just a few cycles of infection, the population establishes the hairless type, even if it is subject to greater selection pressure than the hairy type. In this thought experiment involving a deadly infection, the trait therefore benefits only others; it has a posthumous effect, so to speak.
This will be achieved solely through stochastic processes that arise naturally in group-structured populations. It requires neither competition between the groups nor any ties or relationships between the group members.
In a nutshell
The simulation model proves that humans may have lost their hair for the good of the group.
A more detailed analysis is in progress.
References and further reading
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Curtis, Val, and Mícheál de Barra. "The structure and function of pathogen disgust." Philosophical Transactions of the Royal Society B: Biological Sciences 373.1751 (2018): 20170208.
Cremer, Sylvia, Christopher D. Pull, and Matthias A. Fürst. "Social immunity: emergence and evolution of colony-level disease protection." Annual Review of Entomology 63 (2018): 105-123.
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Steiner, K.F., 2024. Altruism pays off in group-structured populations through probable reciprocity. bioRxiv 2024.01.20.575560. doi: https://doi.org/10.1101/2024.01.20.575560
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Wilson, David Sloan, and Edward O. Wilson. "Evolution" for the Good of the Group": The process known as group selection was once accepted unthinkingly, then was widely discredited; it's time for a more discriminating assessment." American Scientist 96.5 (2008): 380-389.