Denim and Tweed: No room for group selection in disease evolution?: "Charles Darwin originally proposed it to explain the evolution of human moral systems: in a tribal society, helping your neighbor might cost you, but it might still help your whole tribe to compete against other tribes. So natural selection on individuals within a tribe may act in one way, but be opposed by group selection arising from competition among tribes."
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trade-off between transmission and virulence [PDF]. Simply put, if it's easy for a disease-causing critter to spread through a host population, it tends to do more damage to its hosts; and if it is less easy to spread,
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Kin selection takes into account the effect of natural selection on not just the copies of an individual's genes within that individual's body, but also the copies borne by close relatives; if you're a parasite that reproduces inside your host, making more offspring also means making more competitors for your offspring, and thereby reducing the fitness of the genes that you share with the next generation. So, unless it's easy to disperse to new resources -- uninfected hosts -- natural selection can actually favor prudent reproduction by a parasite, which keeps the host alive longer.
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