Models of collective gain to see if a gene for altruistic behavior can persist
in a population if it induces a large enough benefit for other organisms of
the same species. Also called the "Givers & Takers" models.

Ma is a deterministic model consisting only of differential equations that
track the number of carriers and non-carriers of a gene that, eith a certain
probability, turns an organism into a sterile giver.

Mb is a stochastic model in which organisms live on a grid with torus
topology.

Mc is the oldest Givers & Takers model. It's more complex than the others
and stochastically models organisms living on a rectangular grid, competing
for sunlight, and subject to predation.

Md is a new, further simplified stochastic model in which organisms live on a
fully continuous toroidal world.