Lichen is a combination of algae and fungus living together. This relationship allows them to survive in locations they could never live on their own.
If evolution is all about "survival of the fittest", how does cooperation evolve? Living things form cooperative relationships all the time. Research done by applying mathematics to biology is helping us understand the evolution of cooperation. Cooperation has caused some of the major evolutionary steps in the history of life, including the step from single celled creatures to multi-cellular plants and animals.
Scientists used mathematical models to find scenarios in which natural selection will actually favor cooperation.
Many species of fungus feed on lichen (paper)
Kin selection combined with budding can give rise to cooperation (paper)
Math of kin Selection (paper)
Math of Direct Reciprocation (paper)
Math of Indirect Reciprocation (paper)
Math of Network Reciprocation (paper)