Lichens: a lasting relationship like farmers and crops
With their stable symbiosis between a fungus and an alga, lichens are a prime example for lasting relationships. But these lichen fungi do not simply host their algae, they actually grow them as much as farmers grow their crop. And just as farmers select the best crop to propagate, lichen fungi do the same. Not consciously, but through the process of selective evolution. If a fungus and an alga come together to form a lichen, the "fitness" of the lichen, its ability to survive in a competitive environment, depends on the "fitness" of each of its component and how the two of them work together. So, among the genetic variation that exists for both the fungus and the alga, there are better or worse combinations. This is analogous to humans having a strong or weak immune system, depending on the genetic makeup of their parents. Since the more competitive lichens have a higher probability of survival and propagation, the fungal and algal genotypes that participate in these combinations will increase in relative frequency in the population. Just like a farmer preferably grows a particular strain of crop and gets rid of those that don't do well.
It doesn't stop here. Lichens also "sell" their improved photobiont strains. Well, not literally. But commonly photobionts get transferred accidentally to form another lichen with another fungus. These fungi are often not even related. As if a farmer in the United States would sell his best crop to another farm in New Zealand. The lichens depicted here form part of such an "exchange" group, since they all have the same photobiont type, a cyanobacterium in the genus Rhizonema (shown in the lower right corner). But the lichen fungi belong to very different entities: Acantholichen in the upper left corner is somehow related to the fly agaric, whereas Stereocaulon in the upper right corner is a distant relative of Penicillium. So, over millions of years of evolution, these lichen fungi have created highly improved and specific algal strains that do very well in lichens, but might not be able anymore to survive on their own.
