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Reintroducing history into evolutionary thinking has already begun at other biological scales. The cell, once an emblem of replicable units, turns out to be the historical product of symbiosis among free- living bacteria. Even DNA turns out to have more history in its amino- acid sequences than once thought. Human DNA is part virus; viral encoun- ters mark historical moments in making us human. Genome research has taken up the challenge of identifying encounter in the making of DNA. Population science cannot avoid history for much longer.
Fungi are ideal guides. Fungi have always been recalcitrant to the iron cage of self- replication. Like bacteria, some are given to exchanging genes in nonreproductive encounters (“horizontal gene transfer”); many also seem averse to keeping their genetic material sorted out as “individ- uals” and “species,” not to speak of “populations.” When researchers studied the fruiting bodies of what they thought of as a species, the ex- pensive Tibetan “caterpillar fungus,” they found many species entan- gled together. When they looked into the filaments of Armillaria root rot, they found genetic mosaics that confused the identification of an individual. Meanwhile, fungi are famous for their symbiotic attach- ments. Lichen are fungi living together with algae and cyanobacteria. I have been discussing fungal collaborations with plants, but fungi live with animals as well. For example, Macrotermes termites digest their food only through the help of fungi. The termites chew up wood, but they cannot digest it. Instead, they build “fungus gardens” in which the chewed- up wood is digested by Termitomyces fungi, producing edible nutrients. Researcher Scott Turner points out that, while you might say that the termites farm the fungus, you could equally say that the fungus farms the termites. Termitomyces uses the environment of the termite mound to outcompete other fungi; meanwhile, the fungus regulates the mound, keeping it open, by throwing up mushrooms annually, cre- ating a colony- saving disturbance in termite mound- building.
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