Seminar One
Biology, culture and psychology
The symbiosis continuum
A dynamic movement of a colonist and its host move along a continuum between antagonism and cooperation. A pathogen may live with us but not cause disease most of the time, like herpes simplex viruses (that cause fever blisters.) The pathogen occasionally causes serious illness. The movement along the continuum is dynamic and changing.
The bacteria that live in our large intestines can also move along the continuum. Most of the time they are mutualists, benefitting us and being benefitted by living inside us, but can you guess when they might instantly shift to the pathogenic end of the continuum? Answer: If your appendix ruptures, or if you get a gunshot wound to the abdomen, those very same bacteria move into other compartments of your body and become serious pathogens.
Natural selection is the pervasive explanatory paradigm throughout all of these odysseys into the coexistence of hosts and colonists. Both evolve together, or co-evolve. We have two new terms here: coevolution and symbiosis. Coevolution is nothing more than two or more organisms evolving in response to each other, with natural selection working independently on both. Symbiosis is just what the word is constructed to mean: living together. Symbiosis includes the entire continuum between antagonism and cooperation, with commensalism in the middle (in which one organism benefits and the other is more or less unaffected.)
A biologist friend once asked me, “Greg, how many habitats are there?” I answered three: land, fresh water, and marine. “What's the fourth?” he asked. I was dumbfounded—especially when he answered: “Hosts!” When you think about it, hosts are the most common habitat of all. If every parasite has a parasite, often more than one, there are more parasites than hosts. And there are mutualists and commensals too—mutualists are collaborators, such as leafcutter ants and their fungi, and commensals are free-loaders that don 't cause much trouble for their host. Ecology is not just about forests, grasslands, and marine habitats, but also about hosts. Many organisms live only in or on host organisms, which constitute their ecology.
