labor among the cells (which specialize into limbs and organs). A powerful new kind of vehicle appears, and in a short span of time the world is covered with plants, animals, and fungi. 37 It’s another major transition. Major transitions are rare. The biologists John Maynard Smith and Eors Szathmary count just eight clear examples over the last 4 billion years (the last of which is human societies). 38 But 35 Margulis 1970. In plant cells, chloroplasts also have their own DNA. 36 Maynard Smith and Szathmary 1997; Bourke 2011. 37 There is an important flaw in my “boat race” analogy: the new vehicles don’t really “win” the race. Prokaryotes are still quite successful; they still represent most of the life on earth by weight and by number. But still, new vehicles seem to come out of nowhere and then claim a substantial portion of the earth’s available bio-energy. 38 Maynard Smith and Szathmary attribute the human transition to language, and suggest that the transition occurred around 40,000 years ago. Bourke 2011 offers an up-to-date discussion. He identifies six major kinds of transitions, and notes that several of them have occurred dozens of
12 these transitions are among the most important events in biological history, and they are examples of multilevel selection at work. It’s the same story over and over again: Whenever a way is found to suppress free riding so that individual units can cooperate, work as a team, and divide labor, selection at the lower level becomes less important, selection at the higher level becomes more powerful, and that higher-level selection favors the most cohesive superorganisms. 39 (A superorganism is an organism made out of smaller organisms.) As these superorganisms proliferate, they begin to compete with each other, and to evolve for greater success in that competition. This competition among superorganisms is one form of group selection. 40 There is variation among the groups, and the fittest groups pass on their traits to future generations of groups. times independently, e.g., the transition to eusociality. 39 Hölldobler and Wilson 2009. Many theorists prefer terms other than superorganism . Bourke 2011, for example, calls them simply “individuals.” 40 Okasha 2006 calls this MLS-2. I’ll call it selection among stable groups in contrast to MLS-1, which I’ll call selection among shifting groups . This is a subtle distinction that is crucial in discussions among specialists who debate whether group selection has actually occurred. It is too subtle to explain in the main text, but the general idea is this: For selection among stable groups, we focus on the group as an entity, and we track its fitness as it competes with other groups. For this kind of selection to matter, groups must maintain strong boundaries with a high degree of genetic relatedness inside each group over many generations. Hunter-gatherer groups as we know them today do not do this; individuals come and go, through marriage or for other reasons.
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