4.1: Selecting social (cooperative) traits

So how does evolution produce multicellularity? To answer this question, we need to approach evolutionary processes more broadly. The first new idea we need to integrate into our theoretical framework is known as inclusive fitness, which is sometimes referred to as kin selection. For the moment, let us think about traits that favor the formation of a multicellular organism - later we will consider traits that have a favorable effect on other, related organisms. Finally, we consider social situations in which behaviors have become fixed to various extents and are extended to strangers. Humans can, but do not always, display such social (moral) behaviors. The importance of mutual aid in evolutionary thinking, that is the roles of cooperation, empathy, and altruism in social populations, was emphasize by the early evolutionary biologist and anarchist (Prince) Peter Kropotkin (1842–1921)(→).

All traits can be considered from a cost-benefit perspective. There are costs (“c”) in terms of energy needed to produce a trait and risks associated with expressing the trait, and benefits (“b”) in terms of the trait’s effects on reproductive success. To be evolutionarily preferred, that is, "selected for", the benefit b must be greater than the cost c, that is b > c. Previously we tacitly assumed that costs and benefits applied to one and the same organism, but when we consider cooperative (social) behaviors and traits, this is not the case. We can extend our thinking as follows: assume that an organism displays a trait. That trait has a cost to produce and yet may have little or no direct benefit to the organism that produces it; it may even harm it. Now let us assume that this same trait benefits neighboring organisms, a situation similar to the fireman who risks their life to save a child from a burning building. How could a biological system (the fireman), the product of evolutionary processes, display this type of self-sacrificing behavior? The answer (at least in part) is social systems.

An example of this type of behavior is displayed by the social amoebae Dictyostelium discoideum.^[178] Dictyostelium has a complex life style that includes a stage in which unicellular amoeba-like organisms crawl around in the soil eating bacteria, growing and dividing. In this "vegetative" cycle the cells divide asexually (as if vegetables don’t have sex). If or rather when the environment turns hostile, theseamoeba sense the change and secrete small molecules that influence their own and their neighbor’s behaviors. The is amoeba migrating toward one another (←), forming aggregates of thousands of cells.
Now something amazing happens: these aggregates begin to migrate around as multicellular “slugs”. They respond to environmental signals. They move toward light, a behavior known as positive phototaxis, and then settle down and undergo a rather coordinated process of cellular differentiation.^[179] All through the aggregation and slug migration stages, the original amoeboid cells remain distinct. Upon differentiation ~20% of the cells in the slug specialize to form non-dividing "stalk" cells that go on to die through a process known as programmed cell death or apoptosis. Before they die the stalk cells act together, through changes in their composition and shape, to lift the non-stalk cells out of the soil. The non-stalk cells then go on to form spores, specialized cells that can survive harsh conditions. When the spores are released they float in the air and can be transported by the wind and other mechanisms into new environments. The stalk cells "sacrifice" themselves so that non-stalk cells can form spores.

When a spore lands in a new, and hopefully hospitable environment, it converts back into a motile, unicellular amoeba and begins to feed and reproduce vegetatively. The available evidence indicates that within the slug the “decision” on whether a cell will form a stalk or a spore cell is not pre-determined, it arises from stochastic molecular level processes. The decision is not based on genetic (genotypic) differences - two genetically identical cells may both form spores, both stalk cells, or one might become a stalk and one a spore cell within the differentiating slug.^[180]

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