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THE EVOLUTION OF POLYMORPHIC COMPATIBILITY MOLECULES

Several primitive colonial organisms distinguish self from nonself by means of polymorphic compatibility molecules 
bearing similarity to the major histocompatibility complex (MHC). The evolution of such polymorphisms is 
generally explained in terms of resistance to parasites. Ignoring parasites, I develop two mathematical models. One 
model is inspired by slime mold aggregation. The other is a caricature. The results show that parasites are not 
required to explain polymorphisms. The fact that allorecognition allows organisms to maintain their “genetic 
identity” (e.g., to preserve evolved adaptations) suffices for the evolution of polymorphisms. The degree of polymorphism 
that ultimately evolves is equal to the diversity of phenotypes in the population. In these models I 
consider “cells” growing in particular environments. The “genotype” of a cell codes for an “ecological preference,” 
which determines the growth rate of the cell in the environment, and for a “compatibility molecule.” Cells expressing 
the same compatibility molecule form one “colony.” Thus, colonies may be chime& with respect to the ecological 
preferences. The reproduction rate of each cell in a colony is weighted by the colony’s composition of ecological 
preferences. These conditions turn out to be sufficient for the evolution of polymorphic compatibility molecules.