  Most conspicuous organisms are multicellular and most multicellular organisms
develop somatic cells to perform specific, non-reproductive tasks. The ubiquity
of this division of labor suggests that it is highly advantageous. In this
paper, I present a model to study the evolution of specialized cells. The model
allows for unicellular and multicellular organisms that may contain somatic
(terminally differentiated) cells. Cells contribute additively to a
quantitative trait. The fitness of the organism depends on this quantitative
trait (via a benefit function), the size of the organism, and the number of
somatic cells. This model allows one to determine when somatic cells are
advantageous and to calculate the optimum number (or fraction) of reproductive
cells. I show that the fraction of reproductive cells is always surprisingly
high. If somatic cells are very small they can outnumber reproductive cells but
their biomass is still less than the biomass of reproductive cells. Only for
convex benefit functions can the biomass of somatic cell exceed the biomass of
reproductive cells. I discuss the biology of primitive multicellular organisms
with respect to the model predictions. I find good agreement and outline how
this work can be used to guide further quantitative studies of
multicellularity.
