Abstract:
All life is organized hierarchically. Lower levels, such as cells and zooids, are
nested within higher levels, such as multicellular organisms and colonial animals. The
process by which a higher-level unit forms from the coalescence of lower-level units is
known as “individuation”. Individuation is defined by the strength of functional
interdependencies among constituent lower-level units. Interdependency results from
division of labor, which is evidenced in colonial metazoans as zooid polymorphism. As
lower-level units specialize for certain tasks, they become increasing dependant on the
rest of the collective to perform other tasks. In this way, the evolution of division of
labor drives the process of individuation.
This study explores several ways in which polymorphism evolves in colonial
marine invertebrates such as cnidarians, bryozoans, and urochordates. A previous
study on the effect of environmental stability on polymorphism is revisted and
reinterpreted. A method for quantifying colonial-level individuation by measuring the
spatial arrangement of polymorphic zooids is proposed and demonstrated. Most
significantly, a comparison across all colonial marine invertebrate taxa reveals that
polymorphism only appears in those colonial taxa with moderately to strongly
compartmentalized zooids. Weakly compartmentalized and fully compartmentalized
taxa are universally monomorphic. This pattern is seen across all colonial marine
invertebrate taxa and is interpreted as a “rule” governing the evolution of higher-level
individuation in the major taxa of colonial marine invertebrates. The existence of one
rule suggests that there may be others, including rules that transcend levels of biological
hierarchy. The identification of such rules would strongly suggest that new levels in the
hierarchy of life evolve by a universal pattern that is independent of the type of
organism involved.
