WHAT IS AN INDIVIDUAL ORGANISM? A MULTILEVEL SELECTION PERSPECTIVE

Most biologists implicitly define an individual organism as one genome in one body 7 his definition is based on physiological and genetic criteria but it is problematic for colonial organisms We propose a definition based instead on the evolutionary criteria of alignment of fitness export of fitness by germ soma specialization and adaptive functional organization We consider how these concepts apply to various putative individual organisms We conclude that complex multicellular organisms and colonies of eusocial insects satisfy these three criteria but that, in most cases (with at least one notable exception) colonies of modular organisms and genetic chimeras do not While species do not meet these criteria they may meet the criteria for a broader concept-that of an evolutionary individual-and sexual reproduction may be a species level exaptation for enhancing evolvability We also review the costs and benefits of internal genetic heterogeneity within putative individuals demonstrating that high relatedness is neither a necessary nor a sufficient condition for individuality and that in some eases genetic variability may have adaptive benefits at the level of the whole