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individual. How does a cell group evolve into a multicellular individual? This is the central question asked in this article. Although kinship has long been appreciated as a necessary precondition for the transition to multicellularity (Maynard Smith, 1988, 1991; Maynard Smith and Szathmáry, 1995; Michod, 1999), there are colonial species with high degrees of kinship that have not evolved true individuality (based on specialization of cells at reproductive and vegetative functions). For example, in all colonial members of the volvocine green algae (Fig. 7.1), all cells in the colony are clonally derived from a single cell, often by just a few cell divisions, yet true individuality based on specialization of reproductive and somatic functions emerges only in the larger colonies. What additional factors are required for the evolution of reproductive altruism, that is, specialization at vegetative somatic functions? Specialization of reproductive and vegetative viability-enhancing functions, what we term germ soma

FIGURE 7.1 Examples of volvocine species varying in cell number, colony volume, degree of specialization, and proportion of somatic cells. (A) Chlamydomonas reinhardtii, a unicell. (B) Gonium pectorale, a flat or curved sheet of 8–32 undifferentiated cells. (C) Eudorina elegans, a spherical colony of 16–64 undifferentiated cells. (D) Pleodorina californica, a spherical colony with 30–50% somatic cells. (E) Volvox carteri. (F) Volvox aureus. Where two cell types are present (DF), the smaller cells are somatic cells and the larger cells are reproductive cells. Photos were taken by C. Solari (University of Arizona).



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