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Abstract:
A simple model, illustrating the transition from a population of free swimming, solitary cells (Chlamydomonas-type) to one consisting of small colonies (Gonium-type) serves as a basis to discuss the evolution of the cooperative group. The transition is the result of a mutation of the dynamics of cell division, delayed cell separation leads to colonies of four cells. With this mutation cooperative features appear, such as synchronised cell divisions within colonies and coordinated flagellar function which enables the colony to swim in definite directions. The selective advantages under given, environmental conditions are defined and the periods necessary for complete allelic replacement in small populations are calculated for asexual and sexual reproduction. The assumption of a steady-state population during allelic substitution is critically considered, particularly under conditions of competition. It is shown that density-dependent population control must operate in the process of selection, Sexual reproduction slows down the rate of selection even though all cells are haploid. This phenomenon can be explained in general terms of "organizational dominance', where individual units coordinate the function of their neighbours which may be of a different allelotype. Cooperativity is pointed out as an a priori systemic feature which resides in the sub-units of systems, group formation and coordination appears thus as an almost inevitable event. A particular type of system described as 'closed cycle of positive fitness interaction' is discussed in more detail. It has the remarkable feature that its members cannot compete with each other; selection takes place between whole cycles (in analogy to Eigen's 1971 model). Gonium has a wide spectrum of 'somatic plasticity' which enables it to assume various colonial configurations depending on physiological and environmental conditions. This feature can be explained as the result of dynamic flexibilities on the macro-molecular level. The particular relationship between the vast, molecular complexity and the relative simple dynamics of the cell cycle must lead eventually to the genetic fixation of an environmentally induced phenotype.