binding module involved in gene transcription regulation). This example is perhaps the only example of a social gene specifically associated with reproductive altruism, whose origin can be traced back to a solitary ancestor.
Having considered how an altruistic gene might originate (by cooption of a life-history gene in a unicellular ancestor), we now ask why this happens, that is, what are the selective forces favoring soma and reproductive altruism. We wish to understand why it is that soma evolves only in the larger members of this lineage, given that in all species the groups are clonally derived from a single cell and hence of high genetic relatedness. We hypothesize that the selective pressure for soma stems from the increasing cost of reproduction to survival as colonies increase in size.
Flagellar action is an important component of survival. Volvocine algae are denser than water and need flagellar beating to avoid sinking and to find nutrients. These algae are found in quiet, standing waters of transient vernal puddles or in permanent lakes when thermal stirring stops and the lake becomes stratified (Reynolds, 1984; Kirk, 1998). For example, Volvox colonies migrate vertically several meters at night, presumably in search of higher phosphorous concentrations (Sommer and Giliwicz, 1986). In addition to motility, flagellar action provides for mixing