Dasycladalean algae, a group of tropical to subtropical shallow-marine chlorophytes, rank among the largest and most highly differentiated unicellular- uninucleate-grade organisms. A computer model based on physiological and reproductive factors indicates that both the large size and morphology of the basic dasycladalean bauplan — a compound cylinder composed of an upright central cylinder (main axis) surrounded by whorls of secondary cylinders (laterals) — are consistent with the maximization of "R", the number of gametangia produced per unit time. The model further reveals that potential variations on the basic dasycladalean bauplan, such as increases or decreases in the length/diameter of main axis and/or lateral components and changes in the number and disposition of laterals, have a direct and often pronounced effect on R. In the model, R is linked to two design parameters: surface area to volume ratio (SA/V) and gametangia storage capacity (G). Significantly, both SA/V and G can be determined from aspects of exterior thallus morphology; thus, R can be calculated for fossil dasycladaleans, which usually provide data only on external form. Because R is a measure of reproductive potential, it provides an indication of relative fitness among dasycladalean thalli of different form in the absence of differential selection pressures (e.g., selective grazing). A computer generated fitness landscape for dasycladaleans based on R reveals a number of localized "peaks," each of which is developed at the end of a markedly steep-sided "ridge." All ridges radiate from the same localized area on the landscape, representing a cluster of small, simple morphotypes. Preliminary morphometric analysis of fossil taxa indicates a tight correspondence between realized form and the locations of peaks and ridges on this landscape, and empirical SA/V data obtained from both fossil and living taxa suggest that the intervening valleys may be "bottomless" (i.e., represent nonviable forms).