These topics are addressed by life history theory, which was originally developed for macroscopic organisms ( 10– 15), such as animals and higher plants. Our findings demonstrate that a number of seemingly unconnected phenomena observed in unrelated species may be different manifestations of the same underlying process. Finally, we show that the loss of organism mass during sporulation can explain the macroscopic sizes of the formally unicellular microorganism Myxomycetes plasmodium. For primitive multicellular species, these considerations can explain why rosette cell colonies evolved a mechanistically complex binary split reproduction. This has important direct implications for microbial life: For unicellular species, the interplay between cell shape and kinetics of the cell growth implies that the largest and the smallest possible cells should be rod shaped rather than spherical. We found that mass conservation can fundamentally limit the number of possible reproduction modes. While the size of an organism at fragmentation, the number of offspring, and their sizes may vary a lot, the combined mass of fragments is limited by the mass of the parent organism. We present a model of the evolution of reproduction modes, where a parent organism fragments into smaller parts. The most prominent transition between two such modes is the one from unicellularity to multicellularity. These modes are not only subject to evolution, but may drive evolutionary competition directly through their impact on population growth rates. Multiple modes of asexual reproduction are observed among microbial organisms in natural populations.