The units ofselection.

The individual has long been considered by evolutionists to be the only level at which selection acts. Recent advances in molecular biology have forced them to accept at least one other, the molecular level. This paves the way for a broadening of perspective in evolutionary thought, but renders it more necessary to clarify the debate about the level at which the process of evolution occurs. This debate cannot be resolved without clarifying the premises on which the argument is based. To this end, it is helpful to distinguish between that which is transmitted and that which transmits. The discrimination of genetic information from its material support (avatar) can be most useful, particularly when placed in the context of the hierarchical organization of biological systems. The utility of this approach is exemplified by its application to the case of the evolution of sex.