A critical approach to the definition of Darwinian units of selection

What are the biological units of selection? In fact, the notion of "unit of selection" (UOS) is blurred by ambiguity and controversy. To further evalu ate the biological entities that are the objects of natural selection, thre e novel conceptual criteria (holism, minimalism, functionalism) are critica lly applied; they reveal, in addition to the self-evident case of the "indi vidual," at least six distinct types of UOSs. These UOSs do not always have a defined structural organization; they can be parts of a living organism, a cohesive group of conspecifics, a multiunit entity, a totipotent cell, a DNA fragment, or a whole organism. UOS types diversify by amalgamation or parcelation processes of apparent entities. Therefore, previous attempts to characterize the UOSs solely on some morphological levels (gene, individua l, group) without applying stringent criteria have failed to cope with the structural variations of natural phenomena and have led to the ambiguity of terms used.