Molecular sequences may themselves be UOSs. "Doctor there is a fly in my genome" was the title chosen by the journal New Scientist (Vol.
Other elements that are completely integrated in the host's genome, replicating when the entire collective of genes reproduce and contributing to basic functions and processes derived by the host cells (such as the LINE elements that preserve the telomeres of Drosophila; literature cited in Flavell, 1999), are clearly not UOSs.
In chimeras or mosaics, two or more UOSs amalgamated to form a single distinct UOS.
As UOSs, however, many mosaic cases have evolutionary relevance because they are hereditarily transmitted and may manifest a variety of costs (Benirshke, 1981; Hall, 1988; Gill et at., 1995; Rinkevich, 1996a).
Incubated entities, as in the above cases, are the evolutionarily successful UOSs, whereas the incubator entities are those with the role of directly interacting with the environment.
The situation wherein several conspecific UOSs combine to form a morphologically new structure is best represented by certain primitive crustaceans (order Rhizocephala in the subclass Cirripedia, the barnacles).
Each single rhizocephalen organism is therefore an amalgamated structure, consisting of three distinct conspecific UOSs (two form only spermatozoa, one the soma and eggs).
However, the six types of UOSs characterized in this essay, in addition to the whole organismic level as a UOS (No.
Former attempts to identify the particular entities that are the targets of natural selection (Wynne-Edwards, 1962; Lewontin, 1970; Mayr, 1970, 1997; Dawkins, 1976, 1989; Hull, 1980; Buss, 1982; Gliddon and Gouyon, 1989; Sober and Wilson, 1994; Wilson and Sober, 1994; Williams, 1996; Michod, 1997; Gould, 1998; Gould and Lloyd, 1999) have suggested three or four potentially "structural" UOSs -- the gene, the individual, the group, and the metapopulation -- but there has been no consensus.
(1990), on the other hand, have correctly pointed to a major pitfall in the concept of the UOS by advocating that biologists "assume that for each selection episode, there is a unique account that will identify the level of selection." When the descriptions of UOSs in the literature are aligned with the organizational levels, they fail, in many cases, to grasp the structural comprehensiveness of other UOSs and no consistency emerges (Hull, 1980; Kitcher et al., 1990; Sober and Wilson, 1994; Mayr, 1997; Shanahan, 1997).
In this essay, I have focused on the argument that real UOSs should evince a kind of holism and should possess the properties of independent functionalism.