Since the publication and the scientific recognition of the synthetic theory of evolution (Huxley, 1942), natural selection (i.e., the preservation of beneficial individual differences or variations and the disappearance of those that are deleterious in a given environment; Darwin, 1859) is recognized as the main engine of evolution. This natural selection acts on the individual, the latter being traditionally defined by the simultaneous and invariable presence of physiological unity and autonomy, genetic uniqueness, and genetic homogeneity (Santelices, 1999). However, this definition of the individual is disputable (see Pineda-Krch & Lehtila, 2004) and many examples challenge it, such as intra-organismal genetic heterogeneity (IGH; i.e., the presence of more than one genotype in a single organism; Rinkevich, 2001; Rinkevich & Weissman, 1987). Usually, two kinds of IGH are distinguished, depending on the mechanism of formation: mosaicism and chimerism (Pineda-Krch & Lehtila, 2004; Santelices, 1999). Mosaicism refers to organisms that are subject to intra-organismal genetic modifications [e.g., somatic mutations, mitotic recombination, mitotic gene conversion (Otto & Hastings, 1998; Youssoufian & Pyeritz, 2002), or gene duplications (Santelices, 1999)], while chimerism designates a single organism resulting from the fusion or exchange of genetically distinct parts from different organisms (Rinkevich & Weissman, 1987).