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Among the various kinds of biostratigraphic units, lineage-zones are especially interesting and meaningful, not only in stratigraphy but also in evolutionary biology. The chronological shift of morphology in evolution is fundamentally uninterrupted, but sometimes it seems discontinuous or even saltational. Such breaks have been commonly attributed to gaps in sedimentation, but could be explained, at least in some cases, by more essential mechanisms of evolution. The difference in the patterns of morphological change may be primarily related to the nature of non-sex-associated genetic variation. continuous or discontinuous, as well as the strength of selection pressure. Some theoretical models for phenotypic substitution are presented here in terms of population genetics, though these are not the only possible explanation for breaks in fossil records.