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In contrast, Gould sees selection as usually acting on organisms in a local population, although in theory and practice, it can occur at many levels, with change at one level often affecting future options at other levels. Selection can occur at the group level, with some species lineages having characteristics which make extinction less likely, or speciation more likely. And while rare, selection can occur on genes within an organism. While selection is important, and requires understanding, it is just one of many factors explaining microevolutionary events and macroevolutionary patterns. Further, complex adaptations are but one phenomenon explanations in evolutionary biology. Extrapolationism is not a good theory, with large-scale patterns in the history of life not explainable by extrapolating from measurable events in local populations. Evolutionary biology needs a theory of variation, explaining the effect of variation supply on change potentiality. While humans are evolved animals, attempts to explain human behaviour using techniques from evolutionary biology have largely failed, "vitiated by one-sided understanding of evolutionary biology. They have often been biologically naive." (p. 170)