 If we take such a view of the individuating conditions for the species Homo sapiens, what are the consequences for the question of which organisms belong to the species? It might appear that it leaves open the possibility that speciation has resulted in some intrinsic property or set of properties establishing the cohesion specific to the taxon and that such properties count as necessary and sufficient for belonging to it (cf. Devitt 2008: 17ff.). This appearance would be deceptive. To begin with, no intrinsic property can be necessary because of the sheer empirical improbability that all species specimens grouped together by the relevant lineage segment instantiate any such candidate property. For example, there are individuals who are missing legs, inner organs or the capacity for language, but who remain biologically human (Hull 1986: 5). Evolutionary theory clarifies why this is so: variability, secured by mechanisms such as mutation and recombination, is the key to evolution, so that, should some qualitative property happen to be universal among all extant species specimens immediately after the completion of speciation, that is no guarantee that it will continue to be so throughout the lifespan of the taxon (Hull 1984: 35; Ereshefsky 2008: 101). The common thought that there must be at least some genetic property common to all human organisms is also false (R. Wilson 1999a: 190; Sterelny &amp; Griffiths 1999: 7; Okasha 2002: 196f.): phenotypical properties that are shared in a population are frequently co-instantiated as a result of the complex interaction of differing gene-regulatory networks. Conversely, the same network can under different circumstances lead to differing phenotypical consequences (Walsh 2006: 437ff.). Even if it should turn out that every human organism instantiated some property, this would be a contingent, rather than a necessary fact (Sober 1980: 354; Hull 1986: 3).