not help predict specific outcomes. Dawkins has consistently been sceptical about non-adaptive processes in evolution (such as spandrels, described by Gould and Lewontin) and about selection at levels "above" that of the gene. He is particularly sceptical about the practical possibility or importance of group selection as a basis for understanding altruism. Altruism appears at first to be an evolutionary paradox, since helping others costs precious resources and decreases one's own chances for survival, or "fitness". Previously, many had interpreted altruism as an aspect of group selection, suggesting that individuals are doing what is best for the survival of the population or species as a whole. British evolutionary biologist W. D. Hamilton used gene-frequency analysis in his inclusive fitness theory to show how hereditary altruistic traits can evolve if there is sufficient genetic similarity between actors and recipients of such altruism, including close relatives. Hamilton's inclusive fitness has since been successfully applied to a wide range of organisms, including humans. Similarly, Robert Trivers, thinking in terms of the gene-centred model, developed the theory of reciprocal altruism, whereby one organism provides a benefit to another in the expectation of future reciprocation. Dawkins popularised these ideas in The Selfish Gene, and developed them in his own work. In June 2012, Dawkins was highly critical of fellow biologist E. O. Wilson's 2012 book The Social Conquest of Earth as misunderstanding Hamilton's theory of kin selection. Dawkins has also been strongly critical of the Gaia hypothesis of the independent scientist James Lovelock. Critics of Dawkins's biological approach suggest that taking the gene as the unit of selection (a single event in which an individual either succeeds or fails to reproduce) is misleading. The gene could be better described, they say, as a unit of evolution (the long-term changes in allele frequencies in a population). In The Selfish Gene, Dawkins explains that he is using George C. Williams's definition of the gene as "that which segregates and recombines with appreciable frequency". Another common objection is that a gene cannot survive alone, but must cooperate with other genes to build an individual, and therefore a gene cannot be an independent "unit". In The Extended Phenotype, Dawkins suggests that from an individual gene's viewpoint, all other genes are part of the environment to which it is adapted. Advocates for higher levels of selection (such as Richard Lewontin, David Sloan Wilson, and Elliott Sober) suggest that there are