During evolution on Earth there has been transitions toward reproductive units of ascending inclusion, where higher-level units arise from co-operative reproduction among lower-level units (Vrba and Gould, 1986; Buss, 1987; Maynard Smith and Szathmary, 1995). At the lowest level the reproductive unit is the asexual self-replicator. At higher levels the reproductive unit can take the form of a cell, a multicellular organism, or a group of multicellular organisms, with groups including sexual pairs, co-operative breeders, and eusocial colonies.

As defined by interactors, or interacting units, natural selection is a second dimension that exists in a multi-level hierarchy that resembles the hierarchy of reproductive units (Hull, 1980, 1981; Brandon, 1988; Wilson and Sober, 1994; Lloyd, 1988; Sober and Wilson, 1998; Gould and Lloyd, 1999; Keller, 1999). At the lowest selection level interactors are given by the hereditary material. For self-replicators at the origin of life, with no distinction between genotypes and phenotypes (Michod, 1983, 1999; Szathmary and Maynard Smith, 1997), the lowest interacting level is the self-replicator itself. For higher organisms, with genotypic and phenotypic distinction, the lowest interacting level is the gene, which may act as an interactor when genes can replicate differentially within organisms. At higher selection levels the interacting unit can be formed by cells, multicellular organisms, groups, or species.

Buss (1987) proposed and Maynard Smith and Szathmary (1995) elaborated the hypothesis that transitions to higher reproductive units are associated with transitions in the rules of natural selection. A comparable hypothesis was formulated by Michod (1999) , who describes that transitions to higher-level reproductive units are likely to be induced by transitions to higher levels of selection. Transitions to higher levels of selection can induce transitions to higher reproductive units by promoting co-operative reproduction among lower-level units. Co-operative reproduction among lower-level units is typically fitness costly at that lower level, but co-operation may provide a fitness benefit at the next higher-level, and this benefit may outweigh the cost.

Although it is well established that higher-level selection may outbalance lower-level costs of co-operation it remains a relatively open question why selection at higher levels arises in the first place. But in the theory of Malthusian Relativity it is clear that it is selection by density dependent competitive interactions that selects for evolutionary transitions to higher levels of selection (Witting, 2002). Here the interacting unit that defines the level of selection is the group of individuals that co-operate in their competitive interactions with other groups. Thus, by describing selection on the number of individuals in the interacting unit the theory also describes selection on the level of selection. The higher-level benefit of co-operation is then given by the interactive advantage to interacting units with an increasing number of individuals, while the lower-level cost is given by the cost of sharing the resource among the individuals in the unit. It then follows that the level of selection is defined by this trade-off balance, which is pushed towards higher levels of selection when the number of competitive interactions increases and it becomes more important to be interactively superior. And for the four equilibrium states of the major evolutionary transitions it follows that the level of interactive competition is so high that the interacting unit coincides with respectively the asexually reproducing organism, the sexually reproducing pair, the co-operatively reproducing unit, and the fully evolved eusocial colony.

References

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