Interacting unit

The interacting unit is the unit that interacts as a cohesive whole with the environment and other interacting units in such a way that replication is differential across units (Hull, 1980). It is the evolution of interacting units that defines the level of selection.

The unfolding feed-back selection is generating a resource bias in favour of the interactively superior units. But when does the optimal unit include a single individual, and when should it be composed of several individuals? When other things are equal, the group of individuals will gain a competitive advantage over a single individual or a smaller group, but it is also faced with the cost of resource sharing, especially if resources are sparse. Hence, in the absence of interactive competition we expect interacting units of single individuals, and more generally we expect the number of individuals in the interacting unit to increase with the level of interactive competition in the population.

When formulated mathematically and solved for the evolutionary equilibrium (Witting, 1997, 2002), we find that the number of co-operating individuals in the interacting units is

n^** = ψ ι + 1

if we assume that the cost of resource sharing is about maximal, which is likely the case when resources are sparse. When the level of interference for the attracting fix-points is inserted into the group size equation, we find, as illustrated in Fig. 1, that 1) self-replicating cells evolve individual interactors, 2) that multicellular animals with equilibrium attractors have interacting units with two individuals, 3) that steady state attractors have interacting units with three or four individuals, and 4) that attractors with upwards constrained masses have very large interacting units. In terms of the number of individuals, these units resemble the asexual self-replicator, the sexually reproducing pair, the co-operatively reproducing unit, and the fully evolved eusocial colony.

Fig. 1 The spatial distribution of individuals into interacting units as predicted by selection by density dependent competitive interactions. Uniformly distributed (a) individuals, (b) pairs, (c) co-operative, and (d) eusocial units. From Witting (1997).

Evidence

There is no doubt that the individuals of many natural populations organise themselves into groups or units, and that some of these are used in the defence of natural resources against other such units. It is also evident that the deduced structuring exists; with individual interactors being widespread in self-replicating cells, with pair-wise and co-operative units being widespread in multicellular animals, and eusocial units existing only in relatively few taxa.

Whether these units have evolved by density dependent interactive competition, or by other mechanisms, is a more open question that is difficult to prove. Note, however, that the organisation of individuals into co-operative kin-groups is a function of population density in some species (Stacey and Koenig, 1990; Watson et al., 1994; Piertney et al., 2008). And eusocial insects, although few in number of species, may comprise as much as 75 percent of the total insect biomass (Beck, 1971; Fitthau and Klinge, 1973), in agreement with the extreme population densities that are predicted by the upward constrained competitive interaction fix-point.

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References

• Beck, L. 1971. Bodenzoologische gliederung und charakterisierung des amazonischen regenswaldes. Amazoniana 3:69--132.
• Hull, D. 1980. Individuality and selection. Annual Review of Ecology and Systematics 11:311--332.
• Piertney, S.B., X.Lambin, A.D.C. Maccoll, K.Lock, P.J. Bacon, J.F. Dallas, F.Leckie, F.Mougeot, P.A. Racey, S.Redpath and R.Moss 2008. Temporal changes in kin structure through a population cycle in a territorial bird, the red grouse Lagopus lagopus scoticus. εm Molecular Ecology 17:2544--2551.
• Stacey, P.B., and W.D. Koenig 1990. Cooperative breeding in birds: long-term studies of ecology and behavior. Cambridge University Press, Cambridge.
• Watson, A., R.Moss, R.Parr, M.D. Mountford and P.Rothery 1994. Kin landownership, differential aggression between kin and non-kin, and population fluctuations in red grouse. Journal of Animal Ecology 63:39--50.
• Witting, L. 1997. A general theory of evolution. By means of selection by density dependent competitive interactions. Peregrine Publisher, Århus, 330 pp, URL https://mrLife.org.
• Witting, L. 2002. From asexual to eusocial reproduction by multilevel selection by density dependent competitive interactions. Theoretical Population Biology 61:171--195, https://doi.org/10.1006/tpbi.2001.1561.