Malthusian Relativityι**=7/3ψ
Life history evolution - traits & transitions

Interacting individuals and reproducing units

Interactive competition selects for males (interacting individuals) and non-trivial units of reproduction

When natural selection forms interacting units with several individuals it will either select for continued reproduction in all individuals, or for some individuals to become interactors that allocate energy into interactive competition at the cost of energy allocated to the production of offspring.

In addition to males, the well-known interacting individuals include offspring workers in co-operatively and eusocially reproducing units. When other things are equal, interacting units with interacting individuals can dominate units with replicating individuals during competitive encounters, but they will suffer the fitness cost of reduced replication. This is exemplified by the two-fold cost of the male (Maynard Smith, 1968, 1978), where the growth rate of a sexually reproducing pair is half the growth rate of an asexually reproducing female (Fig 1).

Owing to the cost of non-replication we do not expect interacting individuals to evolve when there are no competitive interactions. More generally, for natural selection by density dependent competitive interactions it can be shown (Witting, 1997, 2002) that the selected fraction of interacting individuals in the interacting unit is

φ** = ψ ι / ( 1 + ψ ι )

When the level of interference at the attracting fix-points is inserted into this equation, we find that 1) self-replicating cells have no interacting individuals, 2) that multicellular animals with equilibrium attractors have interacting units with a single interacting individual and a single reproducing individual, 3) that steady state attractors have one reproducing and two to three interacting individuals per interacting unit, and 4) that attractors with upward constrained masses have one replicating individual and plenty of interacting individuals per interacting unit.

Independently of the size of the interacting unit, we find that each interacting unit evolves into a reproducing unit with a single replicating individual and the rest of the individuals in the unit being interactors. Interacting units with more than one reproducing individual may though evolve if the cost of group formation, i.e., the cost of resource sharing, is low.

Fig. 1 An illustration of the two-fold cost of the male when females (circles) produce two offspring per lifetime. The lineage to the left reproduce asexually without males, whereas the sexual lineage to the right require males (squares) for reproduction. From Witting (1997).


  • MaynardSmith, J. 1968. Evolution in sexual and asexual populations. The American Naturalist 102:469--473.
  • MaynardSmith, J. 1978. The evolution of sex. Cambridge University Press, Cambridge.
  • Witting, L. 1997. A general theory of evolution. By means of selection by density dependent competitive interactions. Peregrine Publisher, Århus, 330 pp, URL
  • Witting, L. 2002. From asexual to eusocial reproduction by multilevel selection by density dependent competitive interactions. Theoretical Population Biology 61:171--195.