Interacting individuals and reproducing units

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).

Evidence

There is no doubt that the individuals of many natural populations organise themselves into reproducing units with a single reproducing female and one, or several, non-replicating individuals; let them be males or offspring workers. It is also evident that the deduced structuring exists; with no interacting individuals in self-replicating cells, with the widespread reproducing units in multicellular animals containing a single male and maybe a few offspring workers, and with the eusocial colony being a special case where the sexual pair is associated with a plenitude of offspring workers.

It is less straightforward to prove that these units evolve by selection by density dependent competitive interactions. But, from a theoretical point of view we note that non-replicating individuals like males and offspring workers necessarily must gain fitness by other means than replication, because there is strong replication-selection against the allocation of energy to individuals that do not directly replicate. Hence, if these individuals are to be maintained by natural selection, they must gain fitness to the reproducing unit from some sort of behaviour that will outweigh their energetic costs. This is possible through interactive competition, with selection by density dependent competitive interactions being the only hypothesis that predicts the evolutionary structuring from the asexual self-replicator to the eusocial colony from first principles of self-replication.

In favour of the interaction hypothesis for the evolution of males and off-spring workers, we note that these individuals may compete for the reproducing female in mobile organisms by defending limited resources. This is impossible in sessile organisms where individuals compete by size and position in a flow of resources. Sessile organisms cannot generally co-operate in interactive competition, and the optimal interacting unit is therefore the individual organism allowing no room for pure interacting individuals to evolve. It is therefore not surprising that males and off-spring workers are found only in mobile organisms.

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References

• 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 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.