A diminishing return, in the interactive quality of sexual reproduction with an increase in the size of the reproducing unit, selects for the evolution of offspring workers
The optimal reproducing unit is the male-female pair in multicellular animals with equilibrium masses, it contains one or two extra interacting individuals in species with exponentially increasing masses at evolutionary steady states, and an almost unlimited number of interactors in species with upward constrained body masses. But should these extra interactors be offspring workers or sexually reproducing males?
We have already seen that sexual males are expected over asexual offspring workers, because the most attractive females can increase the interactive quality of their reproducing units relative to the average unit, by attracting the competitively superior males. With sexually produced offspring workers it follows that a fraction of the interactive quality of the male is inherited by the offspring. But is this enhanced interactive quality of sexually produced workers relative to asexually produced workers so high that sexually produced workers can evolve at the cost of some of the sexual males?
Because the interactive qualities of males are transferred by sexual reproduction to offspring workers, there is a diminishing return in the interactive quality that can be gained by increasing the number of males that participate in sexual reproduction. And it can be shown that it is only for the initial transition from asexual to pair-wise sexual reproduction that the interactive quality of the male can outbalance the cost of sexual reproduction (Witting, 2002). For all potential remaining transitions, the transfer of male interactive quality to sexually produced offspring workers implies that the interactive quality that can be gained by exchanging an offspring worker with an additional sexual male cannot outbalance the extra cost to sexual reproduction associated with that transition. The result is selection for pair-wise sexual reproduction and sexually produced offspring workers, predicting reproductive units of co-operative families and eusocial colonies.
It is not self-evident that large reproducing units necessarily resemble the cooperative families and eusocial colonies on Earth, where there is sexual reproduction between a female and a male, and the remaining non-replicating individuals are sexually produced offspring workers.
Alternatives include asexual systems with non-reproducing offspring workers, sexual systems with asexually produced offspring workers, and higher-level sexual systems where females mate with several males with each sexual individual providing only a small fraction of the genome in the offspring. For a model that allows for the evolution of these alternative systems, it is intriguing that the population dynamic feed-back of interactive competition is selecting for the cooperative families and eusocial colonies on Earth; including differences in the ploidy level of the genome between termites and hymenoptera (see section on ploidy level of genome, and Witting, 1997, 2002, 2007).
Behavioural interactions selecting for symmetry and asymmetry in sexual reproductive systems of eusocial species
From asexual to eusocial reproduction by multilevel selection by density dependent competitive interactions
A general theory of evolution. By means of selection by density dependent competitive interactions.
- 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.
- Witting, L. 2007. Behavioural interactions selecting for symmetry and asymmetry in sexual reproductive systems of eusocial species. Bulletin of Mathematical Biology 69:1167--1198, https://doi.org/10.1007/s11538--006--9112--x.