Associated with the long-term increase in body mass there has been a transition from an asomatic, non-senescing, haploid, and asexually reproducing organism to a somatic, senescing, diploid, and sexually reproducing organism. There has also been a stepwise transition in the reproducing unit, from the individual self-replicator over pair-wise sexual and co-operative reproduction to the fully evolved eusocial colony.

Malthusian Relativity is the first theory where a single selection mechanism explains these major evolutionary transitions (Witting, 2002). The essential component that induces the transitions is a step-wise equilibration of the directional increase in body mass and energetic state. Dependent upon extrinsic and intrinsic constraints the directional increase may equilibrate at four evolutionary states:

The downward constrained equilibrium

exists in low-energy organisms where the energetic state cannot evolve beyond a lower threshold value. This state has a negligible body mass and a low level of competitive interactions.

The evolutionary equilibrium

exists in high-energy organisms where the energetic state has evolved beyond the lower threshold and there are no current increase in energetic state. This state has an equilibrium level of competitive interactions and a non-negligible body mass in evolutionary equilibrium.

The evolutionary steady state

exists in high-energy organisms where the energetic state has evolved beyond the lower threshold and both the energetic state and the body mass evolves free of evolutionary constraints. This state has a higher level of competitive interactions than the evolutionary equilibrium and it is characterised by an exponential increase in energetic state and body mass.

The upward constrained equilibrium

exists in high-energy organisms where the energetic state has evolved beyond the lower threshold and the body mass is situated at an upper limit below the corresponding body masses of the evolutionary equilibrium or the evolutionary steady state. The level of competitive interactions at the upward constrained equilibrium is higher (potentially even much higher) than the level at evolutionary steady state.

The stepwise increase in the level of interactive competition associated with the transitions between the four equilibrium states implies selection for interactive traits at different levels. And as the major life history traits are linked to interactive quality it follows that the major transitions can be explained from transitions among the four equilibrium states of interactive competition. When the life history is allowed to equilibrate at the different equilibria

The downward constrained equilibrium

resembles the simple self-replicator. This organism is asomatic, non-senescing and haploid. It has a negligible body mass and a reproducing unit of a single asexually reproducing individual.

The evolutionary equilibrium

resembles a higher eukaryotic organism, with a non-negligible body mass, soma, senescence, a diploid genome, and pairwise sexual reproduction between a female and a male.

The evolutionary steady state

resembles a co-operatively breeding eukaryotic organism. Life history traits resemble those of the evolutionary equilibrium, except for the reproducing unit where the sexually reproducing pair receives help from a single offspring worker.

The upward constrained equilibrium

resembles an eusocial colony. Life history traits resemble those of the evolutionary equilibrium, except for the reproducing unit where the sexually reproducing pair receives help from up to infinitely many offspring workers.

References

• Witting, L. (2002). From asexual to eusocial reproduction by multilevel selection by density dependent competitive interactions. Theoretical Population Biology 61, 171--195.