From replicating molecules to multicellular animals
In a new release on bioRxiv (Witting, 2016b) I integrate an initial mass dependence of mass specific metabolism into the “allometries are selected by the selection of metabolism and mass”-hypothesis (Witting, 2016a). This shows that a decline in the importance of mass specific metabolism for the selection of mass, and an associated unfolding of interactive competition, select for lifeforms from virus over prokaryotes and larger unicells to multicellular animals with sexual reproduction between female and male individuals.
All organisms have a quality-quantity trade-off where energy can be used on many small, or on a few large offspring. Because of this background selection against mass, it follows that net energy must increase superlinearly with mass before an increase in mass can be selected.
An initial mass can be selected by a mass specific metabolism that depends on the mass of the molecular replicator; let it be the mass of its metabolic molecules, heritable code, and potential cell where the metabolic molecules can concentrate. Although an initial sublinear dependence is selecting for virus-like replicating molecules with no intrinsic metabolism, it follows that a small prokaryote-like self-replicating cell with an internal metabolism is selected by a superlinear dependence.
The dependence of mass specific metabolism on mass will decline with a mass that increases from the evolution of more complete metabolic pathways, and this restricts this form of selection to the evolution of the smallest self-replicating cells. Yet, these cells can be selected into a larger size class by the gradual unfolding of feed-back selection from density dependent interactive competition. And yet another size class is selected when the feed-back is fully developed, and the positive dependence of mass specific metabolism on mass is vanishing with the evolution of complete metabolic pathways. This absence of a metabolic mass dependence selects for a multicellular animal, and the interactive competition of the fully developed feed-back selects for sexual reproduction.
The theoretical analysis is also showing that the selected size-class-transitions are connected with a decline in the importance of mass specific metabolism for the selection of mass. And this decline is calculated to be exactly so strong that it is selecting for the allometric transitions that are observed between prokaryotes, protozoa, and multicellular animals.
This model unifies natural selection from viruses to multicellular animals, and it provides a parsimonious explanation where the allometries and life histories of the major lifeforms evolve from primary selection on metabolism and mass.
- Makarieva, A.M., V.G. Gorshkov, B.Li, S.L. Chown, P.B. Reich and V.M. Gavrilov 2008. Mean mass-specific metabolic rates are strikingly similar across life's major domains: Evidence for life's metabolic optimum. Proceedings of the National Academy of Sciences 105:16994--16999.
- Witting, L. 2016a. The natural selection of metabolism and mass selects allometric transitions from prokaryotes to mammals. Preprint at bioRxiv https://dx.doi.org/10.1101/084624.
- Witting, L. 2016b. The natural selection of metabolism and mass selects lifeforms from viruses to multicellular animals. Preprint at bioRxiv https://dx.doi.org/10.1101/087650.