Selection of lifeforms from virus to mammals
In a new paper on bioRxiv I show how the natural selection of metabolism and mass is selecting for the major size, life history, and allometric transitions that define lifeforms from viruses over prokaryotes and larger unicells to multicellular animals.
Evolution on Earth have created distinct lifeforms with discrete size classes and unique life histories. These include i) virus with no metabolism, no cell, and almost no mass; ii) prokaryotes with a small cell, asexual reproduction, and a mass specific metabolism that increases with mass; iii) asexual unicellular eukaryotes with a larger mass than prokaryotes, and a mass specific metabolism that is first increasing and then declining with an increase in mass; and iv) large multicellular animals with sexual reproduction and a mass specific metabolism that declines with mass.
This macro evolutionary pattern with non-overlapping lifeforms indicates a natural selection that is unfolding with major transitions during the course of evolution. And as the lifeforms can be arranged as a function of mass, it is likely that this unfolding of life on Earth is linked to the natural selection of mass.
This is examined in a new paper on bioRxiv. With mass specific metabolism being selected as the pace of the resource handling that generates net energy for self-replication, there is unidirectional selection for an increase in net energy, resource handling, and mass specific metabolism. But how is this increase in energy selected into mass?
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 therefore 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, with the interactive competition of the fully developed feed-back selecting 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.
The proposed mechanism unifies natural selection from viruses to multicellular animals, and it provides a parsimonious explanation where the allometries and life histories of 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. 2016. The natural selection of metabolism and mass selects lifeforms from viruses to multicellular animals. Preprint at bioRxiv https://dx.doi.org/10.1101/087650.