A 2017 paper in Theoretical Population Biology shows how the primary selection of metabolism and mass is explaining allometric transitions from prokaryotes over unicellular eukaryotes to multicellular animals.
The structure of biology is captured by inter-specific allometries. They show how essential traits like metabolism, reproduction, longevity and the abundance of individuals in habitats have evolved with the natural selection of size from bacteria to whales.
The allometric pattern is like a fingerprint of evolution, a print that can reveal the natural selection laws that have created the diversity of living organisms on Earth. And with the publication in Theoretical Population Biology, biology has its first theory that shows how the natural selection of metabolism and mass is explaining allometric transitions from prokaryotes over unicellular eukaryotes to multicellular animals.
The allometric correlations are not restricted to well-known Kleiber scaling, where the exponent for mass specific metabolism is -1/4. The metabolic exponent is instead increasing with a decline in mass, over an apparent body mass invariance in unicellular eukaryotes, to a strongly positive value of about 0.84 in prokaryotes (Makarieva et al., 2008; Delong et al., 2010).
The TPB-paper finds that metabolism is selected as the speed (pace) of the resource handing that generates net energy for reproduction, with the selected net energy generating population growth with an interactive selection of body mass. The overall life history is evolving with this selection, with rate dependent traits being scaled by the selection of metabolism, the selection of mass being driven by the net energy that is generated by the selected metabolism and resource handling, and the selected mass inducing a secondary mass-rescaling where the life history is selected to maintain net energy on the per-generation time-scale of natural selection.
This mechanism explains i) the metabolic scaling in bacteria by a mass that is selected from primary variation in mass-specific metabolism, ii) the allometries in multicellular animals by a mass that is selected from primary variation the handling and/or densities of the underlying energetic resources, and iii) the allometries in protozoa and protists as a transition between the two forms of selection.
- DeLong, J.P., J.G. Okie, M.E. Moses, R.M. Sibly and J.H. Brown 2010. Shifts in metabolic scaling, production, and efficiency across major evolutionary transitions of life. Proceedings of the National Academy of Sciences 107:12941--12945.
- 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.