Malthusian Relativityι**=7/3ψ
The natural selection of lifeforms

Macro evolution across major taxa

The natural selection of metabolism selects a macro evolutionary invariance between metabolism and mass

We have seen that the correlation between mass specific metabolism and mass is changing across the tree of life; from the absence of metabolism in virus-like replicating molecules, over a positive 5/6 exponent in minimum self-replicators like prokaryotes, and a change from 5/6 to -1/6 in interacting self-replicators like protozoa, to negative 1/4 and 1/6 exponents in multicellular animals, dependent upon the spatial dimensionality of the interactive behaviour. But how is the scaling evolving on the macro evolutionary scale across the major taxonomic groups?

This is maybe best understood by comparing the prediction between the three mass scaling components of the pre-mass, mass-rescaling and post-mass allometries. This relationship is illustrated in the left plots in Fig. 1 for the potential range of pre-mass selection on mass specific metabolism. Blue lines represent pre-mass selection on metabolism, green is local mass-rescaling, and red the final post-mass allometry. The pre-mass selection on metabolism is generating the metabolic span of the blue line (in time or across species), and selection on mass is generating the span in mass, with the green lines illustrating local mass-rescaling, and the final post-mass relation evolving along the red line.

With typical post-mass allometries in multicellular animals resembling mass-rescaling, the local mass-rescaling of the solid green lines can be seen as species distributions of different taxa, with the body mass variation within the taxa originating from an evolutionary diversification in the handling of resources across a variety of ecological niches. The red lines are then the different routes for an evolutionary divergence between the major taxa, and this macro evolution is dependent on the natural selection of metabolism. This interpretation is illustrated in the right plot in Fig. 1, together with the predicted scaling in prokaryotes and protozoa, and the predicted transition to multicellular animals with sexual reproduction.

Fig. 1 Left: The post-mass allometries for mass specific metabolism (red) evolve from pre-mass selection on metabolic pace (blue) and local mass-rescaling (green); shown here in plots a to d as a function of pre-mass exponents from 0.02 to 1. Right: The corresponding predictions for a to d of the macro evolutionary relationship from prokaryotes (black), over protozoa (yellow), to major taxa of multicellular animals (blue, red and green). b is the expected pattern where mass-rescaling on the macro evolutionary scale is balanced by pre-mass selection on metabolism with an overall invariance of mass specific metabolism. From Witting (2016).

For the potential range of pre-mass selection on metabolism, the case with no pre-mass selection (a) is in conflict with the predicted increase in metabolism. The limit with maximal pre-mass selection (d) is unexpected as it would imply that all the mass variation between the major taxa is induced by differences in metabolism, while all the variation within the taxa is induced by differences in the handling and availability of resources. A more likely scenario is c, where natural selection on pace and handling are equally important. But expected the most is b, a case where the mass specific metabolism of a taxa is selected to an upper limit, and mass-rescaling on the macro evolutionary scale is balanced by pre-mass selection on metabolism (Witting, 2016). This is also the observed pattern on the macro evolutionary scale from prokaryotes to birds (Fig. 2).

Fig. 2 The macro evolutionary relationship between mass and mass specific metabolism among heterotroph organisms. From Witting (2016) with data from Makarieva et al. (2008).

References

  • 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. bioRxiv http://dx.doi.org/10.1101/087650.