# The evolution of natural selection time

Dependent upon the relative importance of metabolic selection, the timescale of natural selection may contract, or dilate, with the natural selection of mass

To predict body mass evolution over millions of years, as captured by the fossil record, we need to transfer the predicted increase [ d w / d τ = r_{w} w ] on the per generation timescale of natural selection into physical time [ d w / d t = r_{w} w / τ ]. To do this we need first of all to predict the correlated evolution [ ∂ ln τ / ∂ ln w = τ^{•} ] between generation time (τ) and mass (w).

For this we will use the inverse relationship between biotic time and mass specific metabolism [ τ ∝ 1 / β ], and the importance of the pre-mass selected exponential increase in mass specific metabolism

r_{ββ} = d ln β_{β} / d τ = σ^{2}

for the exponential increase in net energy

r_{ε} = r_{ββ} + r_{α}

and mass (Witting, 2017b), r_{w} = r_{ε} / ε^{•} = (r_{ββ} + r_{α}) / ε^{•}.

Let us define

β_{β}^{•} = r_{ββ} / r_{w} = r_{ββ} ε^{•} / (r_{ββ} + r_{α})

and express the per generation exponential increase in mass and the pre-mass component of metabolism as a function of the rate of increase in mass (r_{w}):

w_{τ} = w_{0} e^{rw τ} , and β_{β,τ} = β_{β,0} e^{ββ• rw τ}

We may then solve the mass equation for time τ=ln(w_{τ}/w_{0})/r_{w} and recall the allometric deduction (Witting, 1995, 2017a) where ε^{•}=(2d-1)/2d. Insert these two equations and β_{β}^{•}=r_{ββ} ε^{•} / (r_{ββ} + r_{α}) into the exponential expressions for pre-mass metabolism and obtain the following exponent

β_{β}^{•} = (2d-1) / 2d (1+r_{α}/r_{ββ})

for the metabolic-rescaling [ β_{β,τ} ∝ w_{τ}^{ββ•} ] of the allometry for mass specific metabolism [ w^{β•} ∝ w^{ββ•} w^{βw•} ], where β^{•} = β_{β}^{•} + β_{w}^{•}. Then, as the exponent for mass-rescaling is β_{w}^{•} = -1/2d, we obtain the allometric exponent for mass specific metabolism

β^{•} = [ 2(d-1)/(1+r_{α}/r_{ββ}) – 1/(1+r_{ββ}/r_{α}) ] / 2d

as a function of the rate of increase in the pre-mass component of mass specific metabolism over the rate of increase in resource handling (r_{ββ}/r_{α}; Fig. 1).

_{ββ}) over the selected rate of increase in resource handling (r_{α}). From Witting (2017b)

Now that we have the metabolic-rescaling exponent for mass specific metabolism (β_{β}^{•}) as a function of the r_{ββ}/r_{α}-ratio, we can extend our deduction and obtain all the allometric exponents for the body mass evolution of a phylogenetic lineage in time. This complete set of allometric exponents is given in Table 1 for selected r_{ββ}/r_{α}-ratios and interactive competition in one, two and three spatial dimensions.

The main purpose of our study was a prediction of the evolution of natural selection time, as defined by the selected relationship between generation time and mass. With this relation being given by the inverse of mass specific metabolism, we find that the timescale of natural selection is evolving as

∂ ln τ / ∂ ln w = τ^{•} = [ 1/(1+r_{ββ}/r_{α}) – 2(d-1)/(1+r_{α}/r_{ββ}) ] / 2d

This timescale is dilating to the 1/4, or 1/6, power of mass when the r_{ββ}/r_{α}-ratio is zero and mass is evolving exclusively from an increase in the handling of resources. And it is contracting to the -1/2, or -2/3, power of mass when the r_{ββ}/r_{α}-ratio is infinite and mass is evolving exclusively from metabolic acceleration.

_{ββ}/r_{α}-ratio that describes the relative importance of metabolism for the evolution of mass. Traits include resource handling (α), mass specific metabolism (β), time-periods (τ), survival (p), lifetime reproduction (R), rate of population increase (r), home-range (h), population density (n), and the rate of change in mass in physical time (dt). From Witting (2017b).

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The natural selection of metabolism explains curvature in fossil body mass evolution

## References

- Witting, L. 1995. The body mass allometries as evolutionarily determined by the foraging of mobile organisms. Journal of Theoretical Biology 177:129--137, https://doi.org/10.1006/jtbi.1995.0231.
- Witting, L. 2017. The natural selection of metabolism and mass selects allometric transitions from prokaryotes to mammals. Theoretical Population Biology 117:23--42, https://dx.doi.org/10.1016/j.tpb.2017.08.005.
- Witting, L. 2020. The natural selection of metabolism explains curvature in fossil body mass evolution. Evolutionary Ecology 47:56--75, https://dx.doi.org/10.1007/s11692--020--09493--y.