# The exponential increase in mass

Unconstrained population dynamic feed-back selection selects for an exponential increase in the net energy and body masses of mobile organisms

The feed-back selection of the interactive competition in a population of a multicellular animal will stabilise at the competitive interaction fix-point for a body mass in evolutionary equilibrium when the net energy of the average individual is stable. But unconstrained natural selection is causing an exponential increase in net energy, and this is selecting for an evolutionary steady state where the body mass is increasing exponentially on the per generation timescale of natural selection (Witting, 1997, 2003, 2017b).

To formulate this, from the discrete growth rate [ λ ∝ p ε tr/w ] we have a selection gradient of unity for net energy on logarithmic scale [ ∂ r / ∂ ln ε = 1 ]. This selection is generating an exponential increase

rε = d ln ε / d τ = σ2 ∂ r / ∂ ln ε = σ2

on the pre-generation timescale (τ) of natural selection, whenever the additive heritable variance (σ2) is stable (Robertson, 1968; Taylor, 1996). The increased net energy is allocated into reproduction and an increase in the population dynamic equilibrium and its associated interference (ι*). And this interference is selecting for an increase in mass

rw = d ln w / d τ = σ2 ∂ r / ∂ ln w = σ2 [ ψ ι* – 1 ]

This rate of increase in mass is a consequence of the increase in net energy, and it may thus be rewritten as

d ln w / d τ = [ ∂ ln w / ∂ ln ε ] [ d ln ε / d τ ]

with an invariant selection relation

∂ ln w / ∂ ln ε = 1 / e

that defines mass

w = ∫ [∂ ln w / ∂ ln ε] d ln ε = (ε/ε0)1/e

by the inverse of the mass allometry for net energy [ ε = ε0 we ], where e = (2d-1)/2d from the allometric deduction (Witting, 1995, 2017a).

We may thus write the rate of change in mass as

rw = rε / e

and with rε2 and rw2[ψι*-1], we find the resource bias

ψι** = (4d - 1) / (2d - 1)

and interference [ ι** = (4d - 1) / (2d - 1) ψ ] at the evolutionary steady state with an exponential increase in net energy [ d ε / d τ = re ε ] and mass [ d w / d τ = rw w ]. The attractor of the evolutionary steady state is illustrated for different evolutionary trajectories in the left plot of Fig. 1, and the right plot shows the steady state on the selection profile relative to the evolutionary equilibrium with a stable body mass.

Fig. 1 Left: The exponentially increasing body mass of the evolutionary steady state. The steady state is shown by the thick line, and the two thin curves show evolutionary trajectories that are initialised with the same energetic state but two different body masses. Right: A selection profile for body mass with the position of the evolutionary equilibrium (*) and the evolutionary steady state (S) for an organism that forages in two spatial dimensions. From Witting (1997).

The increase applies to unconstrained selection in stable environments, and as such it is expected for the largest and dominant species in a competitive guild. Other species may increase at a slower rate, or have a stable or even a declining mass if the access to resources is declining because of inter-specific interactions or environmental variation.

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

Inevitable evolution: back to The Origin and beyond the 20th Century paradigm of contingent evolution by historical natural selection

Major life-history transitions by deterministic directional natural selection