The molecular clock of neutral evolution can be accelerated or slowed by asymmetric spatial structure

TL;DR

This paper demonstrates that asymmetric spatial structures can accelerate or slow down the neutral molecular clock, challenging the traditional view that the rate equals the mutation rate, with applications in biology and social networks.

Contribution

It reveals how spatial asymmetry can alter the neutral evolution rate, providing a generalized model applicable to biological and social systems.

Findings

K can be less than, equal to, or greater than u depending on structure.

K ranges from 0 to N u in asymmetric populations.

Applications include genetic mutation accumulation and social network evolution.

Abstract

Over time, a population acquires neutral genetic substitutions as a consequence of random drift. A famous result in population genetics asserts that the rate, $K$, at which these substitutions accumulate in the population coincides with the mutation rate, $u$, at which they arise in individuals: $K=u$. This identity enables genetic sequence data to be used as a "molecular clock" to estimate the timing of evolutionary events. While the molecular clock is known to be perturbed by selection, it is thought that $K=u$ holds very generally for neutral evolution. Here we show that asymmetric spatial population structure can alter the molecular clock rate for neutral mutations, leading to either $K<u$ or $K>u$. Deviations from $K=u$ occur because mutations arise unequally at different sites and have different probabilities of fixation depending on where they arise. If birth rates are uniform across sites, then $K \leq u$. In general, $K$ can take any value between 0 and $Nu$. Our model can be applied to a variety of population structures. In one example, we investigate the accumulation of genetic mutations in the small intestine. In another application, we analyze over 900 Twitter networks to study the effect of network topology on the fixation of neutral innovations in social evolution.