Biology subjects

Schertzer, E.

Publications and source records attributed to Schertzer, E..

3 recordsLinked to original sources

bioRxiv2026

Stabilizing selection on a polygenic trait from the gene's-eye view.

We study a polygenic trait under stabilizing selection at statistical equilibrium, where genetic effect, mutation rate and mutational bias are heterogeneous across loci. The model assumes L biallelic sites subject to reversible mutations, each allele described by its frequency in the population. Using a diffusion approximation, a mean-field approximation and neglecting linkage disequilibrium, we predict consistent phenomena across several regimes of selection: (1) a small deviation {Delta}* of the trait mean from its optimal value appears and persists due to genetic mutations not aligned with selection; (2) while this deviation is often undetectable at the trait level, it leaves a substantial signature at the locus level by favoring alleles reducing it, resulting in genic selection with mean coefficient s* proportional to -{Delta}* acting pervasively; (3) with stronger selection on the trait, (3a) the value of {Delta}* is decreased but the intensity of genic selection is increased in inverse proportion, resulting in an essentially constant, non negligible value of s*. We show how the stationary distribution of allelic frequencies can be obtained from {Delta}*. The latter can then be characterized as the solution to a fixed-point equation. Finally, we quantify several macroscopic observables of interest (genetic variance, description of the fluctuations of the trait mean as an Ornstein-Uhlenbeck process). The orders of magnitude of the macroscopic observables can be derived on a wide region of the parameter space. The model shows good fit and can straightforwardly be extended to accommodate pleiotropy, dominance, and some forms of epistasis. We also discuss the different breakdown which may occur (Bulmer effect, Hill-Robertson effect, breakdown of the Ornstein-Uhlenbeck approximation for the dynamics of the trait mean, depletion of genetic variability due to low mutation rates).

evolutionary biology↗

bioRxiv2023

Opening the species box: What microscopic models of neutral speciation have to say about macroevolution

In the last two decades, lineage-based models of diversification, where species are viewed as particles that can divide (speciate) or die (become extinct) at rates depending on some evolving trait, have been very popular tools to study macroevolutionary processes. Here, we argue that this approach cannot be used to break down the inner workings of species diversification and that "opening the species box" is necessary to understand the causes of macroevolution, but that too detailed speciation models also fail to make robust macroevolutionary predictions. We set up a general framework for parsimonious models of speciation that rely on a minimal number of mechanistic principles: (i) reproductive isolation is caused by excessive dissimilarity between genotypes; (ii) dissimilarity results from a balance between differentiation processes and homogenizing processes; and (iii) dissimilarity can feed back on these processes by decelerating homogenization. We classify such models according to the main homogenizing process : (1) clonal evolution models (ecological drift), (2) models of genetic isolation (gene flow) and (3) models of isolation by distance (spatial drift). We review these models and their specific predictions on macroscopic variables such as species abundances, speciation rates, interfertility relationships or phylogenetic tree structure. We propose new avenues of research by displaying conceptual questions remaining to be solved and new models to address them: the failure of speciation at secondary contact, the feedback of dissimilarity on homogenization, the emergence in space of breeding barriers.

evolutionary biology↗

bioRxiv2023

Weak genetic draft and the Lewotin's paradox

Recurrent selective sweeps reduce diversity at linked neutral loci, a regime known as genetic draft. Most theoretical work has focused on the tight draft regime, where the selected and neutral loci are closely linked, leading to Multiple Merger Coalescents and a diversity largely insensitive to population size. Here, we investigate the neglected regime of loose genetic draft, where sweeps at a distant linked locus have individually negligible effects but collectively drive diversity. To explore this regime systematically, we make extensive use of the RIF model (Random Initial and Final conditions), a semi-deterministic approximation of selective sweeps that is 103 times faster than standard Wright-Fisher simulations and equally accurate. Using this framework, we derive novel analytical approximations for the coalescence probability under a single sweep, valid for a wide range of recombination-to-selection ratios A = c/s, and show that they outperform several previous approximations from the literature, which are only accurate for small A. Under recurrent loose draft (0.1 < A < 0.5), the effective population size scales as a power law of census size, Ne {propto} N 2A, which could contribute to the observed non-linear dependence of diversity on population size. Crucially, despite this strong reduction in diversity, the genealogy converges to a Kingman coalescent, making loose draft patterns indistinguishable from neutrality by standard tests.

evolutionary biology↗