Simple model for the Darwinian transition in early evolution (1501.05073v1)

Abstract: It has been hypothesized that in the era just before the last universal common ancestor emerged, life on earth was fundamentally collective. Ancient life forms shared their genetic material freely through massive horizontal gene transfer (HGT). At a certain point, however, life made a transition to the modern era of individuality and vertical descent. Here we present a minimal model for this hypothesized "Darwinian transition." The model suggests that HGT-dominated dynamics may have been intermittently interrupted by selection-driven processes during which genotypes became fitter and decreased their inclination toward HGT. Stochastic switching in the population dynamics with three-point (hypernetwork) interactions may have destabilized the HGT-dominated collective state and led to the emergence of vertical descent and the first well-defined species in early evolution. A nonlinear analysis of a stochastic model dynamics covering key features of evolutionary processes (such as selection, mutation, drift and HGT) supports this view. Our findings thus suggest a viable route from early collective evolution to the start of individuality and vertical Darwinian evolution, enabling the emergence of the first species.

• The paper formulates a minimal model explaining the theoretical dynamics of the Darwinian transition, the shift from horizontal gene transfer (HGT) to vertical gene transfer and natural selection.
• The model demonstrates two metastable evolutionary states, high-entropy HGT-driven and low-entropy speciated states, with stochastic switching based on competence for HGT.
• Reduced HGT competence below a critical threshold promotes the shift towards speciated, selection-driven evolution, suggesting HGT's pivotal role in pre-Darwinian periods.

A Minimal Model for Understanding the Darwinian Transition in Early Evolution

The paper "Simple model for the Darwinian transition in early evolution" by Hinrich Arnoldt, Steven H. Strogatz, and Marc Timme formulates a minimal yet mathematically robust model to illuminate the theoretical dynamics involved in the hypothesized "Darwinian transition"—the shift from collective evolution driven by horizontal gene transfer (HGT) to modern evolution governed by vertical gene transfer and natural selection. This model provides a framework for revisiting and expanding upon Woese's conjecture of early life's communal genetic exchanges.

Modeling Pre-Darwinian and Darwinian Evolutionary States

The authors leverage a binary sequence model to represent progenotes, each with genomes subject to mutation, selection, drift, and HGT on a Mount Fuji fitness landscape. By introducing stochastic HGT hyperlinks—three-point interactions among genotypes—the model simulates horizontal transfer dynamics as mechanisms for collective evolution. Critically, the model assigns fitness values to these sequences, allowing for selection pressures to influence evolutionary outcomes.

The model demonstrates two metastable states: a high-entropy, distributed state dominated by HGT and a low-entropy, speciated state dominated by vertical gene transfer and selection processes. High entropy reflects widespread genotype diversity, aligning with the hypothesized communal state of progenotes, while low entropy indicates a concentrated genotype distribution characteristic of Darwinian species.

Stochastic Switching and the Role of Competence

A pivotal finding of the paper is the identification of stochastic switching between these states based on the competence for HGT. At high competence, the population remains largely distributed, preventing adaptation to the fitness landscape. In contrast, reduced competence promotes a shift towards the low-entropy speciated state, with selection driving species emergence as competence falls below a critical threshold.

The paper develops a quantitative measure of population entropy changes, elucidating how competence levels modulate the evolutionary trajectory. Notably, even minimal HGT can sustain distributed states, suggesting HGT's critical role in pre-Darwinian periods. Direct simulations underscore the bistability induced by HGT, thus supporting a scenario where lowering competence through temporary selection shifts enables gradual Darwinian transitions.

Implications and Future Perspectives

The theoretical framework posited by this model underlines the complexity of early evolutionary dynamics and the considerable impact of HGT in shaping life's nascent periods. It introduces a mechanistic pathway supporting the hypothesis of HGT-induced collective evolution transitioning into Darwinian evolution through gradual competence attenuation and frequency of stochastic transitions to selection-driven states.

From a broader perspective, the work implies that systems capable of stochastic switching could inform other transitions in evolving biological systems, not limited to the origin of species. Future research could integrate more complex environmental interactions and biological details, like ribosomal dynamics, to refine this hypothesis and further connect model observations with empirical genetic and paleontological data.

In conclusion, Arnoldt et al.’s minimal model serves as a valuable tool for exploring the nuanced dynamics involved in the evolutionary shift towards Darwinian processes, highlighting the interplay between HGT, genetic diversity, and selection. With this work, the authors contribute significantly to theoretical evolutionary biology, offering insights into the fundamental processes that may have governed early life and set the stage for the vast diversity of species observed today.