Ploidy and the Predictability of Evolution in Fishers Geometric Model

TL;DR

This study uses Fisher's Geometric Model to compare adaptive evolution in haploid and diploid organisms, revealing that diploids are less predictable forward but more predictable backward, due to their ability to maintain polymorphisms.

Contribution

First simulation-based comparison of adaptive walks in haploid and diploid organisms using Fisher's Geometric Model, highlighting the impact of polymorphisms on predictability.

Findings

Diploids are less forward-predictable than haploids.

Diploids are more backward-predictable than haploids.

Stable polymorphisms influence adaptive trajectories.

Abstract

Predicting adaptive evolutionary trajectories is a primary goal of evolutionary biology. One can differentiate between forward and backward predictability, where forward predictability measures the likelihood of the same adaptive trajectory occurring in independent evolutions and backward predictability measures the likelihood of a particular adaptive path given the knowledge of starting and final states. Recent studies have attempted to measure both forward and backward predictability using experimental evolution in asexual haploid microorganisms. Similar experiments in diploid organisms have not been conducted. Here we simulate adaptive walks using Fisher's Geometric Model in haploids and diploids and find that adaptive walks in diploids are less forward- and more backward-predictable than adaptive walks in haploids. We argue that the difference is due to the ability of diploids in our simulations to generate transiently stable polymorphisms and to allow adaptive mutations of larger phenotypic effect. As stable polymorphisms can be generated in both haploid and diploid natural populations through a number of mechanisms, we argue that inferences based on experiments in which adaptive walks proceed through succession of monomorphic states might miss many of the key features of adaptation.