Small-world networks decrease the speed of Muller's ratchet

Area of Science:

• Evolutionary Biology
• Population Genetics
• Network Theory

Background:

• Muller's ratchet describes the accumulation of deleterious mutations in asexual populations, leading to extinction.
• Previous studies focused on panmictic (randomly mating) populations, neglecting spatial structure.
• Understanding the impact of population structure on Muller's ratchet is crucial for various evolutionary phenomena.

Purpose of the Study:

• To investigate the speed of Muller's ratchet in spatially structured populations.
• To analyze the influence of different network topologies (regular, small-world, random) on the ratchet's speed.
• To explore the effects of migration rates and mutation load on the ratchet's dynamics within structured populations.

Main Methods:

• Simulations of Muller's ratchet on various network structures: regular, small-world, and random graphs.
• Analysis of the ratchet's speed as a function of network characteristics (connectivity, path length) and migration rates.
• Comparison of results across different network types and with panmictic populations.

Main Results:

• Small-world networks significantly decrease the speed of Muller's ratchet due to high local connectivity and low path length.
• The reduction in ratchet speed is independent of the number of demes but is amplified by larger network size and stronger deleterious mutations.
• Increased migration generally slows the ratchet, but the effect is less pronounced in stepping-stone models compared to small-world networks.

Conclusions:

• Spatial structure, particularly small-world networks, can substantially impede Muller's ratchet, potentially mitigating extinction risk for asexual lineages.
• These findings have implications for the evolution of non-recombining genomes, sex, and the spread of microbes and epidemics.
• The study highlights the importance of considering realistic contact network structures in evolutionary models.