Contributed talk

The Cascade Effect: Mutation fixation rates over evolutionary time

Acacia Ackles, Clifford Bohm, Vincent Ragusa, Arend Hintze

Genome structure is shaped over evolutionary time by the rise of random mutations and the push and pull of selection upon the effects of those mutations. Different mutation types (insertions/deletions, point mutations, transpositions, etc.) have different effects not only on the fitness of an organism but on the underlying structure of its genome. Understanding how genome structure is influenced by selection on the effects of different mutational types can help us predict future mutational impacts and understand how genotypes navigate genetic space. Unfortunately, complete mutational history is difficult or impossible to study in biological evolution due to limitations of sequence availability and sequencing technology. While phylogenetic reconstruction However, examination of selection on mutation types in digital organisms, where precise per-generation mutation tracking is feasible, can provide insight into the processes acting on biological genomes. Thorough work has previously been conducted on mutational history in digital genomes. Such work often focuses on mutations which change a gene directly from one function to another (e.g. "ADD" to "SUBTRACT"). Here we expand on this work by focusing instead on mutations classified by the underlying structural changes they produce in the genome. In particular we investigate 1. selection on fixation rate when mutation rate is held constant and 2. the relationship between elevated mutation rates early in evolution and suppressed mutation rates later. These two metrics allow us to infer fitness impacts of each mutation type directly from their corresponding fixation rates, even when the site-specific change is unknown. We find that the evolved fixation rate can and frequently does deviate from the set mutation rate, even though this mutation rate itself is not subject to evolutionary change. This result demonstrates that strong selective pressures have the ability to alter fixation rate on a population level, even without the ability to alter the trait on an individual level. Early in evolution, mutations which have small structural effects may appear suppressed not because they cannot generate fitness benefits but rather because other mutations types are more able to generate fitness benefits either at a higher rate or of larger magnitude. Later in evolution, especially once fitness is at or near optimal, deleterious mutations are strongly suppressed. In addition, we see that the degree to which each mutation type is initially favored is inversely proportional to the degree to which it is later suppressed. We call this effect the cascade effect. By observing when mutation types are favored in evolutionary history, we can infer which types of mutations are likely to increase fitness without knowing what specific change a mutation will have on a genome. In order to better understand the dynamics of mutation rates and fixation rates in more complex systems, future work will study systems with low mutation rates and high ratios of non-coding regions evolved in less fitness bounded tasks.