Digital sex: Causes and consequences of recombination

The evolution of sexual reproduction has long been a major problem in biology. It is a vibrant and fascinating area of research, rich with theories but poor in experimental results. The crux of the sexual paradox is the dominance of the sexual mode of reproduction in the natural world in spite of the apparent costs associated with recombination. In this dissertation I test two different theories about the evolution of sexual reproduction and also investigate effects of recombination on genetic architecture.; According to one theory, sex opposes the fitness-destroying process of Muller's ratchet, which occurs by the stochastic loss of high-fitness genotypes in small populations. Sex opposes the ratchet by allowing genotypes with different deleterious mutations to produce mutation-free offspring. In Chapter 1, I used the Avida digital-evolution software to investigate sex in relation to Muller's ratchet. Populations of digital organisms mutated, competed, and evolved in a complex environment. Populations were either asexual or sexual; in the latter case, parental genomes recombined to produce offspring. I also varied genomic mutation rates and population sizes, which at extreme values often caused mutational meltdowns and population extinctions. My results demonstrate that sex is advantageous for population survival under some conditions. Theory suggests that modularity promotes evolvability, and that aggravating (synergistic) epistasis among deleterious mutations facilitates the evolution of sex. In Chapter 2, by contrast, I investigate how genetic architecture is shaped by reproductive mode. I allowed 200 populations of digital organisms to evolve for over 10,000 generations while reproducing either asexually or sexually. For ten randomly chosen organisms from each population, I constructed and analyzed all possible single mutants as well as one million mutants at each mutational distance from two to ten.; In Chapter 3, I demonstrate that sex can evolve de novo and outcompete the asexual mode of reproduction under changing environmental conditions. I evolved large populations of digital organisms for thousands of generations in six environments with different periods of substrate change. Sex was the dominant mode of reproduction when the environments were changing rapidly, with up to 65% of populations evolving to reproduce sexually. The ancestral reproductive mode and genetic architecture had only weak effects on whether populations evolved to be sexual or asexual. I also found that in the environmental conditions where sex was prevalent, the sexual populations on average had higher fitness than asexual ones. (Abstract shortened by UMI.)...