Detailed experimental studies in evolutionary biology are sometimes difficult--even with model organisms. Theoretical models alleviate some of these difficulties and often provide clean results, but they cannot always capture the complexity of dynamic evolutionary processes. Artificial life systems are tools that fall somewhere between model organisms and theoretical models that have been successfully used to study evolutionary biology. These systems simulate simple organisms that replicate, acquire random mutations, and reproduce differentially; as a consequence, they evolve naturally (i.e., evolution itself is not simulated). Here I use the software Avida to study several open questions on the genetic mechanisms of adaptation and speciation.In Chapter 1 (p. 13), I investigated whether beneficial alleles during adaptation came from new mutations or standing genetic variation--alleles already present in the population. I found that most beneficial alleles came from standing genetic variation, but new mutations were necessary for long-term evolution. I also found that adaptation from standing genetic variation was faster than from new mutations. Finally, I found that recombination brought together beneficial combinations of alleles from standing genetic variation.In Chapter 2 (p. 31), I investigated the probability of compensatory adaptation vs. reversion. Compensatory adaptation is the fixation of mutations that ameliorate the effects of deleterious mutations while the original deleterious mutations remain fixed. I found that compensatory adaptation was very common, but the window of opportunity for reversion was increased when the initial fitness of the population was high, the population size was large, and the mutation rate was high. The reason that the window of opportunity for reversion was constrained was that negative epistatic interactions with compensatory mutations prevented the revertant from being beneficial to the population.In Chapter 3 (p. 58), I showed experimentally that compensatory adaptation can lead to reproductive isolation (specifically, postzygotic isolation). In addition, I found that the strength of this isolation was independent of the effect size of the original deleterious mutations. Finally, I found that both deleterious and compensatory mutations contribute equally to reproductive isolation.Reproductive isolation between populations often evolves as a byproduct of independent adaptation to new environments, but the selective pressures of these environments may be divergent (`ecological speciation') or uniform (`mutation-order speciation'). In Chapter 4 (p. 75), I compared directly the strength of postzygotic isolation generated by ecological and mutation-order processes with and without migration. I found that ecological speciation generally formed stronger isolation than mutation-order speciation and that mutation-order speciation was more sensitive to migration than ecological speciation.Under the Dobzhansky-Muller model of speciation, hybrid inviability or sterility results from the evolution of genetic incompatibilities (DMIs) between species-specific alleles. This model predicts that the number of pairwise DMIs between species should increase quadratically through time, but the few tests of this `snowball effect' have had conflicting results. In Chapter 5 (p. 101), I show that pairwise DMIs accumulated quadratically, supporting the snowball effect. I found that more complex genetic interactions involved alleles that rescued pairwise incompatibilities, explaining the discrepancy between the expected accumulations of DMIs and observation.
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