Abstract: We use stochastic population models to study the evolution of Ultraselfish Gene Complexes (USGC's). USGC's are chromosomal regions characterized by segregation distortion: a heterozygote bearing the USGC passes it to more than 50 of offspring. USGC-bearing homozygotes are sterile. USGC's promote themselves at the expense of other genes in the same genome. They have been observed in animal, plant and fungal species and may lie, undetected, in many others. While the molecular drive mechanisms differ, USGC's exhibit similar genetic features, suggesting that the fundamental evolutionary mechanisms that allow their emergence may be shared.
We present Markov models and Monte Carlo simulations of genetic drift in populations of USGC's. Genetic drift, the stochastic behavior of allele frequencies in small populations, is a fundamental force in the process of evolution, yet the role of genetic drift in the evolution of USGC's is not well understood. Our analysis shows how genetic drift causes USGC's to evolve different characteristics in species typified by small and large populations, respectively. We apply our results to two well studied USGC's: the $t$-haplotype in Mus musculus and Segregation Distorter in Drosophila melanogaster.