UBC Theses and Dissertations
Mathematical models of life cycle evolution Scott, Michael Francis
In this thesis, I investigate several aspects of life cycle evolution using mathematical models. (1) We expect natural selection to favour organisms that reproduce as often and as quickly as possible. However, many species delay development unless particular environments or rare disturbance events occur. I use models to ask when delayed development (e.g., seed dormancy) in long-lived species can be favoured by selection. I find that long-lived plants experience `immaturity risk': the risk of death due to a population-scale disturbance, such as a fire, before reproducing. This risk can be sufficient to favour germination in the disturbance years only. I show how demographic models can be constructed in order to estimate the contribution of this mechanism (and two other mechanisms) to the evolution of dormancy in a particular environment. (2) All sexually reproducing eukaryotes alternate between haploid and diploid phases. However, selection may not occur in both phases to the same extent. I use models to investigate the evolution of the time spent in haploid versus diploid phases. The presence of a homologous gene copy in diploids has important population genetic effects because it can mask the other gene copy from selection. A key innovation of my investigation is to allow haploids and homozygous diploids to have different fitnesses (intrinsic fitness differences). This reveals a novel hypothesis for the evolution of haploid-diploid strategies (where selection occurs in both phases), where the genetic effects of ploidy are balanced against intrinsic fitness differences. (3) Many sex chromosome systems are characterized by a lack of recombination between sex chromosome types. The predominant explanation for this phenomenon involves differences in selection between diploid sexes. I develop a model for the evolution of recombination between the sex chromosomes in which there is a period of selection among haploid genotypes in pollen or sperm. I find that a period of haploid selection can also drive the evolution of suppressed recombination between sex chromosomes, which should become enriched for genes selected in the haploid phase. This model predicts that the tempo of sex chromosome evolution can depend on the degree of competition among haploids.
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