THE MACROEVOLUTIONARY DYNAMICS OF VERTEBRATE GENOME SIZE

To make reliable inferences about genome size and biology in extinct species, we must understand genome size diversity in extant species. Multiple competing hypotheses attempt to explain genome size variation across species—each making testable assumptions about the tempo and mode of evolution. The effective population size (EPS) hypothesis postulates that changes in genome size occur primarily during speciation events when effective population sizes are small. The mutational equilibrium (ME) hypothesis states that genome size evolves gradually due to an imbalance in the rates of insertions and deletions over time. Both of these hypotheses evoke neutral processes for genome size evolution, but the EPS hypothesis suggests a punctuated mode of evolution while the ME hypothesis suggests a gradual one. The ME hypothesis also argues that the rate of genome size evolution is proportional to its size. The adaptive genome size hypothesis states that genome size variation is predominately explained by environmental factors implying that natural selection plays a greater role in genome size evolution. However, a large-scale analysis has not been conducted to test these hypotheses in the context of vertebrate genomes. We conduct the largest phylogenetic comparative analysis of genome size evolution across every major vertebrate clade. We collected genome size distributions for ~2,600 species and tested various models of evolution. A variable rate model was tested against a random walk model to compare the influence between neutral and adaptive evolutionary forces. To evaluate the EPS and ME hypotheses, we tested for a model of punctuated evolution and for correlations between the rate of genome size evolution and genome size itself. The adaptive genome hypothesis was further evaluated by estimating phylogenetic signal in relation to genome size. Determining the evolutionary dynamics of genome size is particularly important for understanding the origin of convergently evolving innovations. The concurrence of small genomes and flight has been repeatedly demonstrated in extant and extinct groups. Whether this was the product of adaptive or neutral forces is still debated. By testing hypotheses of genome macroevolution, we can better comprehend the interplay between genome architecture and phenotypic innovation.