Predictability in polygenic adaptation: pleiotropy, divergence, and biological scale

Predictability in polygenic adaptation: pleiotropy, divergence, genetic background, and biological scale

Dagný Ásta V. Þórhölludóttir presents her Phd thesis, done at the University of Veterinary Medicine in Vienna.
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Abstract: Understanding when evolution follows predictable paths is a central question in evolutionary biology, particularly for complex traits shaped by many genes. Here, I examine how pleiotropy, evolutionary divergence, genetic background, and biological scale influence the repeatability of polygenic adaptation in Drosophila. I combine experimental evolution, population genomics, transcriptomics, and trait analyses to compare responses to shared selective pressures across populations and species. Pleiotropy both constrained and reinforced adaptation. Genes affecting many traits were recruited less often under laboratory selection but contributed more to adaptive gene expression divergence in natural populations. Their involvement in nature was also associated with greater parallelism between species. Nevertheless, overall gene expression parallelism between species was low and declined with divergence. Differences in allele frequencies, linkage, regulatory architecture, and evolutionary history can thus lead to distinct molecular responses under similar selection. This raises a broader question: where along the genotype-to-phenotype hierarchy does predictability begin to emerge?