Cai, H.

The alignment between mutational variance, standing genetic variance, and macroevolutionary divergence in Drosophila wing shape presents a rate paradox: evolution follows mutational lines of least resistance, yet proceeds orders of magnitude slower than quantitative genetic theory predicts. Two explanations have been proposed: "deleterious pleiotropy," where stabilizing selection on unmeasured traits outside the wing morphology complex constrains evolution, and "correlational selection," where selection acts directly on trait covariances. However, recent empirical work has found little evidence of fitness costs associated with wing shape variation beyond effects on flight performance, undermining the deleterious pleiotropy hypothesis. Here, I propose and evaluate a pleiotropic hitchhiking model, in which natural selection targets a single functional trait (wing size) while the complex geometry of wing veins evolves as a correlated response shaped by the structure of mutational variance. Using individual-based simulations parameterized with empirical mutational covariances, I show that univariate selection on a primary trait can reproduce the observed alignment among mutational variance, genetic variance, and divergence. Importantly, the pleiotropic hitchhiking model can also generate slower-than-neutral divergence rates, offering a resolution to the rate paradox without invoking hidden pleiotropic costs on unmeasured traits.