A fundamental aspect of evolutionary biology is natural selection on trait variation. Classically, selection has been estimated primarily on external morphological traits such as beak size and coloration, or on easily-assayable physiological traits such as heat-tolerance. As technologies and methods improved, evolutionary biologists began examining selection on molecular traits such as protein sequences and cellular processes. In a From the Cover manuscript in this issue of Molecular Ecology, Ahmad et al. (2021) continue this trend by estimating parasite driven selection on the molecular trait of transcript abundance in a wild population of brown trout (Salmo trutta) by uniquely combining a mark-recapture experimental design with non-invasive RNA sampling. Using transcript abundance to estimate selection allows for many different traits (each unique gene’s transcript counts) to be tested in a single experiment, providing the opportunity to examine trends in selection. Ahmad et al.(2021) find directional selection strength on transcript counts is generally low and normally distributed. Surprisingly, transcripts under non-linear selection showed a disruptive selection bias contradicting previous comparative studies and theoretical work. This highlights the importance of within-generation selection studies, where mechanisms may differ from longer time frames. Their manuscript also highlights the benefits of an improved 3’ RNA sequencing technique to measure gene expression.