Cline Widths, Barriers to Gene Flow and Reproductive Isolation: A Comment on Gippner et al. 2026

ABSTRACT Hybrid zones provide opportunities to study the operation of barriers to gene flow as they play out in natural environments. However, comparative analysis is difficult because both hybrid zones and the methods used to analyse them vary widely. Gippner et al. (2026) brought together data on 108 animal hybrid zones. They argued that cline width in these zones provides a proxy for reproductive isolation (RI). Here, we comment on what cline widths do and don't tell us about the evolutionary progression towards the completion of speciation.