The composite theory as the explanation of Haldane's rule should be abandoned

16 In 1922, JBS Haldane discovered an intriguing bias of postzygotic isolation 17 during early speciation: the heterogametic sex of F1 hybrids between closely related 18 species or subspecies is more susceptible to sterility or inviability than the homogametic 19 sex. This phenomenon, now known as Haldane’s rule, has been repeatedly confirmed 20 across broad taxa in diecious animals and plants. Currently, the dominant view in the 21 field of speciation genetics believes that Haldane’s rule for sterility, inviability, male 22 heterogamety and female heterogametic belongs to different entities; and Haldane’s rule 23 in these subdivisions has different causes, which operate coincidentally and/or 24 collectively resulting in this striking bias against the heterogametic sex in hybridization. 25 This view, known as the composite theory, was developed after many unsuccessful quests 26 in searching for a unitary genetic mechanism. The composite theory has multiple sub27 theories. The dominance theory and the faster male theory are the major ones. In this 28 note, I challenge the composite theory and its scientific validity. By declaring Haldane’s 29 rule as a composite phenomenon caused by multiple mechanisms 30 coincidentally/collectively, the composite theory becomes a self-fulfilling prophecy and 31 untestable. I believe that the composite theory is an ad hoc hypothesis that lacks 32 falsifiability, refutability and testability that a scientific theory requires. It is my belief 33 that the composite theory does not provide meaningful insights for the study of speciation 34 and should be abandoned. 35