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