  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.
