[2] This hybrid displays reduced fitness, marked by decreased rates of survival and reproduction relative to the parent species.
[3] In the 1970s, Allan C. Wilson and his colleagues first investigated the evolution of hybrid inviability in tetrapods, specifically mammals, birds, and frogs.
This mammalian characteristic suggests that, although mammals are genetically similar, dramatic changes in regulatory genes caused distinct developmental differences.
[6] The Regulatory Hypotheses specifically attributes hybrid inviability in mammals, birds, and frogs to differences in gene regulation.
Moreover, because the development of the zygote depends on maternal characteristics, such as cytoplasmic determinants, the regulatory traits of the mother may not support the hybrid's developmental needs.
This hypothesis stems from the immunological characteristics of the placenta, where the growing fetus is in constant contact with the fluids and tissues of the mother.
These different placenta types possess divergent immunological systems, and consequently, they cause varying degrees of hybrid inviability.