E” of the sexual mechanism. This constitutes a very different approach than available so far to the question of how sex is “maintained” despite its “costs” [24]. I have claimed earlier that it is incorrect to discuss sex as something whose “costs” and “benefits” determine its existence, because it is a matter of necessity. And here I claim that it is not actively “maintained”, but rather no substantial adaptive evolution occurs without it, and so PD150606 dose obligate asexuality cannot gradually and adaptively evolve. It can only arise non-adaptively. The rest of this section will consider evidence and predictions regarding this point. Note that, among vertebrates, all known unisexual lineages according to Avise [64] have arisen from hybridizations, which is a sudden, breakage event. Indeed, it is thought that hybridization probably disrupted the meiotic operations by reducing chromosomal homology enough to disrupt synapsis [64-67]. While this fact is in accord with the new theory as just stated, in retrospect, one may try to argue that it is consistent with the traditional one too, because if sex is already well established in two separate sexes, then it is hard to see how it will evolve into asex except by breakage. Let us therefore take the battle to the flowering plants: there, most species are capable of both selfing and outcrossing (selfing being akin to asexuality). According to traditional theory, the entire range from pure selfing to pure outcrossing is open to them, and adaptive evolution should be able to push species all the way to pure selfing or pure outcrossing [68]. Indeed, from that theory, based on inbreeding considerations, the paper that initiated the modern interest in this field predicted that pure selfing and pure outcrossing are the only stable equilibria under adaptive evolution [68]. But that approach gives no clear reason why there are overwhelmingly more species at the outcrossing end of the spectrum than at the selfing end [69-74]. This empirical fact supports the theory proposed here while standing uncomfortably with the traditional one. That is, according to my theory, evolution in a mixed selfing-outcrossing system is possible, but pure selfing can only be reached by breakage. Pure selfing is rare because it requires breakage, which can occur only under very specific conditions. (In the unisexual vertebrates, for example, it has been argued PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/27735993 that the hybridizing species need to be genetically close enough to produce a viable hybrid but far enough to disrupt meiosis [43,75] and/or satisfy more specific restrictions [76].) To be sure, other explanations have been offered in the plant-mating literature for the lack of pure asexuals (e.g., [77-84]), but the explanation proposed here is both simpler and more general. Indeed, it predictsresidual outcrossing in regular biparental inbreeding animal species, which goes beyond hermaphrodites. Because the new theory holds that obligate asexuality is arrived at by breakage, it predicts the lack of fine-tuned adaptations ensuring obligate asexuality. In contrast, from traditional theory, one would expect adaptations for pure asex in like manner as for pure sex. This suddenly renders of particular importance the empirical question of what obligate selfers are like. Pannell [85] mentions two notable examples of obligate selfers. One example involves the loss of males in populations or species of androdioecious animals. In these animals, such as the mangrove killifish (Kryptolebias ma.
