Another killer:
There are two subspecies of rice, indica and japonica if you must know. When they are crossed, the hybrids are infertile. That’s not good if you want to grow ice; it’s the seed you would eat, isn’t it? The mechanism has been worked out (A killer-protector system regulates both hybrid sterility and segregation distortion in rice Jian Gyi Yang et al SCIENCE vol 337 no. 6100 September 14, 2012 page 1336).
It turns out to be three tightly bound genes, two of them work together to do the damage. A third acts as a protector but the protector produces problems with the offspring.
In other words, there is a specific mechanism that induces the hybrid infertility. Charles Darwin’s word on the origin of species – happenstance – simply is not the case, at least in rice.
Of course our interest here is not in hybrid infertility. That is a matter of infertility between species, and it is genetic. Our interest is in infertility within species and it appears to be epigenetic.
Still, it is interesting that there is a parallel. In both cases there appears to be a dedicated mechanism causing inconvenience. Between species, or in this case subspecies, the inconvenience is that it would be nice to be able to cross lines or rice at will in an effort to make a better line for agriculture. The mechanism puts a limit on that. Within species the inconvenience is that one cannot have an arbitrarily large mating pool, much as we have been told that this is the way to peace and prosperity.
Of course during speciation there may be an overlap. Consider an animal that feeds off tree leaves but lives on the ground, a giraffe wannabe if you will permit me to call it that. A longer neck would be an advantage. Well you might be able to get a somewhat longer neck by fine tuning the epigenetic effects of the wannabe. Necks are going to vary in length, and no doubt some of this is or can be epigenetic. So there is a selective advantage for animals with long-neck-friendly epigenetic arrangements. It might be enough in and of itself to let the animal begin to move into the niche.
Over time, if the stratagem seems to be working, altered genes might move in and take over the process. This would be a great advantage since the genes change so slowly and it would take less selective pressure to keep the new arrangement in action.
None the less, differences between species seem to be genetic and are to some extent different from differences within species.
Epigenetic effects that limit population size will, in the long run, prevent genetic diversity effects from rendering the population unviable. That makes sense. And since speciation is needed to expedite evolution and the adaptation to new niches as they arise, that make sense, too. But I still have the feeling that there is far more that we don’t know than that we know.
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