The environment is changing. I notice you only used examples that are mammals. You are dodging the truth of the situation, which is that most are NOT sterile. It is much more rare that mammal hybrids make it(which is why those are teh only examples you use). The reason they aren't common is because climates and environs are somewhat stable. Tiger salamander hybrids have fixed and out competed local populations because of permanent pools.
http://news.nationalgeographic.com/news/2009/06/090629-salamanders-hybrid.html
What I said is that you can have leaps in evolution thru the combinations made by hybrids and that probably happened most when climate or environments are changing. The rare become not so rare when they have adaptations that no one else has to fit in with the changes in environment. Phenos that are beneficial in the changed environment take over when these dominant phenos are no longer tops for the environment present.
In times of great change in the environment hybrids have a chance to leap forward better than fixed traits. Rare or not my statement stands as a beneficial gadget of evolution. You diminishing it doesn't matter to science and reality.
You said the most common outcome is sterility(wrong) followed by severe defects(wrong again). If 10% create hybrids it's not really that rare. And you CAN breed hybrids with hybrids. But you are right that improvements in a stable environment are rare and social ques may keep the hybrids from fitting in and breeding with either parent, that's different than sterility. The social cues which prevent hybrids are much more formed in mammals than reptiles, fish and amphibians. Yet still there are multiple hybridization events thru human history with multiple different species. Rare but has happened and given us great adaptations like improved lung capacity, disease resistance, etc.
It's done all the time in captivity. You should put down the books and breed some fish and reptiles and amphibians and see if what you speak of is true instead of believing everything you read in a book. While you have the second statement SOMEWHAT right you were very wrong on the first statement and very general. In captivity most everything you say is untrue because of habitat change. If you read about hybrids you may think they are predisposed to be sterile but if you have bred them you would realize otherwise. Sorry the books you read don't line up in any way to the experience of anyone who breeds fish, reptiles, amphibians and probably birds.
Another problem with mammals is low #'s of offspring. Het traits aren't a problem when you spit out 1000 eggs. The probability works out from sheer #'s of offspring.
And as far as plants go I just remembered I personally documented over a dozen chisos agave(hybrid century plant and lechiguilla) in Big Bend Nat'l Park when I worked there as a geologist on the spring survey.
http://aggie-horticulture.tamu.edu/ornamentals/nativeshrubs/agaveglomer.htm
The truth is that most people only know animals from captivity as we haven't even documented all species let alone spend enough time with other wild species to really know how prevalent it is and how much it has shaped animals thru evolution. We supposedly harbor over 20% of the neanderthal genome thruought the species from numerous hybridization events in numerous different regions we inhabited.
I personally believe that it's a major contribution to evolution when continents collide or landmasses connect and new plants and animals are able to breed again that have been separated for thousands of years. Or when climate change forces animals to migrate into new territories where disease resistance is needed or adaptability to changing climates. It would only make sense.
Also, the more they start to catalog species by genome the more tehy will find that hybridization is more normal than was previously thought.
https://en.wikipedia.org/wiki/Hybrid_(biology)
I understand it's wikipedia but shows how much more widespread hybridization is and how our changing the earth is contributing to it being much more widespread and how much more widespread it could've been at times in teh past. It's serves as a good general overview.
You should gain some real world experience to help with what's lacking with the book smarts. You'd see that holdanes rule doesn't hold true and there are more examples that can breed together than can't and sterility is not that common. You would realize this if you had ever bred animals/fish/insects, etc for a living. You'd also see there are many examples of better fitness, more phenotypes, the problem with incomplete dominance is quickly fixed with amphibians and reptiles which have thousands of offspring, etc. I'll leave you something to read about in Anoles in Fla. I'd also recommend doing some recent research in birds, since there is a culture around watching them in the wild. And you should at least make a few friends in the reptile hobby. You'll find the real world is a bit different than what you learned who knows how long ago.
http://www.anoleannals.org/tag/hybrid/
In the end it seems as if you are caught up in your own belief bias.
Chromosomal differences. Basic genetics.
A thousand hybrids from a billion generations is still a rare event. There's a reason why hybrids aren't that common in nature and why parents of a hybrid have different characteristics, fit for their specific environments. Seems you don't know that hybridization also causes a significant loss of traits, especially phenotypes that would have been beneficial but become useless due to incomplete dominance.
You've just restated what I said - that it's possible but rare. Vicariance among other post-zygotic mechanisms prevent prolonged successful hybridizations. Prezygotic barriers generally nullify the embryo. The liger, the tigon, the mule, the grolar, the zorse - pick any hybrid you want - they all have the same problem which prevents the profileration of hybrids: the heterogametic sex is inviable (male in mammals, female in fish/birds). In genetics, this is called the Haldane's Rule. You can't cross a hybrid with another hybrid of the same species. The only thing you can do is cross a hybrid with a parent specie which dilutes the hybrid's unique genes and inevitably merges it back with the parent species over successive generations.
Moreover, the intermediate traits that result in offsprings are usually useless and detrimental to survival. The grolar can't swim for shit, and the narluga doesn't have a tusk to hunt with. The only place hybrids would fit in well are hybrid environments that don't exist. And these are just the physical traits. The psychosocial effects are even worse. Zebras are territorial, wild horses are not; how would a zebroid fit in with either community? There's no intermediate between territorial and not - you either are or aren't. And that portrays another problem that can arise from genetic inheritance: non dominant.
In the end, it's a numbers game. You have a thousand genes from each parent and you pick one allele from each to recombine into one. The probability of producing a healthy offspring is very, very low, but possible. Just like it's possible to get hit by lightning twice and survive.
