(10-07-2013, 10:20 PM)Thor Wrote: Make sense. Thanks for clarify.
Please correct me if I am wrong. 
Same species can have different breeds. Obviously these different breeds of the same species can "cross-breed" together, and their offspring can still reproduce without a problem.
Absolutely. That is why when you have different "breeds" of dogs or cats they can breed quite successfully and the offspring is fertile. I really hate to use this analogy, but you could use the analogy of different "races" of people. A "black" person can have children with a "white" person, that are as healthy as if they weren't "crossed". Because we are all the same species, even the same subspecies. No matter the outward differences.
The concept of breeds is slightly crossing into reptiles. Green tree pythons are a god example, having many different phenotypes that can be crossed and bred into specifically desired appearances, with all of the offspring being completely viable. Though few of the breeders like that comparison.
(10-07-2013, 10:20 PM)Thor Wrote: It is not the same case when the "cross" is between different species. Think about liger (cross between tiger and lion), or mule (cross between donkey and horse). These cross-species breeding usually result in offsprings without the ability to reproduce again.
Now, in true hybrids, it is far from simple. Reptiles seem to hybridize much easier than mammals. And the results can be anywhere from seemingly infertile, to fertile, to in-between. Though, in most cases, reduced fertility seems to be the case. Especially with males. For example, there is a hybrid reptile, commonly called a "carpondro", which is a cross of a carpet python bred to a green tree python. In most cases, the female carpondros can be bred to either a male green tree python or carpet python. But male carpondros have had very few successful breeding attempts with any other animals.
Which leads one to wonder whether or not it is just the "odds" that are against the hybrids you mentioned. If a snake can only hatch out of 2-5 of 10-20 eggs, that is roughly a 25% successful hatch rate. In mammals with small numbers of babies (mules for example), that would mean that each single baby only has a 25% chance of developing full term. Obviously there are other mitigating factors, and this over simplifies things, but you get the idea. Hybrids have extremely reduced fertility as far as the evidence shows, and the actual fertility is is variable in each hybrid combination.
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