put two rabbits in a field with everything that they need for an easy life and go away for a hundred years.... It happens, and all you need for a mouse infestation is a single pregnant mouse in a building the offspring will sort themselves out
Errm perhaps not. You can't just mass produce a species by back-crossing it causes severe problems with the immune system. I will explain why.
question.
Why bother with sexual reproduction at all? all number of worm species and fish species exist which don't bother... They are hermaphrodites, and they have sex with themselves and make more copies of themselves.
The answer is evolution.
We are all different I'm different from you and so fought this isn't just in appearance but even down to our immune systems they behave differently this is why we have problems with tissue matching when transplanting organs. The immune system of the host doesn't recognize the different tissue even if it's functionally the same as the lost organ.
As such when I get infected with a virus my body will behave in a unique way. As would your body. This is due to the HLA & MHC genes, which are highly diverse in humans.
If however you keep crossing back with a common ancestor in reproduction this diversity is rapidly eliminated and data is deleted from the gene pool. As a result the ENTIRE populations immune system will be compromised as of credit of "behaving" the same way.
This is the testament of what Victorian misconceptions like yours have left certain species.
Cheetahs were extremely endangered even in the 1900's to make more they would inbreed two cheetahs into a population back crossing the offspring with the parent.
These animals were then released into the wild.
Nearly all of them died out within a few generations.
http://en.wikipedia.org/wiki/Feline_infectious_peritonitis
This virus had emerged from a rare disease effecting 1-5% to wiping out over 40% of their population.
BECAUSE they were all inbred and the original progenitor was vulnerable to this illness thus all of the subsequent offspring were equally vulnerable. however had they been outbred then the additional genetic information would most likely have protected them.