oaty

Ok, but could that happen in a single generation? And does that mean people with conditions that affect their chromosomes cannot breed?!

I have a very basic understanding of evolution, and find it fascinating but still have gaps/questions. When for example a certain species population is split geographically, and given enough time they slowly begin to differ, due to demands of environment etc there must come a point when they are no longer able to breed with the other group they were split from. But something tells me that the 'cut off line' could not be that neat, as everything else in this process is so gradual and I could not imagine one animal being able to breed with the other group, but who's offspring couldnt I would be grateful for anyone wiser on the subject to shed some light!