Actually, you are quite wrong. Species that are very different are unlikely to merge, but animals of different species with a high percentage of common DNA are able to reproduce. True, in most cases the offspring is sterile, but in some mutant cases they are not. They go on to pass their DNA on to ancestors, and thus, slowly the species changes. Look at lions and tigers. They are different species, Panthera tigris and Panthera leo, but they are capable of reproducing together. Their habitats do not generally cross except in captivity, but again, there are occasions. A freak storm could separate a lion from the pride and, lost, it will venture out alone. Once alone, it will wander, looking for other lions, but it could perhaps migrate and in a single lifetime make it into an area where tigers are indigenous. That’s the beauty of evolution. It is based on the chance encounters and the slow progress of miniscule changes, generation after generation. But I guess anyone who can really look at the world around us and believe that it is only 8,000 years old can’t be taught much about genetics anyway.