Please explain something I don't understand about the evolution of animals that have sexual reproduction. I thought that being a separate species means an animal will have sexual intercourse resulting in fertile offspring only if the partner is of the same species. I shall cite the giraffe as an example because of an explanation about its evolution that was once taught to me. If I understand correctly, the giraffe with a long neck probably came into existence because at one time a mutation occurred inside in a shorter-neck parent, such that the offspring had a longer neck. Am I correct in saying that the new long neck animal will continue as a distinct species only if does not mate with short neck animals? But why? Would two animals, which are otherwise sexually attracted to each other, stop and think, "I'm unwilling to mate with you because I can see that your neck is too long (or short)"? Or did the mutation that made the offspring's neck longer also make the reproductive cells or organs physically incompatable with those of the short neck animals? In either case, how did that long neck animal find a mate, so that the new species could survive? Did there have to be at least two mutations in the same geographical area, in both cases having the effect of causing short-neck parents to have long-neck offspring, so that the long neck animal could find a mate? Thank you. Please keep your explanation simple because my background is physical science, not life science.