The concept of speciation is easy enough, but the details are a nightmare. Darwin commented on it extensively that there is no exact line at which we consider species diverged. It was really hazy in his day, and still hazy now. The problem with such gradual change is that it becomes nearly impossible to pinpoint an exact moment that a species starts being something new, and not just a variety of one thing. That is another common avenue of attack on evolution, though I find it ultimately pointless, because disagreement on what to call benchmarks does nothing to call into question that the process happens. Darwin's critics jumped on this right away, saying that Darwin's finches were not different species at all, and just a variety of one species. The same was said about dogs, that even though they had been drastically changed by centuries of domesticated life, that they were still the same wolf species that they always were. Darwin's answer was to point out that if put back into the wild, dogs would not revert back to their natural state, even over generations. He also pointed out that some dogs types can't even produce offspring with each other, which is about the most solid line you could draw on a difference between species.