The very notion of “species” is even a little misleading — a discrete-sounding artifice created for the convenience of people who live about a hundred years. If you had eyes to see the big picture, and could watch life change on a geologic time frame, you’d see constant gradual change, as generations adapt to circumstance.
It’s that incredibly slow pace that makes it hard for people to grasp intuitively. When you only live long enough to see three or four generations — a few ticks of evolution’s clock — any tiny generational changes, like humanity getting marginally blonder or taller, are dwarfed by differences in the members among any one generation. Pile on enough eons, and tiny pidgin horses gradually become rideable by gradually less hairy apes. But it’s impossible to see for yourself.
That’s evolution left to proceed at its own lazy, trial-and-error pace. But it turns out you can make the gears turn a lot faster — in fact, we do it all the time. Have you ever seen strawberries in the wild? They’re little tiny things, easily missed if you are not a bird or a bee. We bred them to be big and fat, specifically by only allowing the seeds from the biggest, fattest ones in each generation to reproduce. We similarly manipulate almost every other “natural” food we eat today: Take a stroll through any modern produce section and you can see the fruits, literally and figuratively, of evolution turbocharged by human intervention.