Well, if you're ignorant on a topic, it would be wise to listen to the specialists. Scepticism is a good thing, but scepticism in the face of a strong scientific consensus is just denialism. You're not going to be a sceptic and ask questions about the concept of gravity either.
And there is actually strong scientific consensus here. That's not because I say it; I'm not a specialist either. But the author of that book is. Of course, you can't always be sure whether a single book portrays the scientific consensus (if there even is one on the relevant topic), but if you would check the book's reviews by specialists in the field, you'll find that the science is spot-on and inarguable. Take for instance the review I linked: it's from Nature, far and away the world's leading science journal alongside Science. (Fact; but you can look them.up if you don't believe me.) The only criticism the reviewer had, is that Rutherford gives too much credence to the concept of IQ, and that the world doesn't really need another book on theory. Why not? Because this is all well-known stuff.
Hence also the title: the author isn't arguing for a case or adding to an ongoing discussion, he is summarizing well-known scientific information in a neat package for those who want to engage in discussion with people that hold racist ideas (be it knowingly or unknowingly) - exactly as the title proclaims: 'How to Argue With a Racist: What Our Genes Do (or Don’t) Say About Human Difference'.
Not sure why you'd try and bring up suggestive scenarios if you declare yourself ignorant, but ok. 100 generations span some 2000-3000 years. That's a blink of an eye in terms of evolution; giraffes will have had virtually exactly the same neck length 3000 years ago. Selective breeding can, of course, emphasize certain traits, like they do for cattle. But that's not how people reproduce in reality; it's very messy. As it happens, Rutherford, the author of the book, wrote a piece in the Guardian on his book and talked about this. I have quoted the relevant paragraphs below, and bolded the most relevant bit (since you're not actually arguing some kind of pure blood theory).
Link:
https://www.theguardian.com/world/2...tics-bigotry-african-americans-sport-linnaeus