Many paths lay open when the finches first arrived, and the smallest flights and trials of their descendants were rewarded. That is why they have traveled in more directions than any other creatures on the islands, that is why they have evolved farther and faster than any other creatures: because they got here early.
Our own line is now radiating farther, faster, and in more directions than any other single species in the history of the planet - and for a similar reason. We are the first creatures to arrive in the strange territory we now occupy. We stumbled into our new niche before any other creatures on the planet. We discovered it.
You may recognize this from The Beak of the Finch (p. 300). Awards given for science writing don't always reflect how well the writing actually conveys the science. The Pulizter Prize-winning The Beak of the Finch frequently had me cringing, and the passage above is a one example of many where the author demonstrates a shaky grasp of an important concept that the book is supposed to convey.
So why is the passage wrong? It comes down to this: when you talk about radiation in the context of evolution, you're implying divergence. Darwin's finches (the main subject of the book) are a great example. One mainland finch species colonized the Galapagos and evolved into more than a dozen species to fill various specialized ecological niches. That's called adaptive radiation - one species diverges into many different species.
The passage above strongly implies that the same thing is happening with humans. Weiner first discusses the adaptive radiation of the finches, and then says that the human line "is now radiating farther, faster, and in more directions than any other single species in the history of the planet." In terms of evolution, that's simply not true. Because of the widespread mixing between human populations, the human gene pool well-stirred, probably more so than at any other time since humans first left Africa. We aren't radiating in any evolutionary sense, even though we're expanding to fill all sorts of environmental niches.
To compare our species' evolutionary trajectory to the radiation of Darwin's finches is simply wrong. What's even more frustrating is that Weiner made a much better human-finch comparison earlier in the book, when he gave the example of a single finch species occupying many different niches on one island, essentially a 'universalist' species. Finches of this species make their living in many different ways, but they remain a single species because so far there aren't any barriers that keep various subgroups from interbreeding. Weiner gets it right in one part of the book, only to get it wrong in another part.
There were multiple instances of this in the book, one passage contradicting an earlier one, and to me that's a sign of a very shaky grasp of the science. Unfortunately, this phenomenon is not uncommon in award-winning science writing. I think it's no accident that Richard Dawkins selected only pieces written by scientists for The Oxford Book of Modern Science Writing - good science writing should consist of both good writing and good science.
Of course I'm not suggesting that scientists never get the science wrong when they write, nor am I suggesting that non-scientists can never write accurately about science. But, as Feynman pointed out, science requires fitting your creativity to the straightjacket of reality, and a non-scientist writing about an unfamiliar science topic has a lot of homework to do.