In Your Loss is Your Gain, Carl Zimmer makes the point that evolution isn’t just about new genes you gain:
Evolution isn’t simply about the genes you gain. It’s also about the genes you lose.The word loss has a painful, grieving sound to human ears, and so it can be hard to see how it can have anything to do with the rise of diversity and complexity in life. And until recently, evolutionary biologists didn’t pay much attention to lost genes because they were preoccupied with the emergence of new ones.
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Some new research suggests that much of the evidence for rampant gene trading may actually be an illusion created by lost genes.