" Most of us have roughly the same number of offspring. Again, differences between people and the opportunity for natural selection to take advantage of it have diminished. India shows what is happening."
1.What is between?
2.What does "it" refer to?
1) Here, "between" is a simple preposition indicating a relationship involving two or more people or things, in this case their differences.
... the differences between people ..."
2) I believe "it" is incorrect.
The text intends to say:
"Again, differences between people, and the opportunity for natural selection to take advantage of them (the differences), have diminished."
That is, "it" refers to "differences" (plural) and requires "them".
I would be inclined to include the comma after "people".
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