Bryan Garner writes the following under the word “if” in his Usage Tip of the Day (which comes to me via e-mail every weekday):

Part B: “If, and only if.” This adds nothing but unnecessary emphasis (and perhaps a rhetorical flourish) to “only if.” E.g.: “Such a ‘homocentrist’ position takes the human species to define the boundaries of the moral community: you are morally considerable if, and only if, [read 'only if'] you are a member of the human species.” Colin McGinn, “Beyond Prejudice,” New Republic, 8 Apr. 1996, at 39. The variation “if, but only if,” which sometimes occurs in legal writing, is unnecessary and even nonsensical for “only if.”

Garner is mistaken. “A if B” says that B is a sufficient condition for A and that A is a necessary condition for B. “A only if B” says that A is a sufficient condition for B and that B is a necessary condition for A. “A if and only if B” says both of these things: that A is necessary and sufficient for B. Colin McGinn (a philosopher) is saying that, according to homocentrism (a.k.a. anthropocentrism), membership in the human species is both necessary and sufficient for being morally considerable. In other words, all and only humans have moral status. On Garner’s reading, homocentrism says exactly half of this: that only humans have moral status. The difference between “if and only if” and “only if” is substantive; it is not merely a matter of emphasis or rhetoric.