(Chip Somodevilla/Getty Images)

That shouldn’t be a surprise. For years, our increasingly polarized political system has been culling moderate members. To see this in video form, click the clip below. It’s a visualization of congressional polarization beginning with the very first Congress and running right through today. And if you fast forward toward the end, where we are, you’ll see the pattern progress right in front of you: The Ds and the Rs in the middle — those are the centrists — keep disappearing, and the masses of Ds and Rs keep moving farther away from each other.

That’s American politics today: two parties, no touching. And that’s part of why Snowe is retiring. “[W]hat I have had to consider is how productive an additional term would be,” Snowe said in a statement. “Unfortunately, I do not realistically expect the partisanship of recent years in the Senate to change over the short term.”

She’s right. The partisanship of recent years is not likely to change over the short term. In fact, it’s likely to get worse. Snowe will probably be replaced by a liberal Democrat. Nelson will probably hand his seat to a conservative Republican. The parties will become that much more unified, disciplined and polarized. And that’s okay. Or, at least, it would be okay if America’s political institutions were prepared for it.

We use “polarization” as an epithet. It’s what’s wrong with America’s politics. It’s what’s wrong with America’s political parties. It’s what’s wrong with America’s politicians. It’s what’s wrong, finally, with America.

And polarization is certainly bad for moderate legislators who want to wield influence by brokering deals between the two parties. But for the political system as a whole, “polarization” is a neutral term. It simply means the two parties disagree, and clearly. It doesn’t mean they disagree angrily or unproductively or in service of extreme ideologies.

To imagine this, consider two political systems. In one, the two parties aren’t polarized, because the Democratic Party is filled with conservative arch-segregationists. In another, the parties are very polarized, but it’s because everyone agrees segregation was a moral blight, and with that out of the way, the conservative Democrats who kept their seats by appealing to racism were replaced by Republicans. Which system is more extreme? Or unproductive? Or hateful?

Polarization doesn’t describe people’s opinions. It just describes how those same people, with those same opinions, sort themselves. For political scientists, it was long a puzzle and a frustration that the Democratic Party contained so many conservatives and the Republican Party so many liberals. But race was the reason for much of that, and as race has receded as a driving force in American politics, the two parties have sorted themselves in a more sensible way. The problem is, the political system hasn’t responded.

Our system, as any historian will tell you, was built by men who hated parties and anticipated their absence from American politics. That didn’t quite work out. But for much of American history, and particularly for much of the 20th century, our political parties have been unusually diffuse and unable to act as organized, ideological units. That left them well-suited to a system that, for reasons ranging from the division of powers to the filibuster, required an unusual level of consensus to function.

But as the two parties have polarized, we’ve learned that a system built for consensus is not able to properly function amid constant partisan competition. The filibuster has gone from a rarity to a constant. Compromise has become rare. Crises of gridlock, such as the recent showdown over the debt ceiling, have become common. And no one can say that this is what the American people want: The approval ratings of Congress have been on a downward slide for decades, and they have never been lower than they are today.

Snowe’s retirement will have many lamenting the endangered moderate and wondering how we can turn back the clock. But we can’t. About that, Snowe is right. Polarization is with us now and will be with us for the foreseeable future. The question is whether we will permit it to paralyze our political system and undermine our country or whether we will accept it and make the necessary accommodations.

Doing so would require taking on cherished, consensus-promoting features of the old system, like the filibuster. But in today’s girdlocked world, those features no longer promote consensus. They simply promote gridlock.