Sometimes you need to draw a distinction between “the changing of the guard” and what might be called “the end of the guard.”

That’s the case with the announcement that David Yepsen, the political columnist for the Des Moines Register, will leave the paper in April to become director of the Paul Simon Public Policy Institute at Southern Illinois University in Carbondale.

Yepsen has been a fixture on the Iowa and national political scenes as a guide to the Iowa caucuses and the anointing of presidential candidates. Candidates, would-be candidates and fellow journalists have valued his word, advice and insights. David began his stint as the paper’s signature political columnist in 1983. He waxed forth in his Register column and on television talk shows in the endless campaigns, usually beginning no later than two or three years before Iowans gathered at Democratic and Republican Party caucuses in quadrennial Januarys of 1984 through 2008.

Commentaries and accolades to Yepsen have speculated on who will replace him.

Here’s the answer: No one.

That’s because he’s the end of the bloodline and because of changes at his paper in particular and the newspaper industry in general.

Consider this: In the past 90 years or so, the Des Moines Register has had but four columnists in the Yepsen role — only one more than the number of editorial cartoonists the paper has had. C.C. (Cy) Clifton and George Mills overlapped a bit in the 1940s and1950s, filling the political columnist role from 1923 to 1971. James Flansburg took over the job in ’71. Yepsen succeeded him when Jim became editorial page editor in’83.

Of the Register political writers and their several counterparts on the Des Moines Tribune,— also a Cowles family publication until it was killed in 1982, Yepsen is the only one to leave the post by a route other than retirement or taking on other duties within the company. (Gannett bought out the Cowles family in1985.)

People with institutional memory talk about how each of the writers — except maybe for Clifton — served an apprenticeship before taking on the political column. Each columnist had an encyclopedic knowledge of the state and its politics. The emphasis and influence of the political columnist changed a bit from focusing on the governor and the legislature to dealing with caucuses, beginning in the 1970s. But the wealth of knowledge was a constant.

No more. The impact of the columnist will go the way of the statewide newspaper and the front-page editorial cartoons. (Brian Duffy, the heir of the cartoonist tradition of Jay “Ding” Darling and Frank Miller, was given his walking papers in December as part of Register cutbacks.) The paper has good reporters, of course, but no one was an apprentice to Yepsen. He’s gone, and so is the role played by him, Clifton, Mills and Flansburg.

Because caucus commentary, like nature, abhors a vacuum, the national TV panels will find an Iowa replacement for Yepsen — probably O.K. (Kay) Henderson, a radio broadcaster, or Mike Glover of the state Associated Press. They’ve been longtime companions of David on the caucus front and on ”Iowa Press,” a program of Iowa Public Television.

But the guard is not changing; it’s ending.