There will be a big change at one network’s evening news desk starting next year.

ABC News announced today that Charles Gibson, who has anchored the news channel’s “World News” nightly newscast since May 2006, will be stepping down from his post at the end of the year and retiring from full-time work. Diane Sawyer, who currently co-anchors “Good Morning America,” will step into Gibson’s seat, making her the nation’s second female nightly news anchor along with Katie Couric.

As the story developed this morning, our colleagues at FishbowlDC obtained an email sent by Gibson today. He explained his decision to leave in part:

“It had been my intention to step down from my job at ‘Good Morning America’ in 2007 but with Peter’s illness, Bob’s injuries, and Elizabeth’s pregnancy, the job at ‘World News’ came open in May of 2006, and [ABC News President David Westin] asked me to step in as anchor. It was an honor to do so. The program is now operating at a very accelerated, but steady, cruising speed, and I think it is an opportune time for a transition — both for the broadcast and for me. Life is dynamic; it is not static.”

As Gibson prepares to depart, TVNewser took a look at the ratings since he took over the anchor desk. During his run, Gibson managed to keep his NBC News rival Brian Williams on his toes. With Gibson in the anchor chair, “World News” became the number 1 evening news broadcast by March 2007, although “NBC Nightly News” reclaimed its title in 2008.