* Two of three network evening news anchors now women

* Sawyer seeks to avoid the ratings slump of Katie Couric

NEW YORK, Dec 11 - Diane Sawyer is changing the landscape of American broadcast television news and is likely hoping history won't repeat itself when she becomes the second woman to solo anchor a major evening news program.

After more than 10 years co-anchoring Good Morning America, Sawyer left on Friday to become anchor of ABC's World News on December 21, replacing Charles Gibson who is retiring.

In the hotly contested time slot, she will go up against CBS Evening News with Katie Couric and Brian Williams at NBC Nightly News, who becomes the lone male anchor.

Skeptics wondered if Couric's history on a morning talk show and her bubbly personality would be a hindrance as she ventured into hard news. She initially received good ratings but the program has since been consistently marred by low ratings.

Sawyer, a pioneer among women in U.S. broadcast television news with a history of serious reporting, will also be scrutinized but not like Couric who became the first solo female anchor of a traditional news program at CBS in 2006.

Her network did announce that they would change the face of evening news. That was a burden that Katie Couric had to bear and that is not happening with the case of Diane Sawyer, said Martin Kaplan, director of the Norman Lear Center and journalism professor at the University of Southern California.

Sawyer's appointment may be seen as less of a gamble than Couric's, but any change is risky with the millions of dollars in advertising revenue at stake.

ABC is owned by Walt Disney Co, NBC Universal is being taken over by Comcast Corp and CBS is part of CBS Corp.

More people go to the bathroom during the commercial break on network evening news than are watching cable news. Compared to other niche audiences, the evening news packs a wallop, Kaplan said.

The evening news show broadcast by U.S. television networks traditionally formed a cultural point of reference in the United States, with the anchor becoming known as the Voice of God.

The late anchor Walter Cronkite of CBS was repeatedly voted the most trusted man in American and ruled the airways from 1969 to 1981. After he left, a three-way competition lasted for years between Dan Rather of CBS, Tom Brokaw of NBC and Peter Jennings of ABC.

Then cable television and the Internet lured viewers away from the traditional networks. The audience for evening news has dropped significantly with viewership down from more than 52 million in 1980 to around 25 million in 2008, according to Stateofthemedia.org.

But evening news programs remain profitable. In 2008, NBC Nightly News generated $146 million in advertising revenue, ABC World News had $159 million in ad revenue, and CBS Evening News recorded $166 million, according to data from TNS Media Intelligence.

Williams, the lone male left among network evening news anchors, welcomes the buzz created by Sawyer's arrival and disputes the perception that network evening news is dying.

While perhaps inconvenient, we have reversed some of the newscast obituaries that have easily been floating around for two decades, Williams told Reuters.

Diane is not going to play any woman card, or Katie. As I won't play the male card. We are all reporting the news of the day. What we do know is there is enough audience out there to go around. (Reporting by Bernard Orr; Editing by Daniel Trotta and Patricia Reaney)

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