Tennesee's Republican governor said Tuesday he will let a bill become law that protects teachers who allow students in their classrooms to criticize evolution and other scientific theories, such as global warming.

Gov. Bill Haslam had said previously he would probably sign the bill. On Tuesday, he disclosed he would let the law take effect April 20 without his signature, saying he believes the legislation doesn't change scientific standards currently taught in Tennessee's public schools.

Tennessee was the state where the nation's first big legal battle over evolution was fought nearly 90 years ago.

Supporters say the legislation is intended to help students think critically.

Critics derided the legislation as the "monkey bill" for attacking evolution. The state held the famous Scopes "monkey trial" in 1925 in Dayton, Tennessee, and opponents of the legislation say evolution is still under attack in 2012.

School teacher John Scopes was convicted of violating a state statute by teaching evolution in biology class and fined $100. The Tennessee Supreme Court overturned it on a technicality a year later. In 1967, Tennessee's anti-evolution law was revoked.

Haslam explained Tuesday why he was letting the bill become law without his signature. He said he doesn't believe the legislation changes scientific standards currently taught in Tennessee nor does it accomplish anything that isn't already acceptable in schools.

"The bill received strong bipartisan support, passing the House and Senate ... but good legislation should bring clarity and not confusion," Haslam noted in a statement. "My concern is that this bill has not met this objective."

Last week, the governor was handed a petition with more than 3,000 signatures urging him to veto the legislation. Some contend the measure could open the door for religious teaching in the classroom.

Meanwhile, backers said it would encourage critical thinking by protecting teachers from discipline if they help students critique "scientific weaknesses."

Scientists in Tennessee and the American Association for the Advancement of Science say evolution is established science that shouldn't be taught as a controversy.

Hedy Weinberg, executive director of the American Civil Liberties Union of Tennessee, said there's no question the incoming law "undermines science education in Tennessee public schools."

The ACLU chapter had asked the governor to veto it.

She said terms such as "strengths and weaknesses," and even "critical thinking," are used by those seeking to introduce non-scientific ideas _ such as creationism and intelligent design _ into the science curriculum.

She added that she thought the measure could eventually be challenged in court and observers would be watching what goes on in science classrooms after it takes effect.

Senate sponsor Bo Watson acknowledged creationism and intelligent design are not part of the state's curriculum, but he said questions about them create a "teachable moment."

"If a student asks a question about it, the teacher should feel comfortable in using that ... to say here's the difference between science and creationism, the difference between evolution and creationism," Watson said. "And here's why evolution is science's best explanation and creationism is not."

Tennessee Education Association lobbyist Jerry Winters said the soon-to-be law sends a bad signal.

"Tennessee has a long history when you talk about the problems of teaching evolution," he said. "And with all the emphasis on science, technology and engineering and math, it just seems to be moving totally in the wrong direction."