Updated

Tennessee Gov. Bill Haslam said Tuesday a new law to protect teachers who allow students to criticize evolution and other scientific theories will go into effect without his signature, because "good legislation should bring clarity and not confusion."

The Republican governor declined to veto the bill, because he doesn't believe the legislation changes scientific standards currently taught in Tennessee schools, nor does it accomplish anything that isn't already acceptable in schools. It takes effect April 20.

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

Haslam previously said he would probably sign the bill. Last week, he was handed a petition with more than 3,000 signatures urging him to veto the legislation, which encourages 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 she respects the governor for not signing the bill, but said there's no question it "undermines science education in Tennessee public schools."

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

"The new law is effectively a permission slip for teachers to violate the First Amendment by allowing them to dress up their religious beliefs on the origin of life as pseudo-science," Weinberg said.

Nevertheless, she believes the measure can be challenged in court. She said individuals from across the state have contacted the ACLU about the legislation.

"And they will be our ears and eyes in the science classroom," Weinberg said. "And with the right set of facts, we will move forward."

With or without the governor's signature, House sponsor Bill Dunn said he's just pleased the bill will soon become law.

"I think the governor looks at it and realizes that it goes right along with what we're trying to do in our classrooms, which is just to produce students who can think critically," said the Knoxville Republican. "And so I'm glad it's going to become law."

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

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

Tennessee Education Association lobbyist Jerry Winters said the soon-to-be-law "sends a very bad signal about this state."

"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 and I think it makes the state look bad."

The legislation is the first Haslam has let become law without his signature.