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Rather quietly, CBS’ “The Big Bang Theory” has undergone a transformation that could affect its long-term future much more than last fall’s headline-grabbing shift to a Thursday timeslot.

When the show began, the cast featured four guys and one gal, and the gal was a character whose airheadedness one critic said made Suzanne Somers’ Chrissy Snow on “Three’s Company” look like the president of Mensa.

Now, not only has Kaley Cuoco’s Penny rounded nicely into a three-dimensional person who is thoughtful (if on her own terms), but three other women have taken on prominent roles on the series. Melissa Rauch and Mayim Bialik are now series regulars, and of late, Aarti Mann (as new girlfriend of Johnny Galecki’s Leonard) has made a great impression and doesn’t seem to be going away soon. The producers aren’t shy about devoting significant story time to the new actresses, a possibility that wouldn’t have even occured to most viewers a couple of years ago.

“Big Bang” is still told mainly from the guys’ point of view, principally that of Leonard and Sheldon (Jim Parsons), but the female presence has definitely given the show an extra gear as it heads toward the end of its fourth season. There was a time not long ago when “Bang” looked like it might become mainly a showcase for the immensely talented Parsons, but now its ensemble seems stronger than ever. All the more reason to look forward to future seasons …