The show originally was to have completely different cast line up. Before The Partridge there was a real famliy band called The Cowsills which The Partridge Family series was based on. They did "The Rain, Park, and other things" { aka I love the flower girl}, Indian Lake, and Hair. The deal breaker was they wanted their real mother in the series and IIRC Shirley Jones had already been casted.
The Partridge Family was a studio musician group with Cassidy and Jones singing. None or the kids in the show were musicians and I read a long time ago Susan Dey was tone deaf.
Look on youtube for The Cowsills singing "I think I love you". There was one song both The Cowsills and The Partridge Family released but I can't remember which one it was. The Cowsills are still an active group minus deseased mother and death of two of the brothers.
Thanks for the backstory on how The Partridge Family came to be. I had heard before about the show being based on the real life Cowsills but didn't realize they were actually planning to cast some of them. Makes perfect sense that a studio band backed Shirley Jones and David Cassidy for the recordings. It was obvious by watching the show that the rest of them had zero musical ability.
Cowsills are an overlooked band in my opinion. Many of their songs still hold up well today.