When Michael McDonald joined The Doobie Brothers in 1975, after the departure of Tom Johnston, it significantly changed their sound. Their more straight forward R&B infused rock style was replaced by a more soul based rock sound. This was in part because of the difference in Johnston’s and McDonald’s vocal and songwriting styles and in part because McDonald played keyboards. The Doobies hardly ever used keyboards on their first five albums.
The Doobie Brothers never intended to use that name permanently. Based on one of their activities in addition to making music, they adopted it after a friend jokingly suggested it. The band thought it was a stupid name, but could never agree on anything else. When you consider that before recording their first album, they once performed under the name Pud, it’s probably a good thing they stuck with The Doobie Brothers.