Question: Who's the lead singer of the doobie brothers?

Disambiguated questions and answers:
context: At some point, drummer John Hartman arrived in California determined to meet Skip Spence of Moby Grape and join an aborted Grape reunion. Spence introduced Hartman to singer, guitarist, and songwriter Tom Johnston and the two proceeded to form the nucleus of what would become the Doobie Brothers. Johnston and Hartman called their fledgling group "Pud" and experimented with lineups (occasionally including Spence) and styles as they performed in and around San Jose. They were mostly a power trio (along with bassist Greg Murphy) but briefly worked with a horn section.
question: Who's the original lead singer of the doobie brothers?
short_answers: Tom Johnston
wikipage: The Doobie Brothers
context: The band's history can be roughly divided into three eras. From 1970 to 1975 it featured lead vocalist Johnston and a mainstream rock and roll sound with elements of folk, country and R&B. Johnston quit the group in 1975, and was replaced by Michael McDonald, whose interest in soul music changed the band's sound until it broke up in 1982. In 1987, the Doobie Brothers reformed with Johnston back in the fold; McDonald, who had previously made several guest appearances since their reformation, returned to the band full-time in 2019 for their upcoming 50th anniversary tour. Every incarnation of the group has emphasized vocal harmonies. The Doobie Brothers were inducted into the Vocal Group Hall of Fame in 2004, and will be inducted into the Rock and Roll Hall of Fame on May 2, 2020. The group has sold more than 40 million albums worldwide.
question: Who replaced the original lead singer of the doobie brothers?
short_answers: Michael McDonald
wikipage: The Doobie Brothers
context: At some point, drummer John Hartman arrived in California determined to meet Skip Spence of Moby Grape and join an aborted Grape reunion. Spence introduced Hartman to singer, guitarist, and songwriter Tom Johnston and the two proceeded to form the nucleus of what would become the Doobie Brothers. Johnston and Hartman called their fledgling group "Pud" and experimented with lineups (occasionally including Spence) and styles as they performed in and around San Jose. They were mostly a power trio (along with bassist Greg Murphy) but briefly worked with a horn section.
question: Who became the lead singer of the doobie brothers again in 1987?
short_answers: Tom Johnston
wikipage: The Doobie Brothers

Long-form answers:
knowledge: content: The Doobie Brothers are an American rock band from San Jose, California, known for their flexibility in performing across numerous genres and their vocal harmonies.
wikipage: The Doobie Brothers
long_answer: The Doobie Brothers are an American rock band from San Jose, California, known for their flexibility in performing across numerous genres and their vocal harmonies. At some point, drummer John Hartman arrived in California determined to meet Skip Spence of Moby Grape and join an aborted Grape reunion. Spence introduced Hartman to singer, guitarist, and songwriter Tom Johnston and the two proceeded to form the nucleus of what would become the Doobie Brothers. Johnston quit the group in 1975, and was replaced by Michael McDonald.