Question: Who is the lead singer of 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 is the first lead singer of doobie brothers?
short_answers: Johnston

Tom Johnston

Charles Thomas Johnston
wikipage: The Doobie Brothers

context: In December 1973, the British music magazine "NME" reported the relatively trivial news that Johnston had been arrested in California on a charge of marijuana possession. More seriously however, following years of a road touring lifestyle and health issues surrounding stomach ulcers which stood as a challenge since high school, Johnston became severely ill on the eve of a major tour beginning in Memphis, Tennessee in 1975 to promote "Stampede". Johnston's condition was so precarious that he required emergency hospitalization for a bleeding ulcer. With Johnston convalescing and the tour already underway, fellow Doobie Brother Jeff Baxter proposed recruiting a fellow Steely Dan alum to fill the hole. This led to the emergency hiring of Michael McDonald, who became the lead singer of the band. Restored to fitness in 1976 and briefly back in the band, Johnston contributed one original song to "Takin' It to the Streets" ("Turn It Loose"), and also added a vocal cameo to Pat Simmons' tune "Wheels of Fortune". He also made live appearances with the band in 1976 (appearing in a concert filmed that year at the Winterland in San Francisco, excerpts from which appear occasionally on VH1 Classic), but was sidelined once again in the fall due to exhaustion. None of Johnston's songs appeared on "Livin' on the Fault Line", though he had written and the band had recorded five of his compositions for the album. Finally, before "Fault Line" was released, Johnston had his songs removed and left the band that he co-founded (though he received credit for guitars and vocals and was pictured on the album's inner sleeve band photo). After a few years of restored health but growing differences in musical direction between band members, Johnston finally left the band in 1977 to pursue a solo career that produced two albums with Warner Bros: "Everything You've Heard Is True" and "Still Feels Good" (reissued on compact disc by Wounded Bird Records), and Billboard Hot 100 hit "Savannah Nights" (#34 Top 100 Billboard Hit −1980).
question: Who is the second lead singer of doobie brothers?
short_answers: Michael McDonald
wikipage: Tom Johnston (musician)

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 is the third lead singer of doobie brothers?
short_answers: Johnston

Tom Johnston

Charles Thomas Johnston
wikipage: The Doobie Brothers

Long 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 Dobbie Brothers band had different lead singer through out the years.  Charles Thomas Johnston was their first and third lead singer while Michael McDonald was their second.