George Ledyard Stebbins , Jr ( 6 January 1906 â `` 19 January 2000 ) was an American botanist and geneticist who was one of the leading evolutionary biologists of the 20th century . George Ledyard Stebbins , Jr. ( January 6 , 1906 â `` January 19 , 2000 ) was an American botanist and geneticist who is widely regarded as one of the leading evolutionary biologists of the 20th century .