The board of trustees of the Rockefeller Foundation yesterday elected Dr. John R. Evans, a prominent public health specialist in Canada, its ninth chairman. Dr. Evans, a former president of the University of Toronto, succeeds Dr. Clifton R. Wharton Jr., former Chancellor of the State University of New York, who is leaving the board after 17 years as a trustee.

Three new trustees also were elected to the board: Frances Fitzgerald, the Pulitzer Prize-winning author; Alice Stone Ilchman, president of Sarah Lawrence College, and Arthur Levitt Jr., chief executive of the American Stock Exchange.

Dr. Evans heads Allelix Inc., a leading Canadian biotechnology firm and developer of agricultural, health and industrial products. He also was a director of the World Bank in Washington, heading the Population, Health and Nutrition Department for four years. Trained as a researcher in internal medicine and cardiology, he also is on the faculty of the University of Toronto.

The 75-year-old foundation, one of the country's wealthiest private philanthropies, with assets of $1.8 billion, is beginning an ambitious, new program to help third world countries, Dr. Evans said.