A morphic field was an energy-like field that defined the entire biology of all living beings. The idea was first put forth by a scientist named Sheldrake in the 20th century. It provided a force that guided the development of an organism as it grew, making it take on a form similar to other species. DNA was not the source of structure, but rather a "receiver" that translated instructions in the field into physical form.
Morphic fields were reinforced by successive generation, explaining why there were so many humanoid races in the universe. The Gallifreyans were the first sentient life to evolve in the universe and so created a morphic field for humanoids. That made it more probable the next race to evolve would be humanoid, which made it much more likely that the following race would be humanoid and so on. The majority of the non-humanoid races developed in environments where two legs, two arms and a head would have been a positive handicap.
The Blessing was connected to the morphic field of humanity, particularly around Shanghai and Buenos Aires. In those cities, the people around the Blessing always had the same lifespan as the world average. In 2011, the Three Families changed all humanity by inserting Jack Harkness' immortal blood at both ends, disrupting the morphic field, turning everyone immortal and starting Miracle Day. (TV: The Gathering, The Blood Line) Jack Harkness guessed from the start a disturbance in the morphic field was responsible for Miracle Day. (TV: Rendition) Jack, Rex Matheson and Gwen Cooper were able to reverse the change and Miracle Day by inserting his now-mortal blood into the Blessing at each end. (TV: The Blood Line)
The Lankin, in the form of Orla'ath, told Andrea Quill that her appearing in a more human skin than the true shape of the Quill had something to do with Earth's morphic fields, being due to "some morphic resonance thing I don't understand" and explaining that "one has to make allowances for other planets." (TV: Nightvisiting)
The Tonska were researching a temporal rift, trying to find a way to allow the Yed-Prior Zaniah to be immortal. When Menkib was experimentally destroyed, they were unable to recreate his morphic field and he was turned into a Cerunnos. (PROSE: ...And Eternity in an Hour)