What did the author mean when noting that Galapagos cormorants split off from other cormorants

The author takes note of the fact that the cormorants of the Galapagos islands are the only species of all the 40 species of the world to have lost the ability to fly. Geneticists have identified the gene mutation that is responsible for this divergence. It is also postulated that the cormorants of the Galapagos may have lost flight due to abundant prey on the island, and lack of predators, hence giving less significance to flight.