Most marine species appear abruptly in the fossil record and persist unchanged for millions of years. Speciation and extinction commonly occur in pulses so that groups of species come and go as ecological units that dominate the seascape for millions of years. Dramatic turnover of mollusc, coral and planktonic foraminifera species occurred throughout tropical America about two million years ago in apparent response to the onset of northern hemisphere glaciation. In contrast, subsequent glacial cycles, temperature fluctuations and sea-level change had little lasting biological effect. There is no necessary correlation between the magnitude of environmental change and the subsequent ecological and evolutionary response.