The Living World 6th Edition by George Johnson, Jonathan Losos,William Ober,Claire Garrison

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What Causes New Forms to Arise
Biologists once presumed that new forms -genera, families, and orders-arose most often during times of massive geological disturbance, stimulated by the resulting environmental changes. But no such relationship exists. An alternative hypothesis was proposed by evolutionist George Simpson in 1953. He proposed that diversification followed new evolutionary innovations, "inventions" that permitted an organism to occupy a new "adaptive zone." After a burst of new orders that define the major groups, subsequent specialization would lead to new genera.
The early bony fishes, typified by the sturgeon (see lower right ), had feeble jaws and long shark-like tails. They dominated the Devonian (the Age of Fishes), to be replaced in the Triassic (the period when dinosaurs appeared) by fishes like the gar pike with a shorter more powerful jaw that improved feeding and a shortened more maneuverable tail that improved locomotion. They were in turn replaced by teleost fishes like the perch, with an even better tail for fast, maneuverable swimming, and a complex mouth with a mobile upper jaw that slides forward as the mouth opens.
This history allows a clear test of Simpson's hypothesis. Was the appearance of these three orders followed by a burst of evolution as Simpson predicts, the new innovations in feeding and locomotion opening wide the door of opportunity If so, many new genera should be seen in the fossil record soon after the appearance of each new order. If not, the pattern of when new genera appear should not track the appearance of new orders.
The graph shows the evolutionary history of the class Osteichthyes, the bony fishes, since they first appeared in the Silurian some 420 million years ago.




Making Inferences Do bursts of new genera appear at these same three times, or later