According to Darwin and Mayr, evolutionary innovations arise from the functional displacement of existing organs that can simultaneously perform two functions but not by selectively favouring a new organ that is not yet functional.
According to Charles Darwin’s theory, evolutionary changes take place through very small steps.
According to the model of gradualism in geology, in which mountains that are several kilometres high unfold a millimetre at a time and the Atlantic Ocean, hundreds of kilometres wide today, only opens up a few centimetres each year as a result of shifting plates, the evolution of living beings conceivably takes place in small steps from one generation to the next.
When changing size, colour and other characteristics that permit a smooth transition, the idea of gradual change does not present any difficulties. But how does it look when the fish suddenly becomes a land animal? When the legs suddenly begin to move and the lungs suddenly begin to breathe, and the skin does not dry out?
When a small predatory dinosaur becomes a bird – where does it suddenly have feathered wings to rise into the air?
How does a mammal give live birth to its young, as opposed to laying eggs? All this seemed to contradict gradualism and puzzled the defenders of evolutionary theory. Ernst Mayr writes:
One of the most common objections to Darwin’s gradualism was that gradualism was incapable of explaining the emergence of “evolutionary innovations” i.e. completely new organs, new structures, new physiological abilities and new patterns of behaviour.
For example, how can a rudimentary wing be enlarged by natural selection before it gives its owner the ability to fly? How can an incipient organ be favoured by selection if it is not fully functional?
Darwin (…) gave the answer to this question by pointing out that the key element in the solution to this problem is a change in the function of a structure. His solution was generally unknown until Dohrn (…), Sevetsov, (…), and Mayr (…) developed it further.
(Mayr 2002, p. 491)