Editing Exaptation (section)

===Evolution of complex traits===
One of the challenges to Darwin's theory of evolution was explaining how complex structures could evolve gradually,<ref>The development of complex structures (i.e., evolution of novelties) occur either by intensification of an existing function or by a switch in functions.</ref> given that their incipient forms may have been inadequate to serve any function. As [[George Jackson Mivart]] (a critic of Darwin) pointed out, 5 percent of a bird wing would not be functional. The incipient form of complex traits would not have survived long enough to evolve to a useful form.

As Darwin elaborated in the last edition of ''[[On the Origin of Species|The Origin of Species]]'',<ref>{{harvnb|Darwin|1872}}</ref> many complex traits evolved from earlier traits that had served different functions. By trapping air, primitive wings would have enabled birds to efficiently regulate their temperature, in part, by lifting up their feathers when too warm. Individual animals with more of this functionality would more successfully survive and reproduce, resulting in the proliferation and intensification of the trait.

Eventually, feathers became sufficiently large to enable some individuals to glide. These individuals would in turn more successfully survive and reproduce, resulting in the spread of this trait because it served a second and still more beneficial function: that of locomotion. Hence, the evolution of bird wings can be explained by a shifting in function from the regulation of temperature to flight.