Looking at complexityLife is full of grand complications, such as aerodynamic wings, multi-part organs like eyes, and intricate chemical pathways. When faced with such complexity, both opponents and proponents of evolution, Darwin included, have asked the question: how could it evolve?
Since many of these complex traits seem to be adaptive, they are likely to have evolved in small steps through natural selection. That is, intermediate forms of the adaptation must have evolved before evolution arrived at a fully-fledged wing, chemical pathway, or eye. But what good is half a wing or only a few of the elements of an eyeball? The intermediate forms of these adaptations may not seem adaptive so how could they be produced by natural selection?
There are several ways such complex novelties may evolve:
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Gull photo courtesy of Dr. Lloyd Glenn Ingles © California Academy of Sciences; Butterfly photo © 2002 John White; Insect microscopy courtesy of Electron Microscope Lab UC Berkeley; Tree frog photo © 2003 Twan Leenders; Flatworm photo courtesy of BioMEDIA ASSOCIATES; Feathered dinosaur photos courtesy of Mick Ellison, AMNH
Understanding Evolution © 2017 by The University of California Museum of Paleontology, Berkeley, and the Regents of the University of California