the equivalent of changing a propeller plane into a jet
airliner one step at a time, while ensuring that each intermediate
step is not only airworthy but actually flies better than
the previous step (Jacob 1977). Nature cannot ‘go back to
the drawing board’, find some new genetics and have a
break while the new design is tested and perfected.
Everything has to be done on the fly, based on the mutations
that happen to be around at the time and in the full glare of
competition from other animals. Artificial selection has the
advantage that some of these constraints can be relaxed
(Sand๘e 2010).