Microevolution (for example, the development of insecticide resistance)
merely selects preexisting variation. It does not demonstrate that
mutations create new variation.
In experiments with bacteria, variation (including beneficial
mutations) arises in populations that are grown from a single
individual (Lederberg and Lederberg 1952). Since the population started
with just one chromosome, there was no variation in the original
population; all variation must have come from mutations.
Furthermore, disease organisms and insect pests have developed
resistance to a variety of antibiotics and pesticides, many of them
artificial and unlike anything in nature. It is highly improbable that
all insects were created with resistance to all pesticides.
Mutation is the only natural process that adds variation to
populations. Selection and genetic drift remove variation. If
mutations did not create new variation, there would now be little or no
variation to select from. In particular, reducing populations to a
single pair of individuals, as Noah's Flood requires, would have
removed very nearly all variation from the world's wildlife in one
stroke.
It is true that much microevolution selects from preexisting variation.
In animals, that kind of microevolution occurs much faster than waiting
for certain mutations to occur, so we often see artificial selection
programs stall when they have selected among all the variation that was
there to begin with. However, if the selection is maintained, change
should continue, albeit at a much slower rate.
References:
Lederberg, J. and E. M. Lederberg, 1952. Replica plating and indirect
selection of bacterial mutants. Journal of Bacteriology 63: 399-406.
Further Reading:
True, Heather L. and Susan L. Lindquist, 2000. A yeast
prion provides a mechanism for genetic variation and phenotypic
diversity. Nature 407: 477-483. (technical)
