Many organisms show features of appallingly bad design. This is
because evolution via natural selection cannot construct traits
from scratch; new traits must be modifications of previously
existing traits. This is called historical constraint. A few
examples of bad design imposed by historical constraint:
In parthenogenetic lizards of the genus _Cnenidophorus_, only females
exist. Fertility in these lizards is increased when another lizard
engages in pseudomale behaviour and attempts to copulate with the
first lizard. These lizards evolved from a sexual species so this
behaviour makes some sense. The hormones for reproduction were likely
originally stimulated by sexual behaviour. Now, although they are
parthenogenetic, simulated sexual behaviour increases fertility. Fake
sex in a parthenogenetic species doesn't sound like good design to me.
In African locust, the nerve cells that connect to the wings originate
in the abdomen, even though the wings are in the thorax. This strange
"wiring" is the result of the abdomen nerves being co-opted for use in
flight. A good designer would not have places flight nerves travel
down the spinal cord past their target, then backtrack through the
organism to where they are needed. That's not good design.
In human males, the urethra passes right next to the prostate gland, a
gland very prone to infection and subsequent enlargement. This blocks
the urethra and is a very common medical problem in males. Putting a
collapsible tube next to an organ that is very likely to expand and
block flow in this tube is not good design. Any moron with half a
brain (or less) could design male "plumbing" better.
Perhaps one of the most famous examples of how evolution does not
produced designed, but "jury-rigged" traits is the panda's thumb. If
you count the digits on a panda's paw you will count six. Five curl
around and the "thumb" is an opposable digit. The five fingers are
made of the same bones our (humans and other most other vertabrates)
fingers are made of. The thumb is constructed by enlarging a few bones
that form the wrist in other species. The muscles that operate it are
"rerouted" muscles present in the hand of vertabrates (see S.J. Gould
book "The Panda's Thumb" for an engaging discussion of this case).
Again, this is not good design.
In addition, organisms betray their history (phylogeny) in their
development (ontogeny). All mammals (including humans) go through a
stage were gills begin to form. Many organisms contain useless
structures, rudiments of traits or organs their species contained in
the past. The rudiments, or vestigal structures, are not evidence of
good design, but of "jury-rigged" design. Likewise, developmental
patterns that form traits an organism will never use are not evidence
of good design. Both point to the fact that present day organisms are
the result of millions of years of evolutionary tinkering, not creation
via fiat of an intelligent designer.