If natural selection is indeed the cause of the diversity of life that we see, then what evidence should we find that supports this?
Primarily we should see “order”, but imperfect order. Natural selection basically cobbles together various random genetic changes occurring in many different individuals and turns them into something that makes the population better adapted to the environment. Because natural selection has a limited number of genetic characteristics to work with, we should see “Rube Goldberg” or “jury-rigged” “designs”.
What do we actually see?
We see “Rube Goldberg” or “jury-rigged” designs!
As one well-known example, there are many flightless birds. There are about 40 known species of such birds. For most flightless birds, the wings are completely non-functional, except for the occasional display functions. In some cases, the wings are very small, as they are in kiwis. The practical effect is that the number of usable limbs is reduced from 4 to 2.
The question might be asked: why would evolution eliminate wings? After all wings increase an animal’s range, increase its ability to act as a predator and to avoid predation. Moreover, frankly, it would be a lot of fun to be able to fly.
The “fun” part isn’t relevant to evolution, but what is often overlooked is that having wings is actually a bit hazardous. The odds that a bird will actually fall out of the sky aren’t very great, but when a bird can fly it becomes very dependent on the weather, particularly the strength of the wind. On islands – particularly small islands – a strong wind can push a bird out to sea where it eventually can very easily perish from fatigue. The benefits of being able to fly in order to evade a predator are great, but if there are no large predators around, staying on the ground is a much safer strategy in the long run.
As it happens, we see flightless birds under exactly these conditions. “New Zealand has more species of flightless birds (including the kiwis, several species of penguins, and the takahe) than any other country. One reason is that until the arrival of humans roughly a thousand years ago, there were no large land predators in New Zealand; the main predators of flightless birds were larger birds”. Flying serves no advantage with no such predators around.
Most people are also aware that blind cave fish (and cave salamanders) exist. These creatures are similar to flightless birds in that they have eyes which serve no function. The blind salamanders have eyes with retinas and lenses, yet the eyelids grow over the eye, sealing them from outside light. These animals live in complete darkness so functioning eyes would not provide any benefits. But functioning eyes would take resources from the rest of the body (blood, for example). Therefore, in the complete absence of light, eyes are really nothing but a liability.
Another fairly well-known example of a “flawed design” is the Panda's thumb. Dr. Stephen J. Gould wrote a book titled “The Panda’s Thumb” that goes into great detail about this appendage. “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 most other vertebrates) 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 vertebrates”. This is not good design. If a Panda could make good use of an opposable thumb, then why not just give it an opposable thumb? Something like the thumb “design” in humans should work quite well.
Humans have many “jury-rigged” designs.
The retina in our eyes (a “design” that we share with all vertebrates) is inverted. In our eyes, the sensory cells lie beneath the nerve fibers; light must pass through the nerve fibers in order to reach the light sensors. In order to get the signal from the sensors to the brain, the nerve fibers must pass through the sensors leaving a “blind spot”. Octopus eyes, in contrast, have the sensory cells above their nerve fibers. As a result, their eyes have no such “blind spot”.
Some creationists argue that this is not actually a bad design. To support that claim, they make two arguments.
First, they say that the photoreceptors are embedded in a layer of retinal pigment epithelial cells which perform important functions. Those cells do perform important functions. But that functionality would not be lost if those receptors were embedded in those cells yet on top of the nerve fibers. Our smell (olfactory) system has very similar components yet the receptors in our noses are not beneath the nerves.
Next, creationists insist that the light receptors have a high metabolic rate that requires a good circulatory system. That’s also true but really irrelevant. There is no reason that such a circulatory system could not exist if the light receptors were on top. More relevantly, the capillaries that provide this blood are also in the light path! This design, which to emphasize again is not present in octopi, diminishes the effectiveness of the photo sensors by blocking a portion of the light! Clearly this is a poor “design”.
Another example of “bad” designs is found in male humans. In men, the urethra passes through the prostate gland. When the prostate gets infected and swells, as it is prone to do, it causes problems not only with reproduction but also with the excretory system. Putting a collapsible tube through an organ that is very likely to expand and block flow in this tube is not good design. As a result of this, an estimated one male in three will require prostate surgery during their lives.
There are numerous other examples present in humans as well as other organisms. Vestigial organs provide many such examples. Most creationists insist that there are no vestigial organs. They say, for example, that even something like the appendix in humans serves a function – there are hints that it has an immunological function. Most specifically, some of its follicles produce antibodies which creationists point to as evidence that the appendix is needed to assist our immune system.
But humans have been found with no appendix and they suffer no ill effects. Furthermore many people have had their appendix surgically removed and they have experienced no ill effect either. (At least people suffer no ill effects from the loss of the appendix itself. All surgeries bring some risk, of course, and people have died from complications of the surgery. But that’s a different thing.)
More to the point, some seven percent of the human population will experience acute appendicitis during their lifetime which is a condition that is generally fatal if the appendix is not surgically removed. (My own father had to have his appendix removed by surgeons for this reason.)
How can something that provides no positive effects which can be demonstrated but which presents a possibly fatal condition in one person in 14 be considered beneficial?
The list goes on and on. Certainly our physical characteristics scream out that we evolved rather than were “designed”.
 http://en.wikipedia.org/wiki/Flightless_Birds, referenced on May 29, 2008.
 Durand, J., Keller, N., Renard, G., Thorn, R., and Pouliquen, Y. (1993) "Residual cornea and the degenerate eye of the cryptophthalmic Typhlotriton spelaeus." Cornea 12: 437-447.
 http://www.talkorigins.org/faqs/jury-rigged.html, referenced on May 29, 2008.
 http://www.talkorigins.org/origins/postmonth/oct05.html, referenced on May 29, 2008
 http://www.talkorigins.org/faqs/jury-rigged.html, referenced on May 29, 2008.
 http://www.answersingenesis.org/creation/v20/i1/appendix.asp, referenced on May 29, 2008
 Hardin, D. M. Jr., 1999. Acute appendicitis: review and update. American Family Physician 60(7): 2027-2034.