The Biological Design Argument
1 Living things evince design
2 Design requires a designer
3 Life could not have designed itself
So a Designer of all life must exist - God
The problem: Natural selection and survival of fittest can explain apparent design without a conscious Designer - that is, random variation/ mutation, combined with nonrandom selection through differential death based on relative fitness, quickly "designs' organisms better and better fitted to their ecological niche, by making them more numerous and their lesser competitors less numerous. Hence, premise 2 (or 3, depending on an ambiguity in the term 'design') is false.
Logic, however rationally compelling, often leaves people cold. So let's use some examples to help. An omniscient and omnipotent and omnibenevolent Designer would create optimal designs (or else would lack at least one of those 3 attributes). So simply showing that biological organisms are suboptimally designed would refute the existence of such a designer.
One of the simplest examples of such suboptimal design is the very feature that traditional creationists often appealed to as evidence of god's ingenuity, the human eye. (And not only Christians are bewitched by the "irreducible complexity" of eyes - stupid Muslims use the same arguments.)
So how dare I impugn the optimality of the design of human eyes? Well, for starters, I'm typing this while wearing glasses. That alone should be plenty of evidence! But for those who need more argument, read the following wonderful synopsis:
That's right, the octopus and squid don't suffer from detached retinas or a blind spot as we poor humans are prone to; the positioning of their photoreceptors and blood supply also means they suffer from blurry vision and blindness less often as well, ceteris paribus. In short, our single most dominant sense, the one so dominant that an entire theory of knowledge (now convincingly argued to be false) depends on thinking of ideas as like visual images - that sense is suboptimally designed in comparison to another species alive right now. What kind of a fuck-up would've done that?
"The retina is the 'screen' at the inside back of each eyeball, onto which is projected the incoming light. It is made up of lots of photoreceptor cells with their associated out-going nerves, and the blood supply to them. The problem is, the photoreceptors are in backwards, pointing away from the incoming light: the 'cable' from each cell is therefore in the way, and trails across the eyeball's inside surface to exit the retina at the correctly-named 'blind spot'.
Now, the brain compensates for this, so we don't usually notice it. But a design that needs compensatory mechanism for some aspect of it, is not a good design.
But to make matters worse, this design actually causes unnecessary problems. The photoreceptors have delicate, hairlike nerve endings, which means they cannot be cemented firmly into place. Instead, they are loosely joined to a layer of cells called the retinal pigment epithelium. This absorbs stray photons that would otherwise blur the image, and contains the retina's blood supply. But the connection between the retina and the epithelium is so fragile that the retina can detach, either due to a blow to the head, or often, spontaneously. Starved of their blood supply, the retinal cells die, causing blindness.
Strangely, the creator was able to put retinas the 'right' way round... in those pinnacles of His purpose, the octupus and squid. Not only do their eyes, which are basically the same design as vertebrate ones, have their photoreceptors pointing towards the light, and so lack a blind spot; with the nerves training behind them and embedded in their blood supply, the cephalopod eye is far less prone to detached retinas."
The short answer: a satisficing historical process called evolution, not a perfect divinity. For those who like reality-based rather than faith-based thinking, it's time to let go of religion and embrace reality - our future may depend on it.
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