Just for the record, I am an evil, satanic, baby-killing evilutionist homosexual-loving ugly fat bull-dyke with a forest of unshaved bodily hair and I dance naked every full moon after bathing in my own period blood.
Translation: I am a neopagan, I am pro choice, I am a feminist and I consider evolution to be the most accurate current theory concerning the development of life on this planet.
Why have I written this? Because I am becoming tired of hearing the same old reactionary responses from people who prefer intelligent design (or, to be more accurate, people who bat for ID not out of any love for the theory but because they think evolution = evil). Now, I love debate. I love meeting people who genuinely disagree with me and who can state why. The opportunity to spar intellectually is stimulating and helps me develop. I am a scientist. This means that I do not "believe" anything and, if evidence is shown, will happily modify my understanding of the universe to fit this evidence. In fact, as a scientist I am compelled to by the very values to which I adhere. However, the evidence is often lacking. And "UR A STOOPID UR GAY I KNOW ID IS RIGHT GOD SEZ SO DARWIN GETS RAPED BY MNOKEYS IN HELL BITCH" is not, actually, intelligent debate.
Point the First
Evolution is not linear.
Evolution does not operate like a mechanic's workshop. People see those charts showing the ascent of man and think it is the whole. Such images are, in fact, simplified. To demonstrate, let us travel to the Planet Slugus; an entirely fictional planet designed to show off my lack of PC-based art skills.

Now, here we see the Ascent of Slug. You can clearly see each stage. Slugs evolve spots. Then they evolve wings. The wings turn into fins when they start living in the water. The fins become more flexible tentacles. The slug grows more intelligent, this allows it to learn to use these tentacles to create tools, write and walk. Slug achieves civilisation.
However, this also creates the false impression that these slugs evolved with purpose. It lends credence to the lie that it was all planned, that the slug was aiming for Slugus Sapiens from day nought, and that previous slug stages were "inferior" and transient.
The truth is that amongst all of this there were slug species which evolved to dead ends, slugs which devolved and slugs which stopped evolving. The later stages are not migrations of all previous slugs, but an off-shoot. Both the off-shoot and the ancestor may coexist for a time, although they may live in different habitats. In some cased slugs devolve and are left with seemingly-useless, vestigial parts, such as Supergenius Slug and it's vestigial wibbly-bits.

Basically, look above and you will see a timeline that shows many offshoots from each species. Some are successful up to the present day, such as the basic Slug. Some are successful for a time, such as the Spiney Sabre-toothed Slug, which proliferated until it's main prey, the Hippy Slug, died out. Others, like the Rave Slug, exist only for a brief time before they die out. In the case of Rave Slug, this is because it was adapted specifically to life in the universe's most tasteless rainforest and could not survive in duller surroundings after the rainforest was destroyed due to being an eyesore.
Point the Second
Evolution can go backwards
Take another look at my big, messy graph. Notice that Sabre-toothed slugs, and similar species, reoccur at different times, and from different branches. This is because of the whole changing-habitat thing. Take Stumpy Sabre-toothed Slug. That slug evolved from snake slug when it's environment changed. Rolling meadows and warm forest died off when a continent shifted, leaving harsh desert terrain and less food. Having big fuck-off teeth served to assist with the killing of prey and as a deterrent to predators. When the desert became more hostile however, food became ever more scarce. Tendencies towards cannibalism resulted in the evolution of a spiney sabre-toothed slug which, died out due to an inability to get up when rolled onto its back in soft sand.
Albino Vampire Slugs evolved their large teeth via sexual selection. Longer teeth would grow on slugs which were healthier, and female slugs preferred long-toothed slugs as mates. For a time the teeth were also used in mating rituals, resulting eventually in a new, aggressive sabre-toothed slug with wings.
And so on. Notice also that the Late Slug looks almost identical to the Slug. All the characteristics gained by previous slug species dissappeared, purely and simply because this slug lived in suc a non-hostile environment that there was no need for large teeth, spines or wings, and such body parts took unneccesary amounts of energy to maintain- simpler slugs could survive easier and were less susceptible to brief famines.
Point the Third
Useless bits sometimes remain
Look at Supergenius Slug. Hyper intelligent but utterly lacking in limbs. This slug species evolved in tandem with Slugus Beefcake and they existed in a symbiotic relationship. Over time Slugus Beefcake took on more and more of the physical duties, being stronger andmore agile, leaving Supergenius Slug with the thinking side of things. Supergenius slug was also hindered by a bulky braincasing, and over time its offspring, which quickly died out or interbred with Slugus Beefcake if not sufficiently intelligent, devolved their limbs which took useful energy away from the brain, leaving just randomly placed little wibbly nubs which may have eventually disappeared, leaving a spherical slug, had the hyper-aggressive Fundie Slugs not eaten them all. In the same manner, some species of snake on earth have vetigial toes, the remainder of lost limbs, which they use during sex.
Point the Fourth
Transient Species
I hear a lot of people asking why, if evolution is true, don't we have transient species- fossils showing animals half-way between other species? Why are there gaps?
- Fossils are only preserved in rare circumstances, and often found by chance. There may be fossils out there waiting to fill the gaps which we simply haven't seen yet, because they are buried deep underneath cities or have been eroded under passing glaciers.
- All species are "transient" in the way I think you mean. This is because each new fossil found which lies between two species is another species in its own right.
- Evolution can do big jumps as well as little ones. Creatures don't just have to change one thing to change species- they can take something away, add something and elongate something else simultaneously, if the environment calls for it.
- If you're imagining a fossil that looks like the child half-way between his transformation in those awful Species movies, then you have utterly misunderstood evolution.
Point the Fifth
Half an Eye?
I believe that the premise behind the "what good is half an eye/bacteria flagellum/mouse trap" argument has been pretty much answered, and answered more elegantly than I could hope to, right here.
http://en.wikipedia.org/wiki/Evolution_of_the_eye
But, suffice to say, half an eye is useless. Luckily no creature ever evolved half an eye- because as I said evolution is not a mechanic's workshop.
Point the Sixth
But you need Predictions!
This is a fair point. In order to be considered real science, a theory needs to be useful for making predictions which can be proved-disproved.
Let us look at Tiktaalik.
The scientists involved knew what they were looking for. They expected to find a fossil showing evolution midway between fish and amphibians. They predicted, based on previous fossil evidence, the likely age of sich a fossil and, based on biology, the environment it was likely to live in. From this they worked out the sort of sediment/rock they would expect to find it in and the age of this rock, and subsequently how deep they would need to dig. They made a list of known areas which held rocks likely to match this period and started digging. They found the fossil they had predicted.
For more information, see below.
http://en.wikipedia.org/wiki/Tiktaalik
I recomment Wikipedia for all your reading needs. Here are a few pages you might like to start from if you wish to debate science.
Scientific Theory
Evolution
Intelligent Design
Transient Fossils
And a list of them
Evolution of the Eye
Irreducible Complexicity