Believers in intelligent design often argue with great passion and conviction that when an organism perfectly fits its environment or its ecological niche, that's proof that it "couldn't have been an accident." Just as it couldn't be an accident that a particular key fits a particular lock, they believe that random chance can't account for any two things in nature fitting together so perfectly.
It's rubbish, of course, but their argument makes intuitive sense if you don't understand natural selection. They think, apparently, that random genetic mutation is the only factor in evolution, and don't fully appreciate how environmental forces can carve a species into shape. I've thought of a few ways to illustrate it.
Cookies. Gingerbread men aren't the result of balls of dough dropping and some of them randomly flattening out into the shape of gingerbread men. Genetic variability would be like dropping a big wad of dough on the table and rolling it out. The shape after the initial plop is random. As you push the rolling pin this way and that, you're not trying to precisely roll out the shape of a gingerbread man. It's just a wider, thinner blob. But then comes the cookie cutter. The cookie cutter could represent any selective force--a storm, a plague, climate change, whatever. The dough inside the cookie cutter perfectly fits the shape of the cookie cutter, and the dough outside it is discarded.
But I decided I didn't like that example, because someone made the cookie cutter. Someone deliberately used it to make a cookie that shape, so that example probably wouldn't really succeed in stretching the awareness of someone who already believes in the existence of a designer.
So then I got to thinking about unintentional impressions, like a footprint in the dirt. When you walk, your shoes (or feet) perfectly match the footprints you leave on the ground. You don't intentionally indent the dirt to match your shoes, it just happens. But even there, we have a sapient agent and a man-made shoe, and we really need to remove people from the illustration for it to work right.
A branch! A branch ripped off a tree by the wind during a rainstorm crashes to the ground and falls in the mud. The impression in the mud perfectly matches the shape of the branch. Nobody had to do anything to make the mud that shape, and yet the shape in the mud is so perfectly suited to the shape of the branch that, according to the reasoning of those who believe in intelligent design, it couldn't possibly have happened by accident. Except that it did. Is it an accident that they match? The match happened because of the accident. If we can document that the branch was weak and the wind was strong and the mud was soft, then the preponderence of the evidence suggests that the stick fell and made an impression in the mud by accident rather than that an invisible, supernatural being broke the stick off and placed it in the mud in a precise position for reasons we can never understand. We have an abundance of evidence suggesting an accident, and absolutely none to support wild speculation about divine intervention.