Darwin's original theory was that different life forms evolved through "variation and natural selection." The offspring of each generation are not exactly like their parents. Some are better suited than others to survive in their environment, and those are the ones that survive in greater numbers in the next generation. If the environment stays the same, the surviving offspring will be much like their parents, and the species will not change. If the environment changes, the surviving offspring will be different from their parents; there will be a gradual drift in their characteristics, and in time a new species will be formed. There is no designer in this process.
The proponents of "intelligent design" say that some structures could not have come into existence without the aid of a designer. If you look down at the road you're walking on, and see a watch among the stones, you can easily see that the watch was designed, they say. And to their eyes, the flower by the side of the road is like a watch, not a stone. I would invite such people to compare the intricate symmetry of a snowflake with one of Jackson Pollock's spatter paintings. If you didn't know, you would think the snowflake must have been designed and the spatter painting just happened. You would be wrong both times.
I would also ask them to think about the finches of the Galapagos Islands, which so intrigued Charles Darwin. These finches, obviously closely related, are just as obviously adapted to different diets by the shape of their beaks. It became clear to Darwin that they had only recently (on the time scale of evolution) evolved into distinct species. Should he have supposed instead that some benevolent designer, when a flock of finches landed on this deserted archipelago, gave them different beaks so they could survive on different diets?
Think about how domesticated animals are bred. The traditional method (before genetic engineering) was to watch for desirable variations and select for them in subsequent generations. This process of variation and artificial selection is so much like the evolutionary process of variation and natural selection, and so effective in producing different forms, that it would be absurd to argue that the beaks of the Galapagos finches could not have evolved as Darwin thought they did.
The "intelligent design" faction argues that even if some features could
have evolved, others are so complex that they had to come into existence all at once.
In the IDEA Center's
Primer: Irreducible Complexity in a Nutshell (referring to the
rotating flagella that some bacteria propel themselves with) they write:
Some scientists believe the flagellum is irreducibly complex because all its parts
must be present to function. There is no plausible scenario under which such biological
complexity could arise through mutation and natural selection. The flagellum works like a
designed machine.
In other words: because they can't imagine how it could have happened,
it didn't happen.