Darwin's theory of natural selection hasn't fundamentally changed, nor is it a simple theory; in fact, natural selection has been directly observed. Here's a classic textbook example: peppered moths live in forests in certain parts of England. They come in two varieties: light-colored and dark-colored. Prior to the early 1800s, the light-colored moths, which blended in well with the color of the tree bark, were present in much higher numbers than the dark-colored moths, which showed up easily against the bark and were eaten by predators. Then came the Industrial Revolution; factories were built and started putting out large quantities of smoke and soot. As a result, the bark of the trees darkened. The moth population started to shift; light-colored moths were now easily visible and frequently preyed upon, while the dark-colored moths blended in with the trees. What had formerly been a hindrance for the dark moths was now an adaptation. Over the last few decades, with the implementation of stricter pollution controls, the trees have become lighter, and the moth population has reversed once again. Natural selection is as simple as that; the organisms best adapted to their environment survive more frequently and are able to pass their genes on to their offspring.
For another example, consider the problem of antibiotic-resistant bacteria, which has been plaguing medicine recently. The few bacteria with mutations that allow them to survive the antibiotic survive it and reproduce; the resulting generations are then all antibiotic-resistant. If you want the most fundamental example of all, just look at the original: Darwin's finches on Galapagos. Each species has a unique beak design specifically adapted for the food source in their own habitat. You may think that macroevolution is questionable, but natural selection is an observable fact that can't be realistically misrepresented. As was posted before, the fossil record also shows evidence for diversification and speciation; while we may be unsure about the specifics, such as whether evolution occurs gradually over time or in fits and starts, the scientific theory itself is sound and is supported by a great deal of evidence.
As a side note, I also went to Catholic grade school and high school, and evolution was a core feature of every biology class I took. Yes, we had theology classes that taught Catholic doctrine and beliefs, but we also had the same science courses that most of the rest of the world uses. You see, it can be done.
