Darwin's idea of natural selection makes people uncomfortable because it reverses the direction of tradition.
The problem is that no ethical system has ever achieved consensus. Ethical systems are completely unlike mathematics or science. This is a source of concern.
I think many people are terribly afraid of being demoted by the Darwinian scheme from the role of authors and creators in their own right into being just places where things happen in the universe.
The juvenile sea squirt wanders through the sea searching for a suitable rock or hunk of coral to cling to and make its home for life. For this task, it has a rudimentary nervous system. When it finds its spot and takes root, it doesn't need its brain anymore so it eats it!