Your premise is not entirely correct. Atheists do not "use evolution as a way to discredit religion". Evolution does not discredit religion, it discredits creationism.
Evolution is a very simple principle, based on mutation and selection. It is NOT systematic - it is the very opposite...
In most cases, extinction happens BECAUSE other, superior species have evolved and displace previous ones. Evolution is a process of continuous change. New species appear all the time due to random genetic changes. When they are better equipped for survival, they will reproduce more and...