It highly depends on the plans for specific career. I am not sure about many other fields, but all pre-meds, no matter what is requirement of UG / Merit scholarships / major, are aware that if you let the GPA slip below certain level, than even if you keep your major / Merit awards or whatever, you are derailed from the pre-med track. The good pre-med program will have a killer weed out class in the first semester of the freshman year, so that students do not waste a lot of time / college credits being pre-med. Pre-med does not require any particular major, however, there are certain number of science/math pre-reqs that they have to have. Average “survivor” rate for pre-meds is about 15%, but the 85% of those who did not survive on the pre-med track maybe very successful at whatever they choose for themselves after they realize that they do not have a chance at medical school. I do not know if you would call them “weeded out” or not and if it is considered “aggressive weed out”. These students will not be dismissed though. Overall, many UG’s change their major / career goals for various reasons, some realize that they cannot keep up with the academic level in their original track - got “weeded out”, others simply discover that they like something else.