As a potential PhD applicant, you have to be flexible in your research interests. In a few years, your research interests may change as you are exposed to new problems, methods, and collaborations. What you begin with is usually only a direction, not a fixed path. The key point is that the research skills you gain (critical thinking, modeling, data analysis, scientific communication etc.) remain useful across topics. They do not disappear when your focus shifts, but instead transfer and continue to grow through your research. #Research