First, graduate departments are looking for students who are smart and work hard. So we look at GRE exam results, undergraduate grades, letters of reference, and so on. Perhaps the most important part of your application is the personal statement. It shows us whether you can write and think clearly.
Second, we look for students whose interests mesh with those of our faculty. We don't accept students who want to study subjects that we don't know much about. The personal statement helps us identify students whose interests match our expertise.
Try to identify a few graduate departments with real expertise in the subjects that interest you. What do you want to study? Population genetics? Kinship? The evolution of cooperation? Genetic disease? Whatever it is, find out who is currently publishing in this area and read some of their papers. If you find a paper you like, consider applying to the author's department.
This will give you a short list of departments. The next step is to look at other faculty in each department. What are their interests? What have they published recently? How often are they cited? If there are two or three people whose work interests you, then apply there. If there is only one, then it is probably not the right department for you. Then read a few publications written by the faculty that interest you. This will make it easy to explain (in your cover letter and personal statement) why you are applying to their program.
If your list contains 30 departments, then you probably don't know much about any of them, and that will come across in your letter and personal statement. So keep the list down to a handful of departments, and do some homework on each one.