What is a species? Despite much attention to this question by evolutionary biologists, differences of opinion still exist. One definition that many subscribe to is called the biological species concept. It focuses on whether individuals can interbreed with each other. Members of the same species can interbreed (mate and produce fertile offspring) whereas members of different species cannot. Members of different species have features, called reproductive isolating mechanisms, that prevent interbreeding. Reproductive isolating mechanisms can be of various sorts as shown in the table.