That would solve everything. However, this solution might not be feasible. Eileen may not have room for an additional student, or Steve may not be interested in plant population genetics. Perhaps Steve is interested in studying lizard species and has joined Bill's group because reptiles are the experimental animal of choice in Bill's lab. I Question 1.
Collaboration carries with it a tension.
Yet, we judge each other based on personal achievement, individual inventiveness and insight. A member of a collaborative team is usually not as highly regarded as a solo researcher who has produced similar results and ideas solo . Maybe it shouldn't work this way, but it often does. The way that this question is written suggests that one may have an obligation to accept a proposed collaboration. That is an interesting idea that merits discussion.