Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:…have considerably advanced our understanding of how selection by parasites in conjunction with sexual selection drives the polymorphism of the classical major histocompatibility complex class I and II genes (Landry et al. 2001; Langefors et al. 2001a; Reusch et al. 2001; Wegner et al. 2003a).
Category: