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:…extensive isozyme (Lessios 1979, 1981a; Bermingham and Lessios 1993), mtDNA (Bermingham and Lessios 1993; McCartney et al. 2000), and bindin (McCartney and Lessios 2004) sampling has never identified a hybrid among postmetamorphic sea urchins in the Caribbean, which suggests that…
Category: