Maximum likelihood tree inferred from the nucleotide sequences of exon 1 of the arginine-vasopressin 1a receptor gene for various mammalian taxa. Bootstrap values > 50 are shown for the maximum likelihood method above branches and for neighbour-joining below branches. Positively selected sites (ω > 1) are shown in black circles. Note that most of these sites are found in the G-protein binding domain (231–274). Only two positively selected sites (191; 228) were detected outside this domain in two species (O. aries and C. familiaris).