Low degree of isozyme variation within and between Herring Gull (Larus argentatus), Lesser Black-backed Gull (Larus fuscus) and their British and Swedish subspecies

27 Lesser Black-backed Gulls (Larus fuscus graellsii) and 15 Herring Gulls (Larus argentatus argenteus) from Great Britain were compared with the Swedish subspecies L. fuscus intermedius and L. argentatus argentatus. 18 enzyme systems representing around 40 loci were investigated. The same isozyme polymorphisms are present in the British and Swedish populations and no differences could be detected between species in the British material, nor between Swedish and British populations of each species. The mechanisms behind this conservative evolutionary pattern in avian proteins are discussed.