MICROSATELLITE VARIATION AND MICROEVOLUTION IN THE CRITICALLY ENDANGERED SAN-CLEMENTE ISLAND LOGGERHEAD SHRIKE (LANIUS-LUDOVICIANUS-MEARNSI)

Polymorphic nuclear microsatellite loci were used to characterize gene tic variation in contemporary and historic populations of the San Clem ente Island loggerhead shrike (Lanius ludovicianus mearnsi), an endang ered bird with a current population of about 30 individuals, that is e ndemic to one of the California Channel Islands. We also compared the population of the shrike with two contemporary populations of the stil l abundant subspecies, L. l. gambeli, which live 120 km away on the ad jacent mainland. The current population of L. l. mearnsi has 60 % of t he genetic variation of the mainland shrike populations and is strongl y differentiated from them. Comparison of living birds with 19 birds c ollected in 1915 shows that most of the variation within the island po pulation was lost before the recent 90% decline in population size, an d the 20 % decrease in variation this century is probably attributable to genetic drift. Mitochondrial DNA control region sequence data from 80-year-old specimens show that there may have been limited introgres sion to L. l. mearnsi, this century, from another island subspecies, L . l. anthonyi, found in the northern Channel Islands. Today, gene flow between L. l. mearnsi and mainland L. l. gambeli is very low, even th ough a few mainland birds visit the island annually. The island subspe cies population has evolved sufficient genetic independence to justify ongoing conservation efforts to counter demographic collapse and gene tic erosion; the course of genetic erosion can now be monitored noninv asively, as demonstrated by this study, based on DNA amplified from fe athers.