We analysed the frequencies of three potentially harmful external traits - partial albinos, developmental cripples and predation-marks - in large and small populations of two species of alpine butterflies, Erebia epiphron silesiana and E. sudetica sudetica within an isolated habitat island of alpine grasslands in the Hruby Jesenik Mts (= Altvatergebirge Mts), E Sudetes. In both species, small populations contained significantly more albinos than large populations, but there was no consistent trend in occurrences of cripples and predation marks. Albinism occurred to some extent in both interconnected populations of E. sudetica studied (population sizes ca 4,500, and ca 500 individuals, respectively), but it was mostly limited to smaller population of the two mutually isolated populations of E. epiphron (ca 4,500, and more than 100,000, individuals). In both species, albinism was associated with crippled wings, and was more common in E. epiphron if analysed across species. We infer that albinism is genetically based, possibly as a result of inbreeding depression and/or drift. These mechanisms may explain 1) the relation between albinism and population size; 2) the absence of albinism in the largest population, where it should be selected against; 3) the different frequencies of albinism in interconnected and isolated populations of approximately the same size.
