Linearity of Geographic Range and Its Possible Effect on the Population Structure of Andean Birds

—Geographic ranges of widespread Andean forest bird species are linear, averaging over 300 times longer than they are wide. Among congeneric species, geographic variation in plumage was negatively correlated to the width (W) of their elevational distributions, presumably because narrower distributions are more easily fragmented by barriers to gene flow. The amplitude of a species' elevational distribution may be limited by zonation of resources and by genetic factors that prevent populations from adapting to local environments. inimum amplitudes may be dictated by demographic consequences. Received 13 April 1987, accepted 28 August 1987.