Partial reproductive isolation of a recently derived resident-freshwater population of threespe stickleback (Gasterosteus aculeatus) from its putative anadromous ancestor

We used no-choice matg trials to test for assortative matg between a newly derived resident-freshwater population (8-22 generations sce foundg) of threespe stickleback (Gasterosteus aculeatus) Loberg Lake, Alaska and its putative anadromous ancestor as well as a morphologically convergent but distantly related resident-freshwater population. Partial reproductive isolation has evolved between the Loberg Lake population and its ancestor with a remarkably short time period. However, Loberg stickleback readily mate with morphologically similar, but distantly related resident-freshwater stickleback. Partial prematg isolation is asymmetrical; anadromous females and smaller resident-freshwater males from Loberg Lake readily mate, but the anadromous males and smaller Loberg females do not. Our results dicate that prematg isolation can beg to evolve allopatry with a few generations after isolation as a correlated effect of evolution of reduced body size.