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Variation and natural selection in a population of sticklebacks Gasterosteus MacLean, James Alexander

Abstract

Threespine sticklebacks are genetically polymorphic for the number and arrangement of bony plates on the sides of the body. The adaptive significance and maintenance of plate variation was investigated in Heisholt Lake, a small British Columbia lake with two separate basins. The population contains the low plated, partially plated, and completely plated freshwater morphs of threespine sticklebacks, and plate number varies considerably within morphs. Frequencies of the plate phenotypes changed in space and time. Morph frequencies change spatially both between depths within an area, owing to segregation of breeding females, and between areas at the same depth. Phenotypic frequencies changed temporally both within and between generations. Phenotypes favored within a generation also increased in frequency from that generation to the next. At most stations in basin 1, low and completely plated sticklebacks increased,and partially plated sticklebacks decreased in frequency both within and between generations. In basin 2, partially plated sticklebacks were favored at many stations both within and between generations. Extreme phenotypes within all morphs increased in frequency both within and between generations, and asymmetrical disruptive selection acted during at least one generation within all morphs. Interactions between genetic variation and structure of the stickleback population appear to explain changes in the frequency jf phenotypes in both space and time. Experiments to investigate the movement pattern of sticklebacks in Heisholt Lake show that the population is composed of resident individuals, which remain in a restricted area and maintain either a feeding or "breeding territory, and non-residents, which move from area to area and do not breed. The phenotype of an individual influences its chances to become a resident. Low and completely plated sticklebacks were favored in competition for territories in basin 1, but partially plated sticklebacks were often favored in basin 2.) Females with extreme phenotypes had the greatest chance of breeding, and asymmetrical disruptive selection acted within all morphs. The phenotype of a stickleback also influences its chances of being infected with Schistocephalus solidus, a cestode parasite that reduces the chances of infected sticklebacks to survive and reproduce. Partially plated sticklebacks had the highest rate of infection in basin 1, but had the lowest rate of infection in basin 2. Differential infection of phenotypes was at least part of the explanation for the observed temporal changes in phenotypic frequencies. Spatial changes in phenotypic frequencies are caused by changes in space in the results of competition for territories. Temporal changes in phenotypic frequencies are explained by differential survival and reproduction of phenotypes as a result of differences between phenotypes in chancegto obtain a territory.

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