Variation and Adaptation in the Imported Fire Ant Solenopsis Saevissima Richteri Forel in the Gulf States

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The imported fire ant, Solenopsis saevissima richteri Forel, is the southernmost race of a widespread and highly variable South American ant. It was introduced into the port of Mobile, Alabama, sometime around 1918 and by 1949 had spread to parts of Florida, Mississippi, and Louisiana. 2. A great deal of color variation from nest to nest has been noted in the Gulf States population. This includes an extreme blackish phase referable to the original description of richteri, an extreme reddish phase referable to no described form, and intermediates between the two. This color variation is correlated with differences in size of the ants and in appearance and proportion of their nests. 3. The variation has a genetic basis. It is suggested in this study that the variation can be explained most readily on the basis of multiple pleiotropic alleles. 4. The history of the variation has been determined as follows: The darkest forms, or richteri s. str., were the ones originally dominant from the time of the ant's introduction until at least 1929 and probably sometime after 1932. The origin of the new form is not known, although it is believed that it originated either through mutation within the population or through a second introduction. IN 1949 it was by far the dominant form. It had apparently replaced the typical richteri partly by rapid expansion and subsequent genetic dilution and partly through natural selection by direct competition. Its predominance in the main population and in at least two smaller isolated populations has evidently been responsible for a much greater success of the species. In the main population in 1949 the typical richteri was mostly limited in distribution to portions of the periphery of the range, forming with the new form roughly the concentric pattern of Matthew's modified Age-and-Area hypothesis. 5. The new form has been interpreted as functioning, regardless of its origin, as a favorable mutation introduced into the population. Its rise to dominance has constituted an extremely rapid, nearly vertical evolutionary change.

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