The Dual Role of Convergent Natural Selection and Evolutionary History in the Morphology of Gammarus minus Populations

An important subject of study in the field of evolutionary· biology is the relationship between phylogeny and adaptation. Many researchers are involved in studies regarding the evolutionary significance of different phenotypic patterns. In environments where strong ( and similar) selective pressures exist many characters display convergence. The similarities between populations from geographically distant but environmentally similar milieu can mislead phylogenetic considerations. This is why the identification and elimination of characters affected by convergent evolution is important before any understanding of evolutionary history is possible. Four Grammarus minus populations from West Virginia were studied. Two populations were cave inhabitants and two populations were spring inhabitants. Each spring population selected was phylogenetically related to a cave population from the same hydrologic basin. However, since both cave and spring environments offer very specific environmental conditions it was expected that many characters to display convergence between populations living in different hydrologic basins. The phylogenetic (basin) effect was surveyed based on characters that display differences between basins while the convergence was surveyed based on resemblances between populations living in similar habitats. Identification of characters displaying convergence only and phylogeny only was also done.Measurements were done on 46 characters, mostly appendages. Since a consistent difference in size exists between cave and spring organisms, a correction for size was done so that the observed differences represented shape differences.The results show that significant differences exist between the investigated populations. Many differences also exist between sexes. Most characters display a habitat effect and only a few show basin effects. After all ch8.1:acters showing habitat x basin interactions were eliminated, eleven characters ( out of 46) show convergence (habitat effect), and only four characters (out of 46) show phylogeny (basin effect). Seven characters shoved convergence only and none showed phylogeny only. Only one character displayed interaction between sex and habitat and non showed interaction between basin and sex. Many characters, ( 12 for males and 34 for females, out of 46), show interaction between habitat and basin suggesting the possible significance of such interactions in adaptation.