Geographic isolation is important in allowing local gene pools to differentiate from one another. Generally, the greater the distance between local breeding populations, less gene flow occurs between them. I developed my research project by wondering if there would be any difference in a certain organism in two different locations. I decided upon studying an organism in local streams. In choosing an organism to study, I looked at availability and quantity, and that is mainly why I chose to study the scud or sideswimmer (Gammarus fasciatus).

Geographic isolation is important in allowing local gene pools to differentiate from one another. Generally, the greater the distance between local breeding populations, less gene flow occurs between them. According to the book, The Science of Evolution, by William D. Stansfield, organisms with little or no gene flow between species that live in different locations, tend to go on their own evolutionary way, adapting to different conditions in their own locations.

Migration is the only other process other than mutation by which a population can receive new kinds of genetic information. There are at least three models to represent migratory schemes. The island model shows populations exchanging genes in various quantities and directions through migrations. The river model shows a linear gene flow, and finally the distance model shows a group of organisms distributed over a large geographic area, where individuals do not have as much of an opportunity to mate. The scuds were collected from Bear's Grass Creek and Thompson Valley Creek (see appendix A). Bear's Grass and Thompson Valley are separate drainage basins in the Lower Eau Claire Watershed, which both empty into the Eau Claire River.

The organisms in ponds, lakes, streams, or rivers might be expected to show considerable differentiation because of their isolation, but quite the opposite may be true. Many fresh-water organisms are widely distributed. One way by which different fresh-water systems may be interchanged is caused by floods that periodically join rivers, lakes, etc.