|MadSci Network: Evolution|
You have a very good, thoughtful question with heavy duty evolution! I had to consult with two benthic-oriented graduate students who study invertebratest for this answer (see signature for their names).
All/most of the freshwater lakes in North America were formed around the same geologic time (post glaciation), hence the fauna will tend to be macroscopically similar. Furthermore, very few lakes are completely isolated. Most are connected via streams/rivers, and flora and fauna can be transported with any current. Even if there is no perceived "real" connection, fauna in many life stages can be transported - for example by webbed duck feet, or on algae strands that are transported via other animals. On a more microscopic note (in terms of benthic creatures), these organisms rely on particular conditions (in terms of temp, O2, pH, substrate size, etc). Hence, if similar water conditions exist in several lakes (which tends to be the case in most eco-geographic regions), they will yield similar benthic communities.
Hope this helps.
- Iwona Ciesielka & Kellie White
Graduate Students of Ecology & Evolution
University of Western Ontario
Try the links in the MadSci Library for more information on Evolution.