Re: How fresh-water fish got distributed to widely separated bodies of water

Although it may be hard to imagine, many freshwater lakes that are far apart might have been connected in the past. Especially just after the last glacial period as the glaciers receded. These glaciers actually cut big chunks of land that became many of the lakes we see today. As the glaciers receded, they melted and dumped A LOT of freshwater over the land. This flowed from lake to lake as rivers and streams and eventually flowed out to sea. They formed corridors between larger freshwater bodies of water whereby some species could disperse from lake to lake (or stream to stream) through them. Once the glaciers receded enough, most of the connecting rivers dried up and the lakes, etc. became isolated from each other (i.e. most species couldn't disperse between water masses anymore).

There are a few other mechanisms by which life-forms can disperse between widely separated bodies of water. One is by another animal carrying them from place to place. I've heard of invertebrate larvae (very small baby inverts), plants and bacteria "hitching a ride" on the feet of birds and other long-range travelling animals. Once the larvae, etc. get dropped off in another body of water (quite unintensionally by the larger animal, mind you), they may be able to grow and reproduce if the conditions are right. This form of dispersal is really slow, but over several thousand years, it is conceivable for some species to get quite a long distance around the globe under the right conditions.

The BIGGEST reason for similar or identical species to exist in widely scattered bodiesof water is the influence of humans, especially for fish. As long as people have been casting a fishing pole into lakes and streams, man has been stocking those bodies of water with any fish that will survive in them and people would want to catch. And of course, if that fish that you want to catch eats other smaller fish, then you have to stock those lakes with their prey as well! And with all those fish come their parasites, algal spores on their scales, etc., etc., etc., Humans have probably introduced more species into freshwater (and saltwater) habitats where they were previously absent, both intentionally and unintentionally, more than any other means of dispersal available to these critters. Some creatures can hitch a ride of the hulls of boats (mussels, barnacles, etc) as boat owners travel from lake to lake (or ocean to ocean!). These creatures don't jump off and colonize the new lakes, but they do release their babies into the water, which grow up in a body of water very far away from their parents' place of origin. Such human-caused introductions are usually very detrimental to the ecology of these systems, eliminating the flaura and fauna that was originally there (which tend NOT to be in widely scattered bodies of water).

Hope this helps!

References:
Wilson, Christopher C.; Hebert, Paul D. N. "Phylogeography and postglacial dispersal of lake trout (Salvelinus namaycush) in North America." Canadian Journal of Fisheries and Aquatic Sciences. April, 1998. 55(4):1010-1024.

Svardson, Gunnar. "Postglacial dispersal and reticulate evolution of nordic coregonids." Nordic Journal of Freshwater Research. 1998. 0(74):3-32.

Hokstad, Ove; Skurdal, Jostein. "Spreading of freshwater organisms in Norway." Fauna (Oslo) 1996. 49(1):10-19.

Stemberger, Richard S. "Pleistocene refuge areas and postglacial dispersal of copepods of the northeastern United States." Canadian Journal of Fisheries and Aquatic Sciences. 1995. 52(10):2197-2210.