Subject: Why fres water life is common to all lakes yet separated by lots of land

I live in minnesota with thousands of lakes all over. Why is it that 
all lakes have very simmilar species of aquatic life in general(not 
just Minnesota but everywhre); if the process of evolution seems to be 
dependant on many random occurneces. Why is there not random 
unconventional species in lakes segregated to a specific area? It 
seems that all fresh water life evolved from the exact same conditions 
even though they are islands of life (not connected). Is there a 
Gillapigose Island equivalent for fresh water lakes?

Re: Why fres water life is common to all lakes yet separated by lots of land