As a person who did their Honours thesis on a related question, I can only put most of my thoughts on the first question and its relation to North American fish ecology, though I can imagine that given how long evolution/speciation takes, that barring the specific case of African cichlids, most lakes have not really existed for very long. In North America, especially Canada, most of our lakes were formed from glaciation a mere 10 000 years ago - a great glacier quite literally scraped away holes in the land, and these became filled by meltwater, forming lakes and rivers, many of which were once far more interconnected than they are now so there was once an opportunity to disperse where none now seemingly are.
Given this scenario, we can then think about how fish get into these lakes, and eventually begin to speciate, but the fact is that, there just hasn't been a great deal of time geologically speaking for fish to even start diverging, because although these lakes may have formed 10 000+ years ago, the fish probably didn't get there 10 000+ years ago. Some of the fish are of highly conserved lineages, and spread out from refugia in the southwest and southern parts of North America, but many more, especially the most important varieties are actually the product of human activity - people literally dumping lakes with fish to stock them in the 19th century. Think about that - even if these lakes are quite isolated, there hasn't been all that much time for speciation (200+ years), and most of these lakes aren't actually fit for habitation and only have fish as they are cyclically restocked, usually on an annual basis.
Lastly, as u/GreenStrong has provided, there has recently been some suggestion that ducks are acting as zoochoric agents, and spreading out fish eggs, but although this is a plausible practical vector of dispersal under lab conditions, it's still unclear (as far as I've heard!) if this actually could have happened enough times for it to have been a meaningful mechanism of dispersal.
DeusKyogre1286 t1_ism2veo wrote
Reply to How do fishes get into isolated inland lakes in the first place? and why don't we see more divergent evolution / speciation given the separation of each group of fishes from each other? by I-mean-Literally
As a person who did their Honours thesis on a related question, I can only put most of my thoughts on the first question and its relation to North American fish ecology, though I can imagine that given how long evolution/speciation takes, that barring the specific case of African cichlids, most lakes have not really existed for very long. In North America, especially Canada, most of our lakes were formed from glaciation a mere 10 000 years ago - a great glacier quite literally scraped away holes in the land, and these became filled by meltwater, forming lakes and rivers, many of which were once far more interconnected than they are now so there was once an opportunity to disperse where none now seemingly are.
Given this scenario, we can then think about how fish get into these lakes, and eventually begin to speciate, but the fact is that, there just hasn't been a great deal of time geologically speaking for fish to even start diverging, because although these lakes may have formed 10 000+ years ago, the fish probably didn't get there 10 000+ years ago. Some of the fish are of highly conserved lineages, and spread out from refugia in the southwest and southern parts of North America, but many more, especially the most important varieties are actually the product of human activity - people literally dumping lakes with fish to stock them in the 19th century. Think about that - even if these lakes are quite isolated, there hasn't been all that much time for speciation (200+ years), and most of these lakes aren't actually fit for habitation and only have fish as they are cyclically restocked, usually on an annual basis.
Lastly, as u/GreenStrong has provided, there has recently been some suggestion that ducks are acting as zoochoric agents, and spreading out fish eggs, but although this is a plausible practical vector of dispersal under lab conditions, it's still unclear (as far as I've heard!) if this actually could have happened enough times for it to have been a meaningful mechanism of dispersal.