sorry the pics didn't come out before

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So, we're looking at goby - sculpin - sculpin - goby in this sequence?

Darters seem to fill the same niche that is filled by gobies and gudgeons(sp?) in other parts of the world. My question is, why doesn't NA have gobies and gudgeons instead of darters? Did the glaciers create a vacuum which was filled by perches evolving into the gobie role?

Percid evolution in North America for the past 60 million-plus years was shaped by the existence of a vast but slowly shrinking inland sea or embayment in what's now the great plains and Rockies. I'm sure others here like Tom Near or Dave Neely would have more fleshed out scenarios, but this set the table for the evolution of our very diverse darter fauna, among others. Gobies remained a marine group, and freshwater sculpins were able to diversify and fill niches, too. And then we get into the existence of a geologically stable southeastern NA which allowed diversification in ancient, isolated river systems.