Hudson river fish evolving in front of us?

The toxicity of the Hudson River has created a massively fast example of evolution, or so the headlines are saying. But as the stories explain, the mutation which allows some Tomcod to survive and reproduce was already present in a small percentage of the population prior to the sudden toxicity of the river. According to one article I saw, as much as 10% of the fish normally carry this gene variant. Before pollution, you have both kinds of fish. After pollution, you have only one kind (almost only). So, what you have in the end is a loss of complexity and variety, not a gain. Where’s the new thing that’s supposed to drive evolution forward? This is a classic blunder that evolutionists make in claiming it’s possible to see the creative power of evolution right in front of us.