How would you go about determining if the subway mice constitute a separate species adapted for life under the subway platforms?

A population of mice exists in the stations of the Boston subway system. They appear to live on the food that people toss down onto the tracks as they await their commuter trains. They have very dark grey, almost black, fur and blend well into the dark grey gravel on which the tracks rest. Normal wild field mice have a lighter grey/brown/white coat. How would you go about determining if the subway mice constitute a separate species adapted for life under the subway platforms?

 

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