The Origin Then and Now: An Interpretive Guide to the Origin of Species available as a free preview:
Evolution Today: The mosquitoes of the London underground
This excerpt describes a wonderful example of speciation in action that I'm sure Darwin would have loved.
Public use of the London Underground began on January 10, 1863. That date, or perhaps some earlier date when the tunnels were being readied for traffic, marks the beginning of the path toward the formation of a new species of mosquito. We often wonder how long it takes to form a new species; Darwin speculated timescales on the order of tens of thousand to hundreds of thousands of generations. The mosquitoes of the London Underground show that if conditions are right, the process can be much faster.
Well worth a quick read. The original research paper on which this chapter is based is Culex pipiens in London Underground tunnels: differentiation between surface and subterranean populations.