A scientific theory can be used to make predictions regarding future discoveries. For example, Darwin's contemporaries and successors first proposed that birds and some dinosaurs could be related. Darwin's theory, if correct, indicated that there would be an "in-between" or "transitional" organism that showed traits of both. This prediction eventually became true with the discovery of Archaeopteryx, a Jurassic Period reptile that had feathers and wings. Although flightless, it is most definitely a forerunner to modern birds.
When other paleontologists suggested that amphibians developed from land-venturing fish, Darwin's theory indicated that there would be a transitional species that shows characteristics of both. Sort of a prehistoric analogue of a modern lungfish. Sure enough, such a fossil was discovered that showed traits of both fish and amphibians.
And finally, there WAS an example of speciation that occurred in modern times. During World War II, many Londoners took shelter in the underground subway systems. At least one population of mosquitoes followed them into the subway, and stayed there. 70 years after the end of the war, the descendants of those mosquitoes still live in the subway, feeding on rats and other animals that end up down there. Because of the separation between the underground and above-ground populations of mosquitoes, genetic drift and mutations led to some members of the respective populations being genetically incompatible. Although some members of each population can mate and produce fertile offspring, other members are completely incompatible and cannot do so. Thus, a new species was created. Within the NEXT 70 years or so, computer models indicate that the two populations will be completely incompatible, and thus two distinct species.