Evolutionary change can be fast – Peter and Rosemary Grant's long-term & ongoing research project on the Galapagos finches documented rapid responses to environmental changes, for example, as does the recent work on cane toads in Australia. And biologists have known since Darwin's time that competition can be a strong driver of evolutionary change. (Take Gause's principle of competitive exclusion & its implications, for example.) A just-published paper about Anolis lizards demonstrates this very well (Stuart et al., 2014).
The way in which different species of this little lizard divvy up their habitat is used as an illustration of niche partitioning by many textbooks (you'll find an example here). Stuart & his co-authors describe some elegant experimental work over a period of 15 years, on artificial islands in a Florida lagoon. Initially they used six of these islands, all of which were already colonised by the green native anole, Anolis carolinensis: three of the islands acted as controls, while brown anoles from Cuba (Anolis sagrei) were introduced to the other three. The two species are described as being "very similar in habitat use and ecology", including diet, so they'd be expected to compete fairly strongly when brought together.
In other areas where the two species are found together, A.sagrei perches lower in trees than carolinensis, which left to itself would occupy most of the tree. So the prediction was that on islands where sagrei was introduced the same thing would happen: carolinensis would come to occupy a reduced niche, perching higher than the 'invader'. And this is indeed what happened, in the space of three months:
by August 1995,on treatment islands already showed a significant perch height increase relative to controls, which was maintained through the study.
The researchers also predicted that this change in niche would be accompanied by a change in morphology; specifically, that there would be selection for larger, sticker feet in A.carolinensis, on the basis that
[toepad] area and lamella number (body-size corrected) correlate positively with perch height among anole species, and larger and better-developed toepads improve clinging ability, permitting anoles to better grasp unstable, narrow, and smooth arboreal perches.
This prediction was tested through observations on 11 islands, five with only the native species and six with both the native and the Cuban invader. Again, carolinensis perched significantly higher in trees on islands where sagrei was also present – and on those islands carolinensis anoles also had "larger toepads and more lamellae" than were found on the same species living without the competitor (an example of character displacement) – and this happened within about 20 lizard generations.
Careful analyses allowed the researchers to rule out other explanations:
In sum, alternative hypotheses of phenotypic plasticity, environmental heterogeneity, ecological sorting, nonrandom migration, and chance are not supported; our data suggest strongly that interactions with A. sagrei have led to evolution of adaptive toepad divergence in A. carolinensis.
So, just as with the cane toads, we are seeing rapid evolutionary change in real time.
Y.E.Stuart, T.S.Campbell, P.A.Hohenlohe, R.G.Reynolds, L.J.Revell & J.B.Losos (2014) Rapid evolution of a native species following invasion by a congener. Science 346 (6208): 463-466. doi: 10.1126/science.1257008