There is a concept in evolutionary biology called sympatry, in which two interbreeding populations develop unique genetics and evolve into two species.
Sympatry is a rare biological event. Conventional biology has assumed that speciation only occurs when a mountain range or some other type of geographical or geological feature separates two populations. Increasingly though, sympatry has been gaining ground as more and more researchers seem to be finding it among their subjects.
Most recently, researchers in Switzerland have discovered that two populations of three-spined sticklebacks in Lake Constance and its watershed streams are evolving along these lines.
Basically, one group favors the streams, and another favors the lakes. They show marked differences in size and armor, which could be chalked up to lifestyle differences, but there are specific genetic differences between the two groups.
The most interesting part though, is that they not only both breed in the same streams, but also continue to interbreed. This clear example of sympatry could tell us quite a lot about evolution.
The general assumption is that evolution requires significant periods of time to occur. However, the three-spined stickleback has only lived in those waters for about 150 years, which is the blink of an eye compared to evolutionary timelines. This isn’t the first example of evolution on this kind of timescale though.
Apple maggots evolved in North America within two centuries of apples being first introduced to the continent. Cancers become resistant to drugs. Insects develop resistance to pesticides. Bacteria develop resistance to antibiotics.
A major factor contributing to this process may be the average lifespan of such organisms, but it doesn’t change the fact that we’re finding, more and more, that evolution is capable of a wide variety of timescales.