Jim - The theory of gravity has been around about as long as evolution. There have been multitudes of advancements and refinements on the theory of gravity. Most of this is due to the comparatively easy experimentation on gravity vs. evolution.
Gravity, even though much more refined and tested than the theory of evolution, is still far from fully understood. Evolution, which is very hard to test experimentally, has many fine details ripe for scientific testing.
Gravity has never been thrown out, but it has been clarified hundreds of times, bringing the understanding down the subatomic level. Evolution does not need to be thrown out, but the understanding can be greatly refined.
In order to refine the theory - careful observation and reasoning must be put to work. These fleas, and their shifts in allele frequency, are not support for evolution.
Even though there is a good theory of evolution, this observation (the fleas) minimally supports, but does not advance the theory of evolution.
And neither Gravity nor Evolution are “settled science.”
PS - Yes, 4 billion years is a long time, but that is not evidence for evolution either. Oh, I happen to think that life will be found anywhere that liquid water exists over billion year time-frames, having studied Archaebacteria from black smokers, and bacteriophages.
3 or 4 billion years gives evolution time to work, if you will. Yes, there are recognized times when evolution is “explosive” and other time when status is the rule. These fleas are demonstrating a core means of evolution. A certain portion of the population increases in number in comparison the the past proportions. Eventually, most of the fleas have this characteristic. The characteristic was likely a benign mutation in numerous individuals over time. This specific change takes a short time. However such changes will mount up over time to become major differences with the original population. Major evolutionary change leaves a track in the fossil record.
It is always change within species (or kinds) but the cumulative product will be very different, a different but related species. All the while other animals and plants are changing, creating new environments which, in turn give nitches to be fill by changed other creatures. The most such nitches are, of course, found following mass extinctions. Then such events as the Cambrian explosion occur.