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.
You just miss the distinction between intra-species variation and evolution.
Read Gould.
There is no smooth line in evolution. It is punctuated. That is the best current theory. These fleas are only demonstrating intra-species allele frequency change, perhaps due to climate change (perhaps due to something else).
There are no new alleles and the fleas did not evolve.