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.
Intra species variations can over time lead to speciation. When an isolated sub population can no longer breed with the larger population, you have speciation by definition. While Gould advocated large surges of evolutionary activity following mass extinctions with stasis in between, he recognized genetic drift occurring when it gave a reproductive advantage. Many intraspecies variations are the result of copying errors (mutations).
Punctuated equilibrium takes time because change can be very fast in a geologic time scale and still take millions of years as in the 20 million year Cambrian Explosion.