I believethat evolutionary change is generally gradual unless it is catalyzed by a supernatural event such as the meteor that caused the extinction of the dinosaurs ora random beneficial mutation. Change is generally gradual because it is a stepwiseprocess that involves species evolving with small variations over millions ofyears rather than large variations over a short amount of time.
The factorsthat might lead to slow change of flora would be environmental conditions suchas carbon dioxide levels or formations of mountain ranges. Millions of yearsago the atmospheric carbon dioxide levels were much higher than they are todaywhich is why broadleaf plants had lower stomata density and larger stomata whencompared with broadleaf plants today. As the atmosphere changed and carbondioxide became a less bountiful gas, the broadleaf plants slowly evolved tohave a higher stomata density and smaller stomata in order to be able tocompensate for the lower bioavailability of carbon dioxide. It can be verifiedthat this change was slow because we are still able to find broad leaf plantfossils indicating that the change occurred over a long enough period to allowthe leaves to become fossilized. The formation of the Cascade mountain rangecreated a difficult barrier for rain clouds to pass over due to sheer weight sothat rain must be shed prior to passing over the mountain range. As a result,this rain shadow effect lowers the amounts of water that will beavailable for plants which over millions of years caused the size of plants andtrees to decrease in order to ease survival with the amount of water theyreceived. Spermophilus columbianus, lateralis and saturatusall have similar tail lengths that account for roughly a third of their total bodylength, small ears relative to their body sizes and sharp claws.
Differencesamongst species include the shorter body lengths and claws that Spermophilus lateralis and saturatus possessas well as their notable stripes running the length of their bodies’. Conversely Spermophiluscolumbianus has the longest body length of the three, long claws fordigging, a flat, fluffy tail and no stripes on its body. These squirrel specieslikely had the same predators because they were all ground dwellers in asimilar area which could account for the shared similarities.
The formation ofthe Okanagan River likely lead to the homology between the three Spermophilus species. I hypothesize thatwhen the river formed, the species could no longer traverse the river and wereconfined to the whichever side of the river they ended up on and this affectedthe evolution of the claws amongst species. The Spermophilus saturatus ended up on the West side of the Okanaganwhere it used its’ short claws to run from predators on the ground just like its’relative Spermophilus lateralis whichended up on the East side of the Okanagan.Spermophilus columbianus ended up in arid central Washington where it usedits’ longer claws to dig burrows that enable it to hide from predators in alandscape that does not offer very much foliage cover.