Tahoe yellow cress is the only species in the Sierra Nevada that is restricted to a single lake and know one really knows why. While other Rorippa are associated with flowing water - inhabitanting rivers, streams, and meadow edges- Tahoe yellow cress is confined to the sandy beaches and dunes at the ever-changing margin of Lake Tahoe alone. Perhaps other lakes are simply too young to have fostered the development of a unique lakeshore plant. Lake Tahoe has existed for about two million years, never displaced by glaciers and never static in size, shape, or other essential qualities. Age and environment have thus conspired to create a singular species in a place as singular as the clear blue waters.

It is impossible to know exactly how Tahoe yellow cress came to be.  Its ancestor, probably from the Columbia River drainage, became stranded many hundreds of thousands of years ago in the ancient basin of the Truckee River beside an immense, glacial-fed lake. Perhaps seeds were carried to this basin by accident, or maybe the ancestral plant descended from a more widespread species that inhabited the Region during a distant geological period.  In either case, its arrival long preceded human occupation of the continent and its evolution was guided by endless cycles of rising and falling waters.

What were the peculiar environmental forces that molded Tahoe yellow cress into the species we see today?  While some have suggested the qualities of the beach sand as being important, there is no evidence that the granitic beaches are particularly unique in the Lake Tahoe basin.  Instead, the lake itself, especially its age, complex history, and dynamics were likely the primary force in the evolution of this remarkable little plant.  Great oscillations in climate caused glaciers to advance and retreat and lake level rose and fell, sometimes slowly and sometimes catastrophically over short periods of time.  Submerged tree trunks indicate lake levels 20 to 40 ft (6 to 12 m) lower in the recent past, but other evidence shows levels were more than 570 ft (175 m) lower over the last 160,000 years.

Imagine the potentially suitable habitat created by lake recession during the distant past. During low lake periods, millions of Tahoe yellow cress-like ancestors could have colonized vast, open areas near the retreating lakeshore.  With the return of high waters most established sites would be inundated and individuals lost, except for those upslope and those which were carried to new shores as floating seeds or rootstocks.  Such catastrophic changes in population size are known to have significant and rapid effects on gene pool composition, and could thus affect the rate and direction of evolutionary change.  Tahoe yellow cress would, as a species, reflect eons of these lake-driven fluctuations in its distribution and abundance.

With such a long history of rapid, unpredictable change it is remarkable that this plant has persisted.  Extreme climate change, extraordinary high waters, even landslides and lake tsunamis could have led to the extinction of Tahoe yellow cress, especially when it was composed of few, small, isolated populations.  This diminutive, unassuming plant has proven itself tenacious in its quest for existence, not only weathering the severe forces of Lake Tahoe for hundreds of thousands of years, but incorporating those forces into a unique physical and physiological form.