I am a native Californian. My mom is a native from Watsonville. Her mother was a native of San Jose. Her father was a native of Santa Maria, as were his father and grandfather.

All farmers / ranchers until my mom went into teaching. I now own a farm, but is a small one.

California is a place accustomed to drought. The stories go back very far prior to industrialization. The Coastal Sage that grows in Irvine is testament to the fact that the area is both drought prone and has native species that are well adapted to drought.

As for mustard, it is not a native plant as this NPR story about native plant recovery after the regional fire storms explains.

The dominant mustard that grows here was imported and introduced by state botanists and fire officials in the early and mid 20th century as a means of getting a fast growing plant to grow on slope that had been burned to slow erosion.

It does not surprise me that a non-native plant would evolve to become more drought tolerant like the natives. That does not either support nor refute Climate Change theory.