Question is ambiguous:
Why do you think this happens
"This happens" could mean why is the plate different temperatures.  Heat conduction is the cause of the change in temperature.
Also, "happens" and "happened" aren't the same thing.  
Happens: why does the plate have different (or changes) temperatures?  Heat conduction.
Happened: why is the rad-facing side cooler?  I'm trying to trick you.

I’m glad you used the image of water flowing down-stream because water is how I often imagine natural selection.

If you have a small basin and put variously shaped blocks in it, water will fill the gaps between them. It will “adapt to its environment”.  Just like animals fills in “niches” in nature, so does the water fill the gaps in the basin.

Someone coming in later and seeing the shapes the water has taken could say “only an intelligent designer could have given the water these shapes” because they failed to see the blocks.