RICE is the staple of South Korean diet while CORN is a staple of the US diet. These preferences for food in these two countries are influenced by historical and social factors. Most of the food in South Korea are usually prepared from rice while in the US most of the staple food are prepared from corn.
I'm assuming this is the same image that was posted before, therefore the answer is "gene flow" which occurs through population groups by natural selection and other factors.
