As scientists have developed more productive crop varieties, farmers have switched from growing many traditional varieties to new high-yield varieties. For example, in Peru, the Indians once grew 500 native varieties of potatoes. That number has been reduced to about twenty major potato varieties, worldwide.

Switching from traditional crop varieties to new high-yielding varieties of potatoes will lead to a reduction in the varieties of potato as well as the potato gene pool.  

Explanation:

This switching led to the green revolution in many countries with increased production of crops. However, the traditional healthier varieties of crops are lost over the period and only the newer high-yielding varieties remain. When the genetic pool of a crop is downsized it leads to declining of the genetic diversity.  

A large gene pool will help to maintain the genetic diversity of a species to withstand environmental challenges and stresses. Smaller a gene pool becomes, greater are the chances for it become wiped out or extinct.  

Hence to maintain the gene pool of a crop and the genetic diversity, traditional varieties should be maintained.