Major changes reduce the fitness of the population. The population has stocks of such alleles that do not bring it any benefit at this time; they remain in a heterozygous state. But when, as a result of changing conditions, they suddenly turn out to be useful, their frequency under the influence of selection begins to increase, and ultimately they become the main genetic material.
Remember: The process of learning a person lasts a lifetime. The value of the same knowledge for different people may be different, it is determined by their individual characteristics and needs. Therefore, knowledge is always needed at any age and position.