Heritability in a population decreases if natural selection acts in the same direction for many generations because natural selection reduces the genetic variability within the population. When selection consistently favors certain traits or characteristics, individuals possessing those traits have higher reproductive success and pass on their genes to the next generation at a greater rate. Over time, this leads to an increase in the frequency of the favorable alleles in the population.
As the frequency of these favorable alleles increases, the genetic variability in the population decreases because fewer alternative alleles or genetic variants are being passed on. This reduction in genetic variability reduces the potential for future evolutionary changes within the population.
Heritability is a measure of the proportion of phenotypic variation in a trait that can be attributed to genetic factors. If there is less genetic variability in the population due to consistent selection, the proportion of phenotypic variation that can be attributed to genetic factors decreases. As a result, heritability decreases.
In other words, when natural selection acts in the same direction for many generations, it leads to a reduction in genetic variability and limits the potential for genetic differences to contribute to phenotypic variation. Consequently, heritability decreases as genetic factors play a smaller role in explaining the observed variation in traits within the population.