oh my friend i am not intelligent enough to give a coherent yet understanding answer so i will give myself the easy way out and say please look it up lol

yes terrible answer but i apologize

There are certain Single nucleotide polymorphisms (snips), which is a single genetic code passed down generations by mutation, that are carried over from individual to individual that scientists can now “track”

If that environment which an individual confides in were to remain constant for over a long period of time then the beneficial mutations are given time to spread and become the dominant trait. Blue Eyes are an example

After scientists discovered the genetic analysis of humans from thousands of years ago, they’re able to measure how much “genetics” are passed down

basically what i’m saying is that there’s a smaller amount of “snips” being passed around this day and age