To explain this it is important to know the meanings of some concepts like allelles which is one of two or more alternative forms of a gene that arise by mutation and are found at the same place on a chromosome. So in other words the concept that the paragraph is referring is about the descendancy that can be traced by these alleles.
In other words, the paragrapgh states that some individuals when having more children leave more genetic load in subsequent generations and this subsequent load is illustratin in part what is called genetic drift which is many times a random effect on biological populations. Its effect is to remove genetic variation from a population of living organisms. In natural populations there are a number of forces acting. There are forces which add heritable variation to the population, such as mutation and recombination.
