Let me say this topic is pure science. I had to read it over again to be be able to understand it properly. Hence I agree with you on this;
The variation between major classificatory units are all heritable and are transmitted from one generation to the next, and it is for this reason that each group maintains its identity. The science of genetics has been built largely on variation that exists at the sub-species level. In fact, the bulk of our knowledge in genetics comes from differences that exist between individuals as species. This has been so for some reasons.
SIncere apologies for I could not find simpler words to use for non-science folks to be able to understand. Thanks for reading through all the same :)