There is no need to discuss "breeding" in this context at all. The study is over 25 years, a single generation. The germline of any species does not change that

There is no need to discuss "breeding" in this context at all. The study is over 25 years, a single generation. The germline of any species does not change that quickly. Lots of things in the environment have changed in the last 25 years, though, correlated with any number of things (e.g. obesity.) It will take a while to establish the cause. All the glib hot-takes on HN about "chemicals" and "plastics" might turn out to be a part of the cause, but we will see.