Final answer:
Atlantic cod populations have adapted to overfishing by becoming sexually mature at younger ages, a genetic change caused by overharvesting and technology leading to the fishery collapse.
Step-by-step explanation:
The striking change that has occurred in the reproduction of Atlantic cod populations due to overfishing is that the populations have evolved to become sexually mature at a younger age than before. This is a direct consequence of selective fishing pressure, which can lead to genetic changes in life-history traits such as ages and sizes at maturity.
Overharvesting and the use of modern fishing technologies such as factory trawlers have led to the collapse of the Western Atlantic cod fishery, which was once highly productive. The resultant overfishing and the tragedy of the commons have not only decimated the cod population, making it a minor player in the marine ecosystem but also led to socio-economic impacts, including the loss of an essential food source and the livelihood of local populations dependent on fishing.