Historical: The Jaredites were in the New World at a time and place to correspond to the people who have been called the Olmec. By the time Alma is speaking, the Olmec culture as a separate political entity has been destroyed, though there are clear connections to Olmec ideas apparent in the post-Olmec Mesoamerican world. The destruction of a people in the Book of Mormon does not necessarily indicate the eradication of genetics, but rather of polity. The political/religious unification of a people has been removed, and those people who lived under that polity are now absorbed under a new one.
The people remain, the organization and the “nation” is gone from around them. It is rather similar to the question so frequently asked on television shows investigating the “unusual:” “where did the Maya go?” The answer, of course, is that they didn’t go anywhere. They are still here, still speak the traditional languages, still have the same genetic makeup. What was lost was the overarching cultural content of the Maya city states. Similarly, the Jaredites were destroyed, but there were certainly Jaredite descendants living.