_ are the remains of members of species that are ancestors to modern species:

Explanation

The correct answer is Fossils. Fossils are the preserved remains or impressions of organisms from the geological past, typically found in sedimentary rock layers. They are crucial for understanding the evolutionary history of life on Earth as they provide direct evidence of past life forms and their development over time. The other options refer to various evolutionary concepts but do not represent physical evidence: a Population refers to a group of living organisms in a specific location; Vestigial organs are reduced or unused structures that hint at evolutionary changes; and Homologous organs illustrate shared ancestry but do not serve as physical remains of species.