Final answer:
Organisms change over time through evolution, which entails genetic variations and natural selection acting over many generations to produce new, often beneficial traits that are passed down, leading to significant changes in species over time.
Step-by-step explanation:
For organisms to change so drastically over time that descendants are completely different from their ancestors, a process called evolution must occur. Evolution involves changes in the genetic makeup of organisms, allowing new traits to arise and become predominant over many generations through natural selection. This selection favors organisms with beneficial traits, enabling them to produce more offspring and pass those advantageous traits on. This process is typically very slow, often spanning thousands to millions of years, resulting in the diverse forms of life we see on Earth today.
Evolution is an ongoing process characterized by genetic variation and the adaptation of organisms over time to environmental pressures. Mutations can introduce new traits, some of which are advantageous and increase an organism's reproductive success. These beneficial characteristics are then passed to subsequent generations, gradually shaping the species as the environment changes. Charles Darwin's theory of natural selection provided the foundational concept that explains how evolution can lead to significant changes in a species over vast timescales.