Final answer:
The theory of biological evolution claims that species evolve over time through natural selection, which favors survival and reproduction of organisms with beneficial traits, explaining both the unity and diversity of life.
Step-by-step explanation:
Two very bold claims made by the theory of biological evolution are that species change over time and that evolution occurs by natural selection. The first idea posits that life on Earth has evolved as different organisms descended from common ancestors, leading to a vast diversity of species we see today. The second contends that through the process of natural selection, traits that improve an organism's chances of survival and reproduction become more common in a population over time.
Natural selection is one of the principal mechanisms driving evolutionary change and it underscores the survival and reproduction of individuals with beneficial traits. Mutations introduce variation, and natural selection works on these variations to shape species across generations.
The result is a unified explanation for the diversity and unity of life. Thus, life is not static but constantly changing, adapting through gradual changes over long periods, a concept supported by vast amounts of biological and geological evidence.