Biology STAAR Practice Test 2025 - Free State of Texas Assessments of Academic Readiness Practice Questions and Study Guide

The theory of evolution emphasizes that organisms change over time through processes such as natural selection, which is a key mechanism of evolution. This means that populations of organisms can evolve as advantageous traits become more common in successive generations, allowing them to better adapt to their environments. Over long periods, these changes can lead to the development of new species.

Natural selection operates on the genetic variation present within a population; those individuals with traits that confer a better chance of survival and reproduction are more likely to pass those traits on to the next generation. This gradual process contributes to the diversity of life forms we see today and explains the adaptation of organisms to their environments.

The other options do not accurately represent the principles of evolution. For instance, the idea that organisms remain unchanged over time contradicts the fundamental concepts of evolution. Similarly, while organisms can indeed change during their lifespan, this is not what the theory of evolution focuses on; it looks at changes across generations. Lastly, while environmental adaptations are important, evolution encompasses a broader range of mechanisms, including genetic drift and mutation, not solely limited to environmental factors. Thus, option C captures the essence of evolutionary theory accurately.