What is the primary theory that explains how species change over time?

The primary theory that explains how species change over time is natural selection. This theory, first articulated by Charles Darwin, posits that individuals within a species exhibit variations in traits and that these variations can affect an organism's ability to survive and reproduce in a given environment. Over time, traits that enhance survival and reproductive success become more common in the population, leading to gradual changes in the species. This process is often summarized by the phrase "survival of the fittest," emphasizing that the environment plays a crucial role in shaping which traits are advantageous.

Natural selection is rooted in observable phenomena, such as genetic variations and competition for resources, making it a robust explanation for how evolutionary processes operate. Unlike catastrophic events or the slow and gradual changes suggested by uniformitarianism, natural selection focuses on the mechanisms driving evolution through differential survival and reproduction of organisms. This concept is fundamental to the modern understanding of biology and evolution, providing a cohesive framework for interpreting the diversity of life on Earth.