The scientific theory that explains the origin of species is the theory of evolution by natural selection. Proposed by Charles Darwin in the 19th century, this theory posits that species evolve over time through a process where individuals with favorable traits are more likely to survive and reproduce. Over generations, these traits become more common in the population. Natural selection helps explain the diversity of life on Earth and has substantial evidence from various fields including genetics, paleontology, and comparative anatomy. This theory remains a foundational concept in biology, providing insight into how species adapt to their environments.