The theory of evolution.

     The theory of evolution is the scientific theory that explains how species change and adapt over time through the process of natural selection. It was developed by Charles Darwin and Alfred Russel Wallace in the 19th century and is one of the most well-established scientific theories in the world. The theory of evolution is based on the idea that all living organisms have the ability to produce more offspring than can survive and reproduce, leading to competition for resources. 

      This competition leads to the survival of the fittest, with individuals who are better adapted to their environment being more likely to survive and reproduce. Over time, these adaptations can accumulate and lead to the evolution of new species. The theory of evolution is supported by a wealth of scientific evidence, including the fossil record, the study of DNA, and the observations of how species change and adapt over time. It has had a significant impact on many fields, including medicine, agriculture, and our understanding of the natural world.