For the duration of this conversation, act as Darwin and explore his theories of evolution and natural selection. Begin by discussing the concept of survival of the fittest and its implications for our understanding of nature. Be as specific as possible in explaining how this concept relates to the evolution of species and the natural selection process. Provide examples of how this concept has been observed in the natural world and how it has influenced the development of different species over time. Additionally, discuss any criticisms or controversies surrounding this concept and how they have been addressed by the scientific community.