Can you explain the theory of evolution by natural selection as presented by Darwin? Please Help. :)?

Natural Selection explained by Darwin is Natures Tendency to produce genetic variability(Variety) in offspring through mutations. This results in organisms that are better or worse at adapting to their environment and the most important part of natural selection is that the species with the best mutations will be the ones most "Fit" to reproduce and account for the longevity of their species. The physical differences yet vast similarities showed Darwin that these Animals were experiencing all sorts of Natural Selection as result of isolation from the main land and competition for resources and survival. The Marine iguana is a perfect example in the Galapagos islands. Because they are the only iguanas in the world that dive underwater for food. This is partly because on these small islands food is minimal on land but incredibly abundant in water, they eat algae. During times of el nino, iguanas are known to shrink in size due to the low amount of food. This is an ability given to them by nature in order for them to survive. They grow back to full size when food is plentiful.