Darwin investigated the diversity of species by observing the physical differences between them. He concluded that all the species came from the same ancestor in common. And that the evolution is given by natural selection (or adaptation to the environment). Only specimens more adapted to the environment could survive, reproduce and pass their genetic traits to their offspring.
For example, the long neck of the giraffes is due to the fact that only the highest ones reached the leaves of the trees for food and with that they managed to survive.
As a majority of soils in tropics are of the Alfisols and the Ultisols. These soils are old and have lower fertility, but due to the presence of the dry season, the nutrients can stay in place. In the tropical rain forest, most of the time have sufficient rains but the soil lacks organic matter as they fast decomposition processes.
