Evolution is simply generation of diversity and the shaping of the diversity by environmental selection. It is not as if the newly generated species are in any way better than the older one. It is just natural selection and genetic drift have together led to the formation of a population that cannot reproduce with the original one, as in case of the evolution of humans and chimpanzees from a common ancestor.
In evolution the new forms evolved are more complex than their ancestors. It is the adaptability of a species to the environment that supports its survival not the complexity of the species. Each species, whether complex or simple is subjected to natural selection. Each species has to go through the process of natural selection to survive and reproduce. In evolutionary terms, we cannot say that a particular species has a better design than another. Each species is well suited and adapted to its environment and hence is good enough to live and reproduce.
