In that Unit you also learnt about Darwin, who based on his extensive observations and interpretations showed that evolution is a simple fact of nature. It was Theodosius Dobzhansky, a population geneticist and one of the trinity to structure the modern synthetic theory of Evolution, who emphatically stated, "Nothing in Biology makes sense, except in the light of evolution". Today the Darwinian concept of evolution through natural selection has stood the test of time. As Julian Huxley rightly pointed out "Darwin rendered evolution inescapable a,s a fact, comprehensible as a process and all embracing as a concept". In Unit 10 of this Block you were cited several examples from studies on palaeontology , biogeography, anatomy, embryology, physiology and biochemistry as evidence in support of the evolutionary process. In this Unit we will more specifically discuss the mechanism of evolutionary change. We will elaborate on the Darwinian concept and look into its basic tenets. There will be a detailed discussion on the sources of variability, since variability is the raw material on which natural selection acts to produce adaptations. We will also explain the concept of Darwinian fitness and relate it to the reproductive success of organisms. Finally you will also become familiar with the different ways in which selection can act. In other words, this Unit will highlight that natural selection promotes structural, functional and ecological adaptations of individuals, populations and species to their existing environments.