Define evolution. How does it occur?

The gradual process of change in the heritable traits of populations over millions of years leading to the formation of new species is known as evolution. According to Darwin's theory of evolution, natural selection is responsible for the formation of new species over the course of time. The organisms which are best adapted to survive in the given environmental conditions are selected by nature and they reproduce. The variations that exist in these individuals are passed to next generation while the individuals with unfavourable variations perish. But the exact cause of these variations is a mutation which leads to speciation and evolution. Fossils are the remnants of the past organisms which get buried in sedimentary rocks. Different fossils are formed in the different aged rock sediments. The study of fossils existing in the different rock layers indicate the time period when they existed. Thus different life forms have been formed at different times in the history of the earth. Fossils indicate how the evolution took place leading to the formation of complex life forms from simple ones by the gradual process. For example, fossil bird Archaeopteryx is considered as a connecting link between reptiles and birds indicating that birds evolved from reptiles. It shows the presence of characters of both birds and reptiles like the presence of teeth, wing feathers, clawed toes and tail.