How does evolution occur?

Hint: Evolution is the modification in the characteristics of a species. It happens over several generations. There is various evolutionary evidence like analogous and homologous organs, fossils, etc which support the theories of evolution.

Complete answer:
Evolution is a theory which explains that all species are related and gradually adapt various characteristics over time. Evolution is the change in the inherited traits of a population from generation to generation. Offsprings inherit traits from their parents but they also differ in several aspects because of genetic recombinations and mutations. These changes in the offsprings can be beneficial or detrimental. So within a given population, advantageous traits become common and unhelpful ones disappear. New traits can also come from the transfer of genes between populations via migration, or between species via horizontal gene transfer. These traits lead to the development and appearance of new species.

Darwin's Theory of natural selection is the process by which traits that enhance survival and reproduction become more common in successive generations of a population. Harmful traits become more rare and even extinct. Organisms produce more offspring than actually survive. Every organism must struggle to survive. There is survival of the fittest meaning that only organisms which can adapt to change and variation in the environment can survive. There is variation within a species. Genetic drift produces random changes in the frequency of traits in a population. Various geographical and topographical conditions can also separate populations. These causes variation after some time which ultimately helps the organisms to adapt to the new environment. The theory of evolution provides a unifying answer to the diversity of species present.

Note: The variety and diversity of all organisms present in this plant is a result of evolution. The theory of evolution was proposed and postulated by Charles Darwin and Alfred Russel Wallace.