->Natural selection- When a trait offers survival advantage to a population it is called as natural selection.
->Genetic drift - When there is a sudden change in the frequencies of genes in a population accidently it is called as genetic drift.
->Mutation- sudden heritable change in the gene.
->Gene flow - It is the exchange of genetic material by interbreeding between two populations.
A particular species have many individuals different from each other in one or more context like some fruit flies have dark brown colour whereas some have light brown colour. Only one of these is able to reproduce, for example light brown is able to reproduce and it produce many offspring of light colour that result in more amount of light coloured fruit flies and less amount of dark coloured. Under stress conditions, the population of light coloured will survive because they have large number of individual and on of them will eventually undergo a mutational process that make it able to adapt that stress condition where as dark coloured one will not survive because they have very less individuals. As a result the one that is able to survive will undergo adaptational changes and produce offsprings carrying same changes and result in the production of new species.