Darwin’s theory of evolution by natural selection starts with the difference that exists between the organisms within a species. The creatures of one origination are qualitatively dissimilar from one another. Development of the species entirely results from the fact that the different types varies in their rates of survival and reproduction. Improved modified types leave more offspring and hence the comparative occurrence of the types changes eventually. Therefore, the three significant components to evolutionary change Darwin stated are variation, selection and time.
Alfred Russell Wallace has reached a very analogous conclusion with an extract from Darwin. Wallace description was the wild animals have the life to struggle for survival, may be all the differences from the usual form of a species must have some exact effect, though slight on the behaviour of the individuals and also apparent that most modifications would influence either positively or negatively the controls of extending survival. But, on the other hand any species be supposed to produce a variety having somewhat enhanced controls of maintaining survival.
Darwin and Wallace proposed a new description to report for the occurrence of evolution. They recognized that the population of a given species at a given time involves individuals of different characteristics. They understood that the population of next generations will have an elevated frequency of those types that most effectively exist and reproduce under the obtainable environmental conditions. Therefore, the occurrence of different types within the species will change eventually.