 After computation and variation, the third postulate of the theory of natural selection is survival of the fittest. So Darwin he observed that when the resources they are limited in any environment, so there would be existence and existence would be a constant and the continuous struggle. So all the individuals present in one ecosystem, in one environment, they are in continuous struggle to survive. And as the size of the population increases, more pressure on resources, resources are consumed by the populations and then there would be a tough competition to survive. And those individuals which may die in this process of competition, in this struggle for survival, these are those individuals who Darwin proposed, he recent and he proposed that they are those individuals who eliminate from any population because they don't have good variations, they don't have stronger variations to survive in that tough, competitive environment. So what he proposed that through the process of reproduction there would be new variations in same traits among the individuals of same species. And these those individuals which have the better traits and they are strong enough to survive in a highly competitive environment, they will reproduce, they will survive and they will reach the age of adulthood to reproduce further. So there are more chances for those individuals having better and stronger variations to survive. But those individuals didn't make any strong or good variations, they are not able to survive in tough environment or they don't have ability to adapt themselves in the new environment, they will be eliminated by nature. So he said this is a survival of the fittest, so the fittest would survive but the weak will be eliminated. And this is called as natural selection because nature is selecting those good variations, those individuals which are more able and stronger to survive in the changed environment. But those who can't adapt themselves, who can't fight and compete, they will be eliminated by the nature. That was natural selection. Fourth postulate of Darwin's theory was transfer of these acquired characters or these adaptive traits to the next generation. So once these adaptive traits they are acquired through the process of reproduction which we now we know this is because of the process of meiosis mutations crossing over these adaptive traits are perpetuated in subsequent generations. But those organisms which are not very well adapted or they are maladaptive these individuals are less likely to reproduce and survive. So the populations with the maladaptive traits, they would be less frequent in population and eventually they will be eliminated by the process of natural selection. So the change in environment, mostly they could be in the form of climate, temperature changes, weather changes, availability of food, availability of water and then there are other stressors for example predators, there could be a parasite, could be disease. There are many other stressors which are making situation difficult for those species present in that habitat and it could lead to the extinction of those individuals which are ill adapted species. But at the same time that was the theory of natural selection and it was supported by the other contemporary evolutionist, other contemporary naturalist. So Alfred Russell Wallace, he was also a co-discoverer of theory of natural selection. Just a Darwin made a different collection scheme or different observations scheme At the same time Alfred Russell Wallace was also working in Amazon and Malay Archipelago and he also proposed the same two points which Darwin proposed and the one was the competition that there is population pressure on the resources so there will be competition between individuals. He also proposed the concept of variations like Darwin proposed that with the passage of the time when an environment changes there would be variations in response to these environmental changes. But at the same time where Alfred Russell Wallace's theory is similar to Darwin's theory but it has certain differences as well. For example Alfred Russell Wallace, he proposed that every adaptation which appears in the next generation that is because of certain change in the environment. So he said that adaptive radiations are always the result of changes in the environment. But Darwin proposed that every change or every adaptive radiation which appears in the next generation or we can say the variations which are appearing in the next generation it is not necessary that its driving force is an environmental change in which the individual has to bring some changes in his genetic make up. So that is the theory which is more closer to the modern information because now we know that in the next generation, in the next generation, in the next progeny, the changes which are not similar to which are not exactly like their parents it could be because of mutation, it could be because of crossing over not necessarily every time that environmental stress, environmental change would be the bias of bringing such changes. So that was the theory of natural selection which changed that study of evolution and it is still considered as the foundation of evolution.