Evolution means the gradual change that helps any organism adapt in the environment around them whereas natural selection means the species or organism that fights with the natural consequences and hence comes out to be the fittest one.
When we talk about natural selection, the first name that strikes our mind is Darwin. Yes, he was the first person who clearly gave theories and examples that connects nature with the evolutionary process. When we go through textbooks, the concept of natural selection can be bit confusing due to elaborated explanation but in the actual view, it is a very simple process to understand.
There is many more mechanism of evolution, but natural selection is the basic among them that is given by Darwin. This concept of natural selection is divided into several facts and inferences. According to Darwin, nature have capacity to allow unlimited reproduction, i.e. every species or organism can reproduce in a large number and this is the reason that fish, frog and many more species overproduce the eggs but among this large number, nature supports only few. Among them only few survives whereas other dies. But the main question was how does some survive whereas others die? How does nature choose some and let go the others? He researched and looked for the answer when he found out that after birth, species struggle for several basic components like food, water, shelter, reproduction and many more. Therefore there is always a competition going on between the species for their needs and demands. Beside this he also observed many more of the facts. He noticed that every species that survives the natural consequences and fulfills their basic need possess some variation. These variations occur in almost all the species and are transferred from one generation to another generation. This variation will help offspring survive and compete. Beside this, they can reproduce in much more easier way. This variation will not only have advantage only for present generation but also for many more upcoming generations which will help to bring change in the population.
One excellent example for this variation can be figured out so as to understand the concept of evolution in broader way. In the population of green beetle and the population of brown beetle, green beetle have less advantage of survival than brown beetle due to its color which is easily noticed by the birds. Hence, green beetle being easily spotted are eaten by birds and slowly this variation of brown color becomes advantage for them to reproduce and survive in the population. Likewise, in the other example studied by Michael Fawcett on Tegula of northern and southern California; he noticed that the Tegula of northern California live in the deep sea because less predator are present there and they have no risk of losing life whereas in southern California due to large number of predator, Tegula lived up in the sea shore. He took Tegula from both the places i.e. northern California and southern California and dropped them in deep water. There he noticed that when Tegula encountered any predator around, it went up towards the higher level of the sea. But the amazing thing in this process is that the Tegula that were from southern California swam faster and much more easily than that of Tegula from northern California and hence Tegula of northern hemisphere became the prey for many animals that were present in the deep sea.
From this example we can clearly understand that when Tegula of different places were put together, fast movement and habit of defending or protecting themselves from the prey have became an advantage for Tegula of one place whereas lack of this character became disadvantage for the other. Now, the fast moving character was the result of either adaption to the environment or the genes that have being received from parental generation, but when there was competition between the two species of same kind, that particular character of fast moving became an advantage for the Tegula from sea of southern California and hence nature selected them and they survived and reproduced. Now, this variation after a period of time will be transferred to next generation thus benefiting them in the same or the similar way. As a result, fitness or advantage of survival and reproduction pave way for the evolution in an organism.
This way, evolution occurred as a result of natural selection and natural selection came into existence due to the variation of one species with the other.