The natural selection is a necessary sub-process during evolution. Charles Darwin was the one who, together with Alfred Russell Wallace, presented the theory of natural selection as an explanation for the observed differences between species. Natural selection is often summarized by the phrase "survival of the fittest". However, it is a less successful description because natural selection is also about successful reproduction. The concept is that individuals with characteristics that make them suited to the current environment often survive and can reproduce, which means that these properties tend to be those that spread to future generations.
Sexual selection is the subset of natural selection related to reproductive success. Let’s pretend that an individual survives for hundreds of years for instance; this will be like an evolutionary dead end if the life doesn't lead to reproduction. Sexual selection is a mechanism that depends on the differences between individuals of the same sex in a population for characteristics that influence mating success. This gives the results that different individuals in a population may get different amounts of offspring.
An individual's reproductive success is measured by the proportion of the next generations gene pool and how much of this is consisting of the reproducers own genes. Genes behind certain behaviors and physical attributes that lead to greater reproductive success will increase in proportion over the generations. Sexual selection explains in this way both the intricate mating behavior and physical attributes that, at first glance may seem expensive or unnecessary. Sexual selection can explain why males and females look different or behave differently even though they live in the same environment and using the same resources.
The sexual selection is due to how the different sexes invest their time and resources in their offspring. The less time an individual invests in her offspring with a partner, the more offspring that individual may have with other partners. The sex that invests most time and effort in the offspring will be the limiting factor for the number of offspring produced in the population.
The evolution is because of this sexual selection so much more than "survival of the fittest", and it is important to see the whole picture in order to get a better view on how the evolution of certain species will be in the future. After all, the most important for any living creatures is to reproduce, this is to be implemented to us humans too; and with that in mind it can definitely put some perspective on our lives and the society we're living in.