For Darwin, only natural selection could explain how "organs of extreme perfection and complication" can arise through a process other than design.
Previously, the exquisite perfection of organs such as the eye had been used as an argument against evolution -- eg. William Paley and the "argument from design. (see teleology.)
In his book On the Origin of Species by Means of Natural Selection (1859) Darwin argued that because "variations useful in some way to each being in the great and complex battle for life" do occur sometimes in thousands of generations in the wild, and because "many more individuals are born than can possible survive," we cannot doubt that "individuals having any advantage, however slight, over others would have the best chance of surviving and of procreating their kind," while "any variations in the least degree injurious would be rigidly destroyed." It is, then, the "preservation of favourable variations and the rejection of injurious variations" that "I call Natural Selection." (from: M.J.S. Hodge, "Natural Selection: Historical Perspectives" in Keywords in Evolutionary Biology, p.212)
The key feature of natual selection is relative reproductive success. Variation in the reproductive success of one individual relative to another is essential for natural selection to occur. In J.B.S Haldane's classic calculations (1927) a variant that produces 1% more offspring than its alternative allele would increase in frequency from 0.1% to 99.9% of a population in just over 4,000 generations. In contemporary usage, natural selection is defined as a process that occurs if and only if these three conditions are present: 1. The population has variation among individuals in some attribute or trait ( phenotypic variation). 2. There is a consistent relationship between that trait and mating ability, fertilization ability, fertility, fecundity and / or survivorship (fitness variation). 3. There is a consistent relationship, for that trait, between parents and their offspring, which is at least partially independent of common environment effects (inheritance.)
Male and female humans pursue different reproductive strategies. Theoretically, males compete for fertilisations, trying to inseminate as many females as possible. There is a strict limit, on the other hand, to how many times a female benefits from insemination. Her reproductive success depends not on number of fertilisations but on the contingencies of her life, the qualities of the mates she chooses, and, above all, how successful she is at keeping alive such infants as she does produce. (Sarah Blaffer Hrdy, Mother Nature, p. 83)
Evolution is a historical process, that includes both the origins as well as the spread of new variants or traits. Evolution may include other effects than natural selection -- eg. changes in environment, genetic drift, "accidents" etc. whereas natural selection in itself is a non-historical process.
For Stephen Pinker, "natural selection is the only scientific explanation of adaptive complexity..." (the characteristic of) "any system composed of many interacting parts where the details of the parts' structure and arrangement suggest design to fulfill some function." As Sarah Hrdy reminds us, this fit is primarily relative. "Never permitted the luxury of starting from scratch to produce the perfect solution, natural selection recycles workable solutions for a "good-enough" fit, meaning simply: better than the competition." (Mother Nature, p. 104)
"Natural selection, while feeding on variation, uses it up."