Evolution and natural selection apply and influence all organisms at all times, and so do selective pressures like sexual selection, which is natural selection driven by the preference of specific traits in a mate. Sexual selection is determined and influenced by a number of factors, including a number of ornaments which typically a male will develop to attract female offspring, such things include the array of colourful feathers a male peacock displays to find a mate.
There are several reasons why a female or potentially a male, would find these features attractive, like the idea that a reproductive ornament handicaps the individual possessing it, which makes it harder for that organism to survive, so the organism would have to have an excess of positive traits in that environment to make up for the handicap and survive; this notion is derived from the handicap principal. Another potential explanation for the development of these reproductive ornaments, is that the presence of these ornaments displays the health, good metabolism, and ability to fight diseases in the owner of the ornament; this idea is present in the good genes hypothesis. Despite being explained by different ideas, the presence of ornaments related to sexual selection is said to be an honest indicator of health and fitness in a mate.
Both the handicap principle and the good genes hypothesis beg the question: why is it almost always the females of each species which conduct most of the sexual selecting, and why is it most often the males which develop ornaments displaying their health and viability as mates? The answer to this question is simple enough, it involves the amount of care and time which a gender of an organism typically puts into supporting their offspring. In poison dart frogs, for example, where typical roles are reversed (ie males take care of offspring and females compete for male mates) males are left behind to defend their eggs after mating, and take an active role in supporting their offspring. This causes males to spend more time with their offspring, which leaves them unable to mate for extended periods of time, which restricts the amount of available males, females have to reproduce with, which essentially creates female competition for the limited number of males left. This puts the males in a position to simply choose the healthiest mate available to them, which is why females in this case would develop ornaments to display their fitness. Even though this example uses reversed roles, the same idea can be applied where males compete and females choose mates.
So members of competing gender develop ornaments to display their fitness, but how do these ornaments manifest themselves in humans, as we don’t have large colourful feathers, or large tails. While human relationships are based on a lot more than physical beauty (hopefully) some universal indicators of attractive human mates are: symmetry, which is supposed to indicate a lack of DNA damage, damage in the womb, or unhealthy habits like alcohol use. The smell of a potential partner is also supposed to indicate immune system health, where complimentary immune system traits are deemed the most attractive. Body shape, where a waist to hip ratio of 7:10 is said to be most attractive in women and waist to hip ratio of 9:10 is deemed most attractive in males. These along with a variety of other characteristics including finger length and even an average of features are said to be determinants of attractive mates in humans.
Despite all of these sexual ornaments and selection using the handicap principle or the good genes hypothesis, there cannot be a perfect organism because natural selection and evolution operate on the scale of individual organisms. This is the case because evolution doesn’t create perfect organisms, it only creates organisms which are good enough in that environment to survive and reproduce. Also because natural selection operates on the scale of the organism, and organisms contain a multitude of various alleles, both positive and negative, and organism with negative alleles can still survive long enough to reproduce and thereby pass on its negative alleles, if they have enough positive alleles to suit the environment. So even natural selection which is the very mechanism by which organisms which don’t suit the environment are selected against, and organisms which do suit the environment, live until they can reproduce and therefore pass on their traits, can still allow negative genetic traits to be passed on which further supports the notion that no organism, including humans can be perfect.