Evolution is an ongoing experiment in which random mutations are selected based on the effects of the resulting phenotypes on reproductive success. As we have discussed, various non- adaptive processes are also involved, which can impact evolutionary trajectories. The end result is that adaptations are based on past selective pressures and i) are rarely perfect and ii) may actually be outdated, if the environment the organisms live in has changed. One needs to keep this in mind when one considers the differences associated with living in a pre-technological world on the African savannah in small groups and living in New York City. In any case, evolution is not a designed process that reflects a predetermined goal but involves responses to current constraints and opportunities - it is a type of tinkering in which selective and non-selective processes interact with pre-existing organismic behaviors and structures and is constrained by cost and benefits associated with various traits and their effects on reproductive success106. What evolution can produce depends on the alleles present in the population and the current form of the organism. Not all desirable phenotypes (that is, leading to improved reproductive success) may be accessible from a particular genotype, and even if they are, the cost of attaining a particular adaptation, no matter how desirable to an individual, may not be repaid by the reproductive advantage it provides within a population. As an example, our ability to choke on food could be considered a serious design flaw, but it is the result of the evolutionary path that produced us (and other four-legged creatures), a path that led to the crossing of our upper airway (leading to the lungs) and our pharynx (leading to our gastrointestinal system). That is why food can lodge in the airway, causing choking or death [→]. It is possible that the costs of a particular "imperfect" evolutionary design are offset by other advantages. For example, the small but significant possibility of death by choking may, in an evolutionary sense, be worth the ability to make more complex sounds (speech) involved in social communication107.
As a general rule, evolutionary processes generate structures and behaviors that are as good as they need to be for an organism to effectively exploit a specific set of environmental resources and to compete effectively with its neighbors, that is, to successfully occupy its niche. If being better than good enough does not enhance reproductive success, it cannot be selected for (at least via natural selection) and variations in that direction will be lost, particularly if they come at the expense of other important processes or abilities. In this context it is worth noting that we are always dealing with an organism throughout its life cycle. Different traits can have different values at different developmental stages. Being cute can have important survival benefits for a baby but be less useful in a corporate board room (although perhaps that is debatable). A trait that improves survival during early embryonic development or enhances reproductive success as a young adult can be selected for even, if it produces negative effects on older individuals. Moreover, since the probability of being dead (and so no longer reproductively active) increases with age, selection for traits that benefit the old will inevitably be weaker than selection for traits that benefit the young, although this trend can be modified in organisms in which the presence of the old can increase the survival and reproductive success of the young, for example through teaching and babysitting. Of course survival and fertility curves can change in response to changing environmental factors, which alter selective pressures. In fact, lifespan itself is a selected trait, since it is the population not the individual that evolves108.
We see the evidence for various compromises involved in evolutionary processes all around us. It explains the limitations of our senses, as well as our tendency to get backaches, need hip-replacements, and our susceptibility to diseases and aging109. For example, the design of our eyes leaves a blind spot in the retina. Complex eyes have arisen a number of times during the history of life, apparently independently, and not all have such a blind spot. We have adapted to this retinal blind spot through the use of saccadic eye movements because this is an evolutionarily easier fix to the problem than rebuilding the eye from scratch (which is essentially impossible). An "intelligently designed" human eye would presumably not have such an obvious design flaw, but because of the evolutionary path that led to the vertebrate eye, it may simply have been impossible to back up and fix this flaw. More to the point, since the vertebrate eye works very well, there is no reward in terms in reproductive success associated with removing the blind spot. This is a general rule: current organisms work, at least in the environment that shaped their evolution. Over time, organisms that diverge from the current optimal, however imperfect, solution will be at a selective disadvantage. The current vertebrate eye is maintained by stabilizing selection (as previously described). The eyes of different vertebrates differ in their acuity (basically how fine a pattern of objects they can resolve at what distance) and sensitivity (what levels and wavelengths of light they can perceive). Each species has eyes (and their connections to the brain) adapted for its ecological niche. For example, an eagle see details at a distance four to five times are far as the typical human; why, because such visual acuity is useful in terms of the eagle’s life-style, whereas such visual details could well be just a distraction for humans110.
106 Evolutionary tinkering: http://virtuallaboratory.colorado.ed...nTinkering.pdf
107 How the Hyoid Bone Changed History: http://www.livescience.com/7468-hyoi...d-history.html
108 Methusaleh's Zoo: how nature provides us with clues for extending human health span: http://www.ncbi.nlm.nih.gov/
pubmed/19962715 and Why Men Matter: Mating Patterns Drive Evolution of Human Lifespan: http://www.plosone.org/article/info%...l.pone.0000785