Many of the apparent design flaws in our bodies owe their existence to the fact that natural selection is a process of differential survival of alternative phenotypes that compete with each other at a given place or time. The better of the two wins no matter how bad it is in absolute terms. Thus, when organisms move to a different environment, natural selection favours those in which small mutations make them even slightly better adapted than the others.
Natural selection simply makes the best of a bad job.
In summary, our bodies are frail and faulty and prone to disease because natural selection may have taken certain historical trajectories, because natural selection also simultaneously works on behalf of disease-causing pathogens, because there is a mismatch between our biology and our culture, because bad genes can have some good effects or because there are often trade-offs that limit its ability to perfect several functions simultaneously.
Related on the SLP
For an evolutionary biologist, this is exciting basic science dazzling us with its sheer beauty and logic. But there can be more. The promise of evolutionary medicine is that viewing our bodies as having evolved by natural selection and understanding the reasons behind health and disease from an evolutionary perspective will strengthen medical science.
