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Table 1 Principles of natural selection generally applicable to understanding human biology in health and disease.

From: Evolutionary explanations in medical and health profession courses: are you answering your students' "why" questions?

Natural selection:
• cannot build perfect designs because it compromises between different adaptations – bipedal walking vs. large brain size.
• will maintain a disease gene if it confers an advantage in a particular environment – sickle cell disease, alpha- & beta-thalassemia, Hb E syndrome, G6PD deficiency, Tay-sachs, cystic fibrosis.
• has shaped many human genes to ancient lifestyles (i.e. a hunter-gathering versus modern life-way) explaining chronic diseases like obesity, and type II diabetes.
• favors genes maximizing reproduction even if they compromise health (PKU, hemochromatosis, fragile X syndrome etc.) or longevity (Alzheimer's, atherosclerosis, prostate hyperplasia)
• explains the "arms race" between pathogens and us. We have evolved defenses both natural – fever, diarrhea, vomiting, inflammation – and manufactured – antibiotics. Pathogens evolve counterstrategies like antibiotic resistance, and manipulation of our defenses for their spread.