1. Selection can act only on existing variations. New advantageous alleles do not arise on demand.
2. Evolution is limited by historical constraints. New structures aren't formed, but instead evolution adapts to the existing structures.
3. Adaptations are often compromises. We can be versatile and athletic but our limbs are prone to sprains.
4. Chance, natural selection, and the environment interact. Alleles are sometimes "left behind" or the organism lives in a constantly changing environment