Nothing is Perfect in Natural Selection

Evolution Is Limited By Historical Constraints All species have a legacy of decent with modification from long lines of ancestry. Evolution co-opts already existing structures and then adapts them to the new situations. Evolution does not, on the other hand, scrap ancestral anatomy and build a new complex structure from scratch.

Adaptations are Often Compromises Each organism must posses the ability to do many different things. An example of this would be in humans. Our flexible limbs and prehensile hands help with our versatility and athleticism. This also makes us more prone to dislocations and sprains. Reinforcement in our structure has been compromised for agility.

Chance and Natural Selection Interact Chance in events can affect the evolutionary history of a population. For example, when a storm blows birds and insects miles and miles across the ocean to another island, the wind doesn't really transfer the species that are best suited for the environment. Because of this, not all alleles that are present in the founding population's gene pool are suited better for the new environment than alleles left behind.

Selection Can Edit Only Existing Variations Natural selection only favors the fittest phenotypes among those in the population, which may not be the ideal traits. Those new alleles do not arise on command.

SOURCE Campbell AP Edition Biology Textbook