The answer to this question, in a word, is no. There are at least four reasons why natural selection cannot breed perfection.
1. Organisms are locked into historical constraints. As we saw in Chapter 20, each species has a legacy of descent with modification from a long line of ancestral forms. Evolution does not scrap ancestral anatomy and build each new complex structure from scratch, but co-opts existing structures and adapts them to new situations. For example, the excruciating back problems some humans endure result in part because the skeleton and musculature modified from the anatomy of four-legged ancestors are not fully compatible with upright posture.
2. Adaptations are often compromises. Each organism must do many different things. A seal spends part of its time on rocks; it could probably walk better if it had legs instead of flippers, but it would not swim nearly as well. We humans owe much of our versatility and athleticism to our prehensile hands and flexible limb, which also make us prone to sprains, torn ligaments, and dislocations; structural reinforcement has been compromised for agility.
3. Not all evolution is adaptive. Chance probably affects the genetic structure of populations to a greater extent than was once believed. For instance, when a storm blows insects hundreds of miles over an ocean to an island, the wind does not necessarily pick up the specimens that are best suited to the new environment. And not all alleles fixed by genetic drift in the gene pool of the small founding population are better suited to the environment than alleles that are lost. Similarly, the bottleneck effect can cause nonadaptive or even maladaptive evolution.
4. Selection can only edit variations that exist. Natural selection favors only the most fit variations from what is available, which may not be the ideal traits. New alleles do not arise on demand.
With all these constraints, we cannot expect evolution to craft perfect organisms. Natural selection operates on a "better than" basis. We can see evidence for evolution in the subtle imperfections of the organisms it produces.
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Natural selection is usually thought of as an agent of change, but it can also act to maintain the status quo. Stabilizing selection probably prevails most of the time, resisting change that may be maladaptive. Evolutionary spurts occur when a population is stressed by a change in the environment, migration to a new place, or a change in the genome. When challenged with a new set of problems, a population either adjusts through natural selection or becomes extinct. The fossil record indicates that extinction is the more common outcome. Those populations that do survive crises often change enough to become new species, as we will see in the next chapter.
REVIEW OF KEY CONCEPTS (with page numbers and key figures)
■ The modern evolutionary synthesis integrated Darwinism and Mendelism: science as a process (pp. 416-417)
■ The development of population genetics, with its emphases on quantitative inheritance and variation, brought