Evolution is a powerful idea that can be used to understand many natural phenomena, however it has limits. For example, individual organisms do not evolve; biological evolution is a population-level phenomenon, and cannot be used to explain changes in a single individual over the course of a life span. When individuals change over time, these changes may be due to development, conditioning, or learning, but they aren’t evolution.
Evolution does not occur on demand, or to meet a pre-conceived goal. A dark insect on a light background may be visible to predators, and would likely benefit from a change that caused the insect to be lighter and thus concealed from danger. However, evolution can only change the frequency of characteristics that actually exist. Thus, if no light-colored insects exist in the population, then the frequency of light coloration cannot change. Similarly, resistance to HIV would be desirable in Sub-Saharan Africa, but resistance alleles do not occur there. If resistance ever does arise in HIV-plagued countries such as Botswana or Kenya, we might see a change in the frequency of HIV resistance in those populations. However, evolution cannot cause desirable features to arise on demand.
Evolution cannot be stopped. Populations are always changing, for better and for worse. Biologists often speak of evolutionary change that involves a population-level increase of desirable features. For example, inland plants that colonize a coastal area may become more salt-tolerant in their new habitat. But sometimes, useful features are lost. Human ancestors could synthesize their own vitamin C, but modern humans cannot. The loss of this ability has led to thousands of human deaths from scurvy, a debilitating disease that results from insufficient levels of vitamin C.
Evolution doesn’t result in perfection. An insect that blends into its background may have extra protection against predators, but is still susceptible to disease-causing parasites, drowning in a flash flood, or getting squished by a human foot. Just like humans who must consume certain foods for their necessary vitamin C, or people who lack HIV-resistant genes, all organisms are biologically imperfect. Instead, we are the product of an evolutionary history that lacked foresight (and the inability to predict the consequences of vitamin-C deficiency), and evolutionary processes that could only work with the genes (such as non-functioning CCR5 alleles) that already existed in the population.