If evolution exists, why do people still get cancer? What is the evolutionary benefit of eyebrows? Why doesn't evolution fix the genes that cause problems? Two of the most commonly Googled questions about evolution are "Why does evolution not produce perfect organisms?" and "Why does evolution not make sense?" Although recent polls say most Americans believe in the theory of evolution (with about 60% of American adults accepting it), frequently asked questions like these suggest that the general public doesn't understand evolution- at least not truly, specifically having several misconceptions about natural selection. To understand evolution, one must first understand that evolution is not a voluntary and purposeful process or choice.
Charles Darwin is credited with "discovering" evolution due to his studies of finches on the Galapagos islands. In the book On the Origins of Species, Darwin theorized that evolution occurs because of natural selection. First, in science, a theory isn't just a mere guess or speculation. Rather, it is the best possible explanation for a phenomenon that has yet to be debunked by experimentation. In the theory of natural selection, organisms produce more offspring than can survive. Those better physically equipped to survive and reproduce pass on their genes. On the other hand, those lacking in such fitness do not reach an age when they can reproduce or produce fewer offspring. Natural selection is just one mechanism of evolution. For example, a phenomenon known as genetic drift can also drive evolution. In genetic drift, some organisms, by chance, produce more offspring than would be expected, although they aren't necessarily the fittest. Realize that when the environment changes, traits that enhance an organism's chance of survival change, meaning organisms with those traits are more likely to survive and reproduce. Organisms do NOT purposely "adapt" different traits to survive.
Arguably, the most common misconception or inaccuracy parroted about natural selection is that adaptation to environments is purposeful. Natural selection leads to the adaptation of species over time, but the process does not involve effort. Natural selection naturally results from genetic variation in a population caused by random mutation, which does not depend on what the organisms want or are trying to do. Due to this genetic variation, some of those variants may be able to create more offspring than other variants. For example, resistance in bacteria evolves because random mutations generate some individuals who can withstand the antibiotic better. These individuals can reproduce more than others, creating a population of more resistant bacteria in the next generation. Organisms also can't physically change or adapt during their lifetime and pass those traits on. For example, the baby of two bodybuilders would be no more muscular than an average baby, and although people tend to relate natural selection with the biggest and strongest organisms, this often isn't an accurate picture.
"Survival of the fittest" is a common term to describe natural selection. However, it is not necessarily the "fittest" who survive. First, a more accurate phrase would be "survival of the fit enough." In most populations, organisms with many different genetic variations survive, reproduce, and leave offspring carrying their genes in the next generation. It is not simply the one or two "best" individuals in the population that pass their genes onto the next generation. These organisms may not be the "fittest" in the population, but they are "fit enough" to pass their genes onto the next generation." Secondly, in evolutionary terms, fitness has a different meaning than most people associate it with. An organism's evolutionary fitness indicates its ability to get its genes into the next generation; this doesn't always correlate with strength, speed, or size. For example, a bunny with fur colored more similarly to its environment will be less likely to be spotted by predators than a bunny with fur that stands out — meaning that the more camouflaged bunny will likely have higher evolutionary fitness than its stronger, larger counterparts.
Considering how it is not necessarily the "fittest" who survive and what makes an organism the "fittest" is variable, why organisms still get cancer and have other ailments can be answered. Natural selection doesn't produce perfect organisms. Living things are often made up of traits resulting from trade-offs —improving one feature for the better may hurt another for the worse (For example, the sickle cell trait can cause a sickle cell crisis if a person exercises too intensely, but the trait protects from malaria). And because organisms have arisen through complex evolutionary histories, their future evolution is often constrained by traits they have already evolved. For the same reason, organisms often have what appear to be useless features. Vestigial organs are body parts that have little to no purpose. They are remnants left from evolutionary ancestors. Humans have more than 100 vestigial organs, the most known of which are the appendix, wisdom teeth, coccyx, and male nipples. Often traits are nonadaptive due to the result of another characteristic. For example, the color of blood is not adaptive. There's no reason that having red blood is any better than having blood of any other color. Blood's redness is a by-product of the chemistry of hemoglobin, which causes it to reflect red light.
Why does evolution allow people to get diseases? Why don't organisms adapt to stop cancer? In summary, during natural selection, which drives evolution, organisms that have good enough traits (that occurred due to random mutations) to survive in their specific environment survive, pass on their genes, and then the next generation has more of their traits, including the ones inherited from their previous evolutionary history even though those traits may be useless or have drawbacks. Biologists continue to use the theory of evolution to study how natural selection and other mechanisms affect the evolution of organisms today.