The Facts and Theory of Evolution
At the start of this Module, you watched evolution occurring on a plate. Evolution has been observed in many other ways. These occurrences of evolution are facts.
Our understanding of evolution is more than these facts, though. Evolution places facts and laws, such as the law of faunal succession, into a larger context. Evolution has been tested and confirmed repeatedly, and predicts as well as explains. Evolution helps us understand what is going on, and helps us use that knowledge to do things that help people. Evolution, like relativity and quantum mechanics, is a scientific theory, the highest level of scientific understanding. The theory of quantum mechanics was used to design your computer and your phone, the theory of relativity is in your phone correcting the GPS data so you know where you are with great accuracy, and the theory of evolution is being used to fight diseases and keep you alive. Please note that calling evolution a scientific theory does NOT mean that it is speculative or uncertain, but that it is highly reliable.
The basis of evolution is diversity. (Modern social scientists and politicians are about 4 billion years behind nature on this one.) We know that kids do not look exactly like their parents—offspring are diverse or different. We also know that kids share more characteristics with their parents than with less-closely-related people from the parent’s generation. That is, we look a lot like our parents, but we are not exact copies. This arises because of genetics; the biological instructions or programs that guide the development of an individual are passed down from the parents, but there are many mechanisms active that serve to experiment a little with the instructions between generations, but not too much.
Suppose that one of these small experiments is successful (say, it gives a young giraffe a longer neck than her neighbors, which allows her to reach leaves that are out of reach of other giraffes). The long-necked giraffe will be better-fed than others and eventually is likely to succeed in surviving to have babies of her own. Some of those babies will grow a little taller than their mother, some a little shorter, and some the same height as their mother, but the offspring will average taller than the kids of other giraffes lacking the initial, successful change.
Those other giraffes lacking this new development will be less successful, and so will leave fewer babies who go on to have babies. Most populations are small enough that, if one individual is even slightly more successful than others, after a few thousand generations, all the survivors will be related to the one with the successful experiment; if one individual is even slightly less successful than others, after a few thousand generations it will have no survivors. You can demonstrate this easily using mathematical models, or with greater difficulty by breeding living types such as fruit flies, but both reach the same answer. Scientists have indeed been successful in causing evolution in the lab, and observing it in the wild. The evolution of antibiotic resistance that you watched at the start of this Module is one small example.
Once all of the members of a species contain the successful experiment, the species has been changed a little. But over those thousands of generations, other “experiments” are conducted, some successful and some not. The slow accumulation of successful experiments is evolution. The mechanism by which the changes accumulate is called natural selection—beneficial experiments allow more survival and reproduction and so are preserved and multiplied. When enough changes have accumulated, we say that a new type or species has emerged. (If a population is split into two or more parts, those parts are called new species when they no longer can interbreed.)
Notice that things that happen to adults, such as having their ears pierced or their behinds tattooed or stretching their necks to reach leaves, are not passed on to children. The changes that are passed on occur during reproduction. Sex helps generate new combinations of genetic instructions. Even species that reproduce asexually by splitting in half have ways (proto-sex?) to exchange genetic material. Sometimes, accidents occur owing to radioactive decay or toxic chemicals damaging the genetic instructions in an egg or sperm or asexually reproducing creature; however, these often are changes that hurt rather than help.
More importantly, the mechanisms of reproduction do experiment a little by moving a few things around in the genetic instructions during reproduction. Some species, and some individuals of species, conduct more experiments than others. Overuse and misuse of antibiotics by humans are producing antibiotic-resistant disease-causing organisms. These antibiotic-resistant types are often those that experiment a lot during reproduction, and so were lucky enough to quickly find an experiment that allows survival despite antibiotics.
The virus HIV that causes the disease AIDS is especially hard to “beat” with a vaccine or antiviral drug because the virus experiments a huge amount (perhaps the fastest known rate of mutation). It took decades for a large number of dedicated researchers to come up with a “cocktail” of drugs that gives long-term survival to HIV-infected patients. The rapid rate of experimentation in HIV is costly to the virus—many of the experiments are failures, which means those offspring don’t succeed. But, this high rate of experimentation allows the virus to respond quickly to challenges such as new drugs or vaccines by producing offspring with new ways to defeat those drugs or vaccines. In an AIDS patient, the viruses infecting different organs may be different. And, given that the AIDS viruses in just one person are so diverse, it is not surprising that the viruses in different people are different. The remarkable advances in molecular biology allow these changes to be measured now, but no effective vaccine has yet been developed to help people completely clear HIV.
Evolution is a well-tested, well-established science. It makes predictions that are borne out every day. Partial speciation has been achieved in the laboratory in fast-breeding types such as fruit flies. The geological evidence of gradual changes is strong, and becoming steadily stronger as more and more samples are collected.
Evolution is also being used routinely in science. The Evolution on a Plate video showed one reason why. A search on the ISI “Web of Science” in July of 2012 revealed over 3000 scientific papers on the subject "evolution and antibiotic resistance," with an ongoing rise in the number of papers on the topic. The same search in 2024 found almost 10,000 scientific papers. A quick perusal of the titles and abstracts of many of those papers revealed that, as microbes evolve to defeat our antibiotics, the scientists who are trying to keep us alive are using the tools and language of evolutionary biology. Antibiotics are quickly losing effectiveness against evolving microbes, and without major efforts, more and more people would be dying of infections picked up in hospitals, thus scientists are increasingly focusing on the problem, informed by a full understanding of evolution, its rates, and processes.
Computer scientists even use evolution—some “artificial intelligence” approaches have been patterned after the natural processes of evolution. Techniques such as genetic algorithms or evolutionary computation successfully solve complex problems, in essentially the same way that nature does. (For more on this, see the Module 11 Enrichment.) The computational pioneer Alan Turing suggested that scientists could mimic evolution to find solutions to complex problems as early as the year 1948, for example.
In the U.S., some groups continue to oppose evolution based primarily on religious grounds. This opposition has the good effect of keeping the experts “on their toes”—the experts work harder and do better science. This opposition has the unfortunate effect of convincing many people that something is fundamentally wrong with evolutionary theory, which may scare some students from entering the field and helping save lives, and may cause some people to ignore scientifically based advice and thus endanger themselves and others. Many of these people seem to believe that evolution is somehow anti-religious when the majority of church members in the U.S. belong to denominations that endorse evolution as the best description of how the biological world works. Evolution is consistent with the major religions on Earth, and even with rather strict readings of the Christian Bible. The idea that the Earth appears young, and was created recently with all of the modern types of organisms present, was tested in the 1700s and 1800s and proved wrong, as we saw in Module 10.
A longer discussion of some issues—how evolution can be consistent with religion, how evolution is consistent with the second law of thermodynamics, why "intelligent design" is not science, etc.—is given in the Enrichment. We strongly suggest that if you are interested in this topic, you read Module 11 Enrichment.