What Evolution Is and What It Isn’t

What evolution is…

…and what it isn’t

Many people who argue against evolution do so because they do not understand it. The straw-man caricatures of evolution commonly presented by creationists are indeed illogical, implausible and unscientific. But they are precisely that – straw men – and do not accurately represent what evolutionary theory really says. When presented in its true form, the theory of evolution is not only simple and plausible, but the only explanation of biological diversity that is scientific and consistent with the facts.

Unfortunately, creationists work through the political process, not the scientific one. They spread misinformation as to what evolution is and work to prevent its teaching in public schools so that people do not learn the truth. These people then reject evolution based on their creationist-generated misunderstandings of it, and a vicious cycle is perpetuated. This essay will attempt to clear the air and show how creationist caricatures of evolution are faulty.

So what is evolution? In its simplest sense, evolution can be defined as change over time, and that is what the theory of biological evolution means at the most basic level: a change in the frequency of alleles in a population over time. Alleles are different versions of genes that control specific traits. To slightly oversimplify, there is, for example, a gene that controls what color eyes you will have. One version of this gene – one allele – codes for brown eyes, another allele codes for green eyes, a third allele for blue eyes, and so on. Evolution states that over time in a population of living things, some alleles become more common and eventually dominate, while others become less common and may disappear altogether.

But of course, there is far more to the theory of evolution than that. It is to the complexities of evolution that this essay will now turn.

Charles Darwin opened his groundbreaking On the Origin of Species with a common-sense observation: namely, that great diversity exists among life, not just between different species, but among members of the same species as well. From the breathtaking mix of hues in a flower garden to the countless different melodies of songbirds and whales, from the millions of different beetles to the vast branching tree of the mammals, from the lowliest of bacteria to the most brilliant geniuses of humanity, modern biologists have more than confirmed Darwin’s observations on this point by classifying the enormous diversity of life into a system comprising hundreds of classes and families and millions of species, with new ones being discovered constantly. For further proof, one need only witness the dozens of remarkably dissimilar varieties of dogs, cats and other domestic animals that human breeders have brought forth in only a few hundred years. As modern geneticists now know, the ultimate source of all this variation is a population’s gene pool, the entire set of alleles collectively possessed by all the members of that species. With thousands or millions of alleles that can be combined in countless ways, the potential for variation among even a small population is almost limitless.

Darwin followed this up with another observation that was both simple and obvious: all life is a struggle for existence. Prey must escape their predators to survive, while predators must catch and kill their prey if they are to keep from starving. Plants growing near each other must compete for sunlight, water and nutrients. Bacteria, viruses and parasites cannot reproduce without a host organism, while the host’s immune system must constantly work to fend them off. And everything must find sufficient food, water and other vital resources, resources which are often scarce. Out of this constant struggle, some organisms emerge triumphant, flourishing and reproducing abundantly. Others lose the struggle, dwindle and die off. This process is readily observable in all things living today, and the fossil record shows that it occurred in the past as well – the losers (and there are many of them) are preserved in stone. Over 99% of all the species that have ever lived are extinct today.

Darwin’s next observation formed the crux of his theory, and yet it too was so amazingly simple that it’s almost hard to believe no one thought of it before him. As stated above, all living things must struggle for existence, must compete for resources and avoid predators and other hazards. Since the hazards are many and the resources scarce, it is inevitable that not all organisms will survive. In fact, in every population – every species – more organisms are born than can survive. But due to the vast amounts of variation in the gene pool, no two organisms of the same species are exactly alike. They all differ from each other, even if only in subtle ways. By chance, some of these slight variations will make some members of a species more adept at competing for survival than others. The lucky few that are more adept – more “fit” – will survive and produce many offspring. Others, which are less fit, will not be as well suited to their environment, will not be as adept at competing in the struggle for existence, and will die off having produced fewer offspring or none at all. Thus, in time, the traits that make organisms more successful, more fit, will predominate, while the traits that make organisms less fit will disappear. This blind process of winnowing is known as natural selection or, alternatively, survival of the fittest. The way in which it interacts with the idea of changing allele frequencies is obvious in the light of modern genetics. The genes of successful organisms, which carry the traits that make them successful, will tend to be passed on and proliferate. The genes of unsuccessful organisms will tend to vanish from the gene pool because less fit organisms reproduce less abundantly or not at all.

Natural selection is not a conscious process; it does not have goals or intent, and it does not “choose” which organisms will survive. It is the result of environmental pressures which eliminate those organisms less suited to survive them and leave alive those that are better suited. To take a concrete example, on the African savanna, the ability to run quickly might be an indication of fitness in a herd of gazelles. Gazelles that can run faster are better at escaping cheetahs and other predators; thus they survive and produce offspring (they are selected for). Gazelles that can’t run as fast are more likely to be caught and killed (they are selected against), and thus do not reproduce. It is important to note, however, that the cheetahs do not “choose” to attack any specific gazelle; they pursue all of them, and it is the faster and more fit ones that escape. Similarly, in a culture of bacteria exposed to an antibiotic, some might have resistance to that antibiotic. Those ones with resistance will survive and reproduce (they are selected for). Those without resistance will die (they are selected against). Again, the antibiotic does not consciously choose to kill any specific bacterium. It targets all of them, and the ones with resistance survive while the ones without resistance die. That is natural selection at work.

Note, however, that natural selection does not necessarily imply that less fit organisms will die. Organisms that are less fit (meaning that they produce fewer offspring) may be incapable of reproducing because of injury or disease, or simply because they cannot attract a mate. For example, female peacocks tend to favor males with larger, brighter tail feathers as mates. Males with shorter or less brightly colored feathers are selected against, and tend to reproduce less abundantly or not at all.

But why should some gazelles run faster than others, or some bacteria have resistance to an antibiotic while others do not, or some peacocks have more colorful feathers than others? To put it another way, where does all this variation – all these different alleles – come from? At the time Darwin proposed his theory, this was its greatest weakness. He could not explain the mechanism or the source of variation. Of course, that information is now known, and the wealth of knowledge provided by modern genetics has been integrated with Darwin’s original ideas in the modern theory of evolution known as the neo-Darwinian synthesis.

Genetics provides for many sources of variation within a population’s gene pool. As an example, there is genetic drift, a statistical “sampling error” of the gene pool: alleles may increase or decrease in frequency purely by chance. In species that reproduce sexually, there is also recombination, where the parents’ alleles are mixed and matched to produce offspring. But, as the creationist might object, neither of these produce new information, only swap around alleles that already exist. This is correct, which leads to the next major component of evolutionary theory: mutation.

Packed within the nucleus of every cell of your body is a tightly wound double helix. DNA, deoxyribonucleic acid, is literally the instruction manual of life. This intricate, complex molecule contains all your genes, all the alleles that make you unique, encoded in a series of nucleotide bases that can be read by the cell’s transcription machinery and converted into proteins that control the functions of life. Every time a cell in your body divides, the new cells must be provided with a copy of these instructions so that they know how to perform their specific duties. For this to happen, the DNA of the original cell must be replicated. The molecular machinery present in every cell does this job with a degree of fidelity that would put any human copyist to shame. Nevertheless, DNA is a long, complex molecule and the replication process is not perfect. Frequently, when DNA is copied, there are small errors, slight changes. A single nucleotide “letter” might be changed to another, or a segment of DNA might be deleted entirely, flipped end-to-end or added where one did not previously exist. These changes are mutations.

Most mutations are silent – that is, they don’t do anything at all. Of the non-silent ones, most are deleterious and impair the fitness of the organism that has them. Natural selection sees to it that these mutations are quickly eliminated from the gene pool. Those that remain – a small percentage of all total mutations, but they definitely do exist – are beneficial. In some way, even a small one, they contribute to the fitness of the organism that has them. Since organisms with this beneficial mutation have a survival advantage, they are selected for and tend to reproduce more abundantly than those without it. In this way, the mutation spreads throughout the population and eventually becomes “fixated” in the gene pool.

It is important to recognize that mutations are changes to the genome, the set of instructions that dictates how an organism develops. Therefore, every non-silent mutation makes the organism that has it slightly different from every other member of its species. It may be something as simple as a tiny alteration in the shape of a blood protein, or something as complex as a change to the skeletal and muscular structure that determines the body’s ultimate shape. The point, however, is that each mutation makes the organism different in some way, and over thousands or millions of years, these differences add up. Eventually, as mutations spread and accumulate, a population of organisms may evolve to a point where it is considered to be a separate species from the population it descended from. This is called speciation or macroevolution, and contrary to creationist claims, it has been observed and documented. See Observed Instances of Speciation and Some More Observed Speciation Events.

It should be noted that in biology, the definition of the word “species” is a little fuzzy; it has several different meanings. It does not necessarily mean that two species will look entirely different or have entirely different body structures (though it can); it does not necessarily mean that two separate species will have very distinct genes (though it can). Indeed, two organisms from different but closely related species may have virtually identical genes and look so alike that only an expert observer can tell the difference. The key concept, and the one most commonly used to define what constitutes a species, is reproductive isolation; that is, two organisms from different species will not be able to mate and produce fertile offspring. Organisms that can mate and produce offspring which are invariably sterile hybrids are considered to be of separate species.

Although species are not required to be drastically different, over time separate species do tend to become distinct, since mutations acquired by one population cannot spread to the other. Living things today possess enormous variety, but it can be shown that separate but closely related modern species all evolved from a common ancestral species. For instance, modern apes and humans both diversified from a primitive line of ancestors called hominids. However, those common ancestors were in turn linked to other organisms by their own common ancestors. If one extrapolates backwards far enough, it is possible to show that all organisms alive today have a single ancestor in common, a simple unicellular organism that existed between four and five billion years ago, from which all modern life descended and diversified. This is known as common descent and constitutes the final major component of evolutionary theory.

Within the scientific community, evolution is as solidly established a fact as gravity, and whether it occurs is no longer a matter of any serious debate. However, there is some debate over how quickly evolution occurs and the relative importance of the different mechanisms by which it proceeds. Until fairly recently, the prevailing interpretation was phyletic gradualism, which holds that evolution occurs constantly at a slow, relatively steady pace. However, in 1974, Stephen Jay Gould and Niles Eldredge proposed the theory of punctuated equilibrium, PE for short, which holds that life consists of long periods of evolutionary stasis punctuated by brief (on the geologic time scale) intervals of rapid development and speciation. PE has won fairly general acceptance, though it is important to note that neither of these models precludes the other, as the rate of evolution may be different at different places and times. Other scientists have made other contributions to the theory as well. For example, Japanese biologist Motoo Kimura proposed the neutral theory of molecular evolution, which holds that genetic drift may play a more important role in the development of some genes than was previously thought. American Lynn Margulis has also contributed the endosymbiotic model of evolution, which holds that certain microscopic structures in the cells of all living things (chloroplasts in plants, mitochondria in animals) were originally separate organisms that colonized host cells and eventually became permanent parts of them. Both of these theories have become more or less accepted additions to the neo-Darwinian synthesis, but healthy debate is ongoing today; the frontiers of knowledge continue to advance.

With that established, we turn to what evolution is not. Contrary to any creationist claims, what is described above is the whole of evolutionary theory. Significant departures from it, such as the more commonly heard ones listed below, are straw-man arguments made up to make it seem ludicrous and improbable.

One of the most common misrepresentations of evolution is to extend it beyond its boundaries, claiming it says more than it actually does. The theory of evolution says nothing about the origin of the universe, the origin of the earth or even the origin of life. Evolution concerns itself only with the subsequent development of life once it already existed. The manner in which life first came into being is irrelevant to evolutionary theory, though it is covered in a related field, abiogenesis. (If God had miraculously created the first living cell in the primordial soup, evolution could have taken over normally from there.) The origin of the universe and other cosmological bodies is not biology at all; it is sometimes referred to as stellar evolution, but it is an unrelated branch of science and has nothing to do with the theory first proposed by Charles Darwin. Statements such as “Evolution says that hydrogen gas turned into people,” “Evolution says that particles develop into people” or “Evolution says that life and/or the universe came out of nothing” are all examples of this faulty type of argument.

Another common misrepresentation is say that evolution proceeds only according to chance, accident, blind luck or random behavior. This is completely false. Evolution is anything but random. It is guided by natural selection, a non-random process that follows a fixed, specific set of rules. It is true that mutations, which provide the raw stuff for natural selection to operate on, are random events, in the sense that they are not predisposed toward increasing fitness. Nevertheless, they are frequent enough for it to be a certainty that a few beneficial ones will arise, and once they do, the process of filtering by natural selection takes over. To say that evolution is a random process would be equivalent to flipping a thousand coins, removing all those that didn’t turn up heads, and then saying that the all-heads result came about purely by chance. (For more on this, see here.)

In a related vein is the “speciation by mutation” argument, often phrased as “It’s impossible that an X could have given birth to a Y,” such as in, “There’s no way a reptile could have laid an egg that hatched into a bird.” This is another argument that demonstrates a profound misunderstanding of evolution. Speciation and other profound changes are not brought about by many sudden, drastic, simultaneous mutations – in other words, they do not occur by saltation. Rather, they are brought about by tiny, incremental changes gradually accumulating and building on each other over long periods of time, with natural selection filtering out those that do not improve fitness. Over time, this process can lead to organisms that are very different from their evolutionary precursors.

Astronomer Fred Hoyle once said that evolution is as likely as a tornado blowing through a junkyard and assembling a fully functional Boeing 747. Hoyle, however, was not a biologist, and this statement demonstrates a severe misunderstanding of the subject. A tornado assembling a jetliner would be an example of a saltationary jump owing purely to random chance in a process of single-step selection with a predetermined goal. This fails as an analogy to evolution on four counts. As stated above, evolution does not operate by saltation, nor is it guided by random chance, nor does it have goals specified ahead of time. And most especially, it does not operate by a process of single-step selection, where an end product entirely unlike the beginning product is assembled in one large chance step. As Richard Dawkins states in his book The Blind Watchmaker, evolution actually operates by a process of cumulative selection, where large changes are brought about through a gradually accumulating series of small changes guided by selection at each step. Rather than a tornado assembling a 747 in one step, send the tornado through the junkyard not once, but thousands of times, with each time representing a single generation. Rather than tearing apart its past work each time, assume that, once assembled, collections of parts that could function as part of an airliner will survive. Do this long enough and a 747 may indeed emerge – or, to complete the analogy to evolution, allow the end product to be not just a jumbo jet, but any functioning piece of machinery. In this way, cumulative selection can achieve in a short time results that would take trillions of years to produce by pure-chance, single-step selection. (For more on this, see The Tornado in the Junkyard.)

Nor does evolution predict “survival of the meanest” – in other words, it does not predict that every species should be a vicious, carnivorous killer bristling with claws, fangs, stingers, spikes, and so on. What evolution does predict is that every species needs a niche, a way to make a living and successfully reproduce, and different species have adapted to different niches. Some are predators; some are not, instead getting their energy from eating plants, or making their own. In fact, it could be argued that it is easier to make a living as a plant or an herbivore than a carnivore. Not only is it easier to eat food that doesn’t try to run away, but due to considerations of usable energy, successively higher levels of the food chain can only support smaller and smaller populations. Thus, it could be said that in terms of reproductive success, the deer and gazelles are more “fit” than the wolves and lions – some of them will certainly be killed by predators, but their counterstrategy is not to fight back, but to breed so rapidly that enough of them survive to allow the population to continue. Evolving strong defensive measures like the ability to fight back would be a significant drain on their energy and resources which could more profitably be used to reproduce. Some prey species have evolved their own defenses where it is cost-effective for them to do so; others have not. They each have their own niches and their own ways of making a living, not all of which involve being the strongest or the meanest.

Another argument is often heard as “Evolution says that we should develop X, but we don’t,” where X is some significant change to bodily structure, such as another arm, feathers or new internal organs. However, these statements are wrong. Evolution does not state that our body structures should change drastically. Rather, it builds on what already exists. A case in point is the basic tetrapod (four-limbed) body form, which has been preserved to some degree in all vertebrate animals. Even some snakes have small vestigial limbs. A radical change to this well-established body form, such as growing a new limb, would not be in keeping with evolutionary theory; nor would the appearance of a new organ, since the vertebrate body structure already has all the organs necessary for survival. The basic body plan, including separate organ systems, evolved at a very early stage of life’s history. The chances of drastic changes arising now in a well-established and complex body plan are virtually nil. Another note: It is absurd to claim “Evolution says I should grow a third arm because I could use one,” or something of the like. Mutations are random events and do not favor what changes would be best for the organism; it is simply that the ones that do turn out to be useful are the ones that are kept.

A subtle point worth noting is that individual organisms do not evolve, not even if a particular organism is the one possessing a beneficial mutation that later spreads throughout the population. Populations evolve; individuals do not. This answers the occasional absurd creationist objection that “Evolution can’t explain how the first member of a new species could find a mate.” This is like saying the first speaker of English wouldn’t have been able to find anyone to carry on a conversation with. As follows from the above refutal of the “speciation by mutations” argument, an individual organism never arises that is an entirely new species from its parents.[1] Populations gradually change over time, and while many of the intermediate stages would probably be able to interbreed, the original species and the end product would not be able to. There is rarely a specific and exactly defined point where speciation occurs; it is a process that usually happens in slow, incremental steps, one step shading into the next.

Related to the last point, it is also important to mention that evolution, although it is not a chance process, does not have long-term goals or targets either. Natural selection has only one immediate short-term goal – to increase the fitness of organisms – and it does not care how that goal is achieved. It cannot traverse a “valley” of temporary decreased fitness to reach a “peak” of increased fitness. A river might be a helpful analogy. The water of the river has a goal, namely, to seek the lowest point – to flow downhill. As long as it is on a level downward slope, the river’s behavior is predictable. If it passes onto a flat plain, the direction it will take can be influenced by many different features of the terrain, causing bends and meanders, and thus is no longer entirely predictable. Nevertheless, the river will continue to follow the general law of always seeking the lowest point. It cannot flow up the side of a mountain – not even if, by doing so, it would be able to get over the top and flow down the other side to a much lower level than it was at before. Like evolution, the river’s goal is only in the immediate short term. Likewise, although the modern vertebrate eye is what exists now, this does not mean that natural selection “intended” to produce it. The modern eye is the result of a long series of chance mutations, guided by natural selection, operating on the original eyespot; different mutations at various points along the line could have produced a completely different result, as long as every intermediate step conferred a survival advantage. Natural selection does not look ahead, which is why it is sometimes referred to as the blind watchmaker; it is a blind natural process without long-term goals.

Another all too frequent misconception is that evolution is a steady march of progress, an increase in complexity or an upward climb towards perfection. This is not the case. This popular but erroneous idea is based on a nineteenth-century teleological concept called the “great chain of being” which has since been discredited. Natural selection guides organisms only in the direction of increased fitness; depending on the circumstances, this can entail either an increase or a decrease in complexity. (An example of the latter would be some types of parasitic intestinal worms which have lost their digestive systems; they no longer need them, since they can directly absorb food from their hosts through their skin.) Nor is there any ultimate standard of perfection toward which evolution builds, as fitness is a strictly regional characteristic determined by local environmental factors. Organisms only adapt to their own environment, by definition, but an organism that is extremely well adapted to one environment may be at a severe disadvantage in another. For example, penguins are well suited for life in arctic climates, but at the equator they would not survive long. Likewise, humans are primarily tropical creatures. It is only technology that has made us able to survive at colder latitudes, but this does not mean that we are more perfect or even more highly evolved than any other living thing on earth. Every species alive today, from bacteria to monkeys to man, is the result of four and a half billion years of evolution – theirs has simply taken different paths than ours.

And as such, it is also worth noting that man did not evolve from monkeys, or frogs, or slime molds, or bacteria. This misunderstanding is very common, perhaps one of the most commonly heard among creationists, but it is wrong. Humans are not directly descended from any species that is alive today; instead, we share common ancestors with them. Humans and apes both descended from an original common ancestor, a variety of hominid, that is now extinct. Likewise, humans and frogs both descended from an common ancestor, albeit one much more distant than the common ancestor of humans and monkeys. The same is true of insects, plants, slime molds and bacteria, though the common ancestor is progressively farther back in each case. The common ancestor of humans and insects dates back to the original emergence of animal life, while the common ancestor of humans and plants or humans and bacteria was not even multicellular. But in every case, these common ancestors are now extinct; they either died out or evolved into an entirely new species themselves. No extant species is evolving into any other extant species, which answers the occasionally heard objection that “if evolution is true, why don’t we see monkeys still evolving into humans?” The answer is that monkeys’ evolution has taken a different path than ours, and the odds against the two paths ever converging again are next to nil. Evolution has a virtually infinite number of possible pathways, and it is extremely unlikely, to put it mildly, that humans would evolve again – that it would take the exact same path – if, so to speak, the tape were replayed. (The claim that monkeys should be evolving into humans also partakes of the erroneous concept of the “great chain of being”, debunked above. Humans are not the ultimate goal of evolution – the evolutionary path of every species has gone its own way, each adapting to different niches.)

Some creationists have claimed that evolution is a religion, but this claim too is false. Evolution is well supported by evidence, and all its basic mechanisms can be observed to operate today; unlike religion, it does not require faith. In addition, no one claims evolution is an inerrant doctrine – like all branches of science, it is being constantly tested and refined, and it could be falsified and rejected if the right evidence turned up. No one prays to evolution. Also like all sciences, evolution is theologically neutral. It says nothing, one way or the other, about the existence of God or the supernatural; it does not require divine intervention, but nor does it forbid it. Atheists can accept evolution without believing there is anything more, while theists can accept evolution and believe that their god controls it. Any god can be given credit for using evolution as the method of creation, and indeed, theists of all denominations accept it – click here for a partial list, or see here for more on atheism, religion and evolution.

In addition to not being a religion, evolution is also not, nor does it pretend to be, a moral guide. Creationists sometimes charge that “if we’re descended from animals, we should act like animals,” but this is an example of a classic logical blunder – the naturalistic or “is implies ought” fallacy. Just because things are some way does not mean that they should be that way, or that it is right that they be that way. It is true that, in nature, there is much pain, suffering and death, even things that seem needlessly cruel. Natural selection can be a harsh and uncaring process, if we insist on subjecting an unintelligent force of nature to a human value judgment. But this does not mean it is right to be cruel and uncaring, just as the theory of gravity does not mean we should push people off tall buildings, or the germ theory of disease does not mean we should not treat sick people. Like all sciences, evolution is descriptive and not prescriptive. It is merely a statement of the way things are, not a statement of the way they should be. And for what it is worth, for every example of cruelty in nature, there is at least one counterexample of love, kindness, or cooperation. Many animals are monogamous, care for their offspring and defend them with their lives. Symbiosis, or reciprocal generosity, has proven to be a very effective survival strategy. Some of our closest relatives among the apes even care for and feed the wounded, sick or crippled among their numbers, displaying an almost human compassion. This is not meant to alleviate the cruelties and violences nature often also displays – it is merely meant to illustrate the uselessness of trying to derive moral rules from scientific theories.

The final creationist distortion is to charge that evolution is “just a theory,” as if this were a point against it. In truth, however, to label it “just a theory” is to support it, not denigrate it. In scientific parlance, “theory” does not mean “wild guess” or “hunch,” but rather describes a scientific idea that is strongly supported and has stood the test of time. Furthermore, evolution is more than just a theory. It has also been directly observed to occur, and thus, in addition to being a theory, it is also a simple fact, as undeniable as the sphericity of the Earth. (For more on this, see here.)

When the distractions of creationist straw men are removed, evolution is a process that, at its core, is both simple and plausible. Knowing what it truly says, and just as important, what it does not say, is a boon both to proponents of evolution who must defend their science against religious attacks, and also to creationists who wish to argue against evolution. While no one is denying them the right to do this, arguing against a faulty caricature of the theory based on misunderstanding, rather than against the theory itself, will earn them only ridicule.


[1] The experienced reader will note that this is an oversimplification. In cases of speciation by polyploidy, it is indeed possible that a new organism that also constitutes an entirely new species will arise in a single step. However, in the majority of speciation events morphological diversification occurs first, eventually followed by reproductive isolation.