How does selection work

Scientists are still working out the answer to that question. While we know that selection can operate at levels above that of the individual, it is still unclear how important this process has been in shaping the history of life.

Subscribe to our newsletter. Email Facebook Twitter. Selection at the individual level To understand how selection works at different levels, first consider the requirements of natural selection in the most familiar situation, at the level of the individual organism, using a beetle population as an example: 1.

Some beetles are green and some are brown. Selection at the cellular level This process requires just three basic features to operate: variation, differential birth and death, and heredity. Selection at the species level Moving up the hierarchy, we can also see how this same process would work on entire lineages of organisms.

As an example, imagine a group of closely related species: 1. They collected data on the occurrence of more than different mollusk species fossilized in late Cretaceous rocks of southeastern North America and found a few key patterns: 1. Footer Connect Email Facebook Twitter. Teachers, in particular, are urged to familiarize themselves with these errors so that they may identify and address them among their students. Much of the human experience involves overcoming obstacles, achieving goals, and fulfilling needs.

In fact, it has been argued that the default mode of teleological thinking is, at best, suppressed rather than supplanted by introductory scientific education. On the one hand, teleological reasoning may preclude any consideration of mechanisms altogether if simply identifying a current function for an organ or behavior is taken as sufficient to explain its existence e.

On the other hand, when mechanisms are considered by teleologically oriented thinkers, they are often framed in terms of change occurring in response to a particular need Table 2. Obviously, this contrasts starkly with a two-step process involving undirected mutations followed by natural selection see Fig. A related conceptual bias to teleology is anthropomorphism, in which human-like conscious intent is ascribed either to the objects of natural selection or to the process itself see below.

Gould described the obvious appeal of such intuitive notions as follows:. Since the living world is a product of evolution, why not suppose that it arose in the simplest and most direct way? The penchant for seeing conscious intent is often sufficiently strong that it is applied not only to non-human vertebrates in which consciousness, though certainly not knowledge of genetics and Darwinian fitness, may actually occur , but also to plants and even to single-celled organisms.

Anthropomorphism with an emphasis on forethought is also behind the common misconception that organisms behave as they do in order to enhance the long-term well-being of their species. Once again, a consideration of the actual mechanics of natural selection should reveal why this is fallacious.

All too often, an anthropomorphic view of evolution is reinforced with sloppy descriptions by trusted authorities Jungwirth a , b , ; Moore et al. Consider this particularly egregious example from a website maintained by the National Institutes of Health Footnote 10 :.

As microbes evolve, they adapt to their environment. If something stops them from growing and spreading—such as an antimicrobial—they evolve new mechanisms to resist the antimicrobials by changing their genetic structure. Changing the genetic structure ensures that the offspring of the resistant microbes are also resistant. Fundamentally inaccurate descriptions such as this are alarmingly common.

As a corrective, it is a useful exercise to translate such faulty characterizations into accurate language Footnote For example, this could read:. Bacteria that cause disease exist in large populations, and not all individuals are alike. If some individuals happen to possess genetic features that make them resistant to antibiotics, these individuals will survive the treatment while the rest gradually are killed off.

As a result of their greater survival, the resistant individuals will leave more offspring than susceptible individuals, such that the proportion of resistant individuals will increase each time a new generation is produced. When only the descendants of the resistant individuals are left, the population of bacteria can be said to have evolved resistance to the antibiotics.

Many students who manage to avoid teleological and anthropomorphic pitfalls nonetheless conceive of evolution as involving change due to use or disuse of organs. This view, which was developed explicitly by Jean-Baptiste Lamarck but was also invoked to an extent by Darwin , emphasizes changes to individual organisms that occur as they use particular features more or less.

Modern evolutionary theory recognizes several reasons that may account for the loss of complex features e. This is because the cells that are involved in reproduction the germline are distinct from those that make up the rest of the body the somatic line ; only changes that affect the germline can be passed on. New genetic variants arise through mutation and recombination during replication and will often only exert their effects in offspring and not in the parents in whose reproductive cells they occur though they could also arise very early in development and appear later in the adult offspring.

Correct and incorrect interpretations of inheritance are contrasted in Fig. A summary of correct left and incorrect right conceptions of heredity as it pertains to adaptive evolutionary change. In all diagrams, a set of nine squares represents an individual multicellular organism and each square represents a type of cell of which the organisms are constructed. In the left panels, the organisms include two kinds of cells: those that produce gametes the germline, black and those that make up the rest of the body the somatic line, white.

In the top left panel , all cells in a parent organism initially contain a gene that specifies white coloration marked W A. A random mutation occurs in the germline, changing the gene from one that specifies white to one that specifies gray marked G B. This mutant gene is passed to the egg C , which then develops into an offspring exhibiting gray coloration D.

The mutation in this case occurred in the parent specifically, in the germline but its effects did not become apparent until the next generation. In the bottom left panel , a parent once again begins with white coloration and the white gene in all of its cells H. During its lifetime, the parent comes to acquire a gray coloration due to exposure to particular environmental conditions I. However, because this does not involve any change to the genes in the germline, the original white gene is passed into the egg J , and the offspring exhibits none of the gray coloration that was acquired by its parent K.

In the top right panel , the distinction between germline and somatic line is not understood. In this case, a parent that initially exhibits white coloration P changes during its lifetime to become gray Q. Under incorrect views of soft inheritance, this altered coloration is passed on to the egg R , and the offspring is born with the gray color acquired by its parent S. In the bottom right panel , a more sophisticated but still incorrect view of inheritance is shown.

Here, traits are understood to be specified by genes, but no distinction is recognized between the germline and somatic line.

In this situation, a parent begins with white coloration and white-specifying genes in all its cells W. A mutation occurs in one type of body cells to change those cells to gray X. A mixture of white and gray genes is passed on to the egg Y , and the offspring develops white coloration in most cells but gray coloration in the cells where gray-inducing mutations arose in the parent Z. Intuitive ideas regarding soft inheritance underlie many misconceptions of how adaptive evolution takes place see Fig.

That it seems intuitive probably explains why the idea of soft inheritance persisted so long among prominent thinkers and why it is so resistant to correction among modern students. Unfortunately, a failure to abandon this belief is fundamentally incompatible with an appreciation of evolution by natural selection as a two-step process in which the origin of new variation and its relevance to survival in a particular environment are independent considerations.

Thirty years ago, widely respected broadcaster Sir David Attenborough aptly described the challenge of avoiding anthropomorphic shorthand in descriptions of adaptation:. Darwin demonstrated that the driving force of [adaptive] evolution comes from the accumulation, over countless generations, of chance genetical changes sifted by the rigors of natural selection.

In describing the consequences of this process it is only too easy to use a form of words that suggests that the animals themselves were striving to bring about change in a purposeful way—that fish wanted to climb onto dry land, and to modify their fins into legs, that reptiles wished to fly, strove to change their scales into feathers and so ultimately became birds.

Unlike many authors, Attenborough admirably endeavored to not use such misleading terminology. However, this quote inadvertently highlights an additional challenge in describing natural selection without loaded language. Darwin himself could not resist slipping into the language of agency at times:. It may be said that natural selection is daily and hourly scrutinizing, throughout the world, every variation, even the slightest; rejecting that which is bad, preserving and adding up all that is good; silently and insensibly working, whenever and wherever opportunity offers, at the improvement of each organic being in relation to its organic and inorganic conditions of life.

We see nothing of these slow changes in progress, until the hand of time has marked the long lapse of ages, and then so imperfect is our view into long past geological ages, that we only see that the forms of life are now different from what they formerly were. Being, as it is, the simple outcome of differences in reproductive success due to heritable traits, natural selection cannot have plans, goals, or intentions, nor can it cause changes in response to need. For this reason, Jungwirth a , b , bemoaned the tendency for authors and instructors to invoke teleological and anthropomorphic descriptions of the process and argued that this served to reinforce misconceptions among students see also Bishop and Anderson ; Alters and Nelson ; Moore et al.

That said, a study of high school students by Tamir and Zohar suggested that older students can recognize the distinction between an anthropomorphic or teleological formulation i.

Moore et al. Some authors have argued that teleological wording can have some value as shorthand for describing complex phenomena in a simple way precisely because it corresponds to normal thinking patterns, and that contrasting this explicitly with accurate language can be a useful exercise during instruction Zohar and Ginossar In any case, biologists and instructors should be cognizant of the risk that linguistic shortcuts may send students off track.

Intuitive models of evolution based on soft inheritance are one-step models of adaptation: Traits are modified in one generation and appear in their altered form in the next.

This is in conflict with the actual two-step process of adaptation involving the independent processes of mutation and natural selection. Unfortunately, many students who eschew soft inheritance nevertheless fail to distinguish natural selection from the origin of new variation e. For example, many students may believe that exposure to antibiotics directly causes bacteria to become resistant, rather than simply changing the relative frequencies of resistant versus non-resistant individuals by killing off the latter Footnote Again, natural selection itself does not create new variation, it merely influences the proportion of existing variants.

Most forms of selection reduce the amount of genetic variation within populations, which may be counteracted by the continual emergence of new variation via undirected mutation and recombination. Misunderstandings about how variation arises are problematic, but a common failure to recognize that it plays a role at all represents an even a deeper concern. Not surprisingly, transformationist models of adaptation usually include a tacit assumption of soft inheritance and one-step change in response to challenges.

A proper understanding of natural selection recognizes it as a process that occurs within populations over the course of many generations. It does so through cumulative, statistical effects on the proportion of traits differing in their consequences for reproductive success.

Natural selection is mistakenly seen as an event rather than as a process Ferrari and Chi ; Sinatra et al. Events generally have a beginning and end, occur in a specific sequential order, consist of distinct actions, and may be goal-oriented.

By contrast, natural selection actually occurs continually and simultaneously within entire populations and is not goal-oriented Ferrari and Chi Misconstruing selection as an event may contribute to transformationist thinking as adaptive changes are thought to occur in the entire population simultaneously. In actuality, it is a probabilistic process in which some traits make it more likely—but do not guarantee—that organisms possessing them will successfully reproduce.

Surveys of students at all levels paint a bleak picture regarding the level of understanding of natural selection. Though it is based on well-established and individually straightforward components, a proper grasp of the mechanism and its implications remains very rare among non-specialists.

The unavoidable conclusion is that the vast majority of individuals, including most with postsecondary education in science, lack a basic understanding of how adaptive evolution occurs. While no concrete solutions to this problem have yet been found, it is evident that simply outlining the various components of natural selection rarely imparts an understanding of the process to students. Various alternative teaching strategies and activities have been suggested, and some do help to improve the level of understanding among students e.

Efforts to integrate evolution throughout biology curricula rather than segregating it into a single unit may also prove more effective Nehm et al. At the very least, it is abundantly clear that teaching and learning natural selection must include efforts to identify, confront, and supplant misconceptions. Most of these derive from deeply held conceptual biases that may have been present since childhood. Natural selection, like most complex scientific theories, runs counter to common experience and therefore competes—usually unsuccessfully—with intuitive ideas about inheritance, variation, function, intentionality, and probability.

The tendency, both outside and within academic settings, to use inaccurate language to describe evolutionary phenomena probably serves to reinforce these problems. Natural selection is a central component of modern evolutionary theory, which in turn is the unifying theme of all biology. Without a grasp of this process and its consequences, it is simply impossible to understand, even in basic terms, how and why life has become so marvelously diverse. The enormous challenge faced by biologists and educators in correcting the widespread misunderstanding of natural selection is matched only by the importance of the task.

For a more advanced treatment, see Bell , or consult any of the major undergraduate-level evolutionary biology or population genetics textbooks. Much of the additional material is available in Darwin and Stauffer Ridley points out that Darwin's calculations require overlapping generations to reach this exact number, but the point remains that even in slow-reproducing species the rate of potential production is enormous relative to actual numbers of organisms.

Humans are currently undergoing a rapid population expansion, but this is the exception rather than the rule. This is because not all evolution occurs by natural selection and because not all outcomes of natural selection involve changes in the genetic makeup of populations. Instructors interested in assessing their own students' level of understanding may wish to consult tests developed by Bishop and Anderson , Anderson et al. Even more alarming is a recent indication that one in six teachers in the USA is a young Earth creationist, and that about one in eight teaches creationism as though it were a valid alternative to evolutionary science Berkman et al.

Strictly speaking, it is not necessary to understand how evolution occurs to be convinced that it has occurred because the historical fact of evolution is supported by many convergent lines of evidence that are independent of discussions about particular mechanisms. Again, this represents the important distinction between evolution as fact and theory.

See Gregory a. Likewise, mechanisms involving organisms' conscious desires to change are often incorrectly attributed to Lamarck. For recent critiques of the tendency to describe various misconceptions as Lamarckian, see Geraedts and Boersma and Kampourakis and Zogza It is unfortunate that these mistakenly attributed concepts serve as the primary legacy of Lamarck, who in actuality made several important contributions to biology a term first used by Lamarck , including greatly advancing the classification of invertebrates another term he coined and, of course, developing the first albeit ultimately incorrect mechanistic theory of evolution.

For discussions of Lamarck's views and contributions to evolutionary biology, see Packard , Burkhardt , , Corsi , Humphreys , , and Kampourakis and Zogza Alters B.

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Can students be taught to distinguish between teleological and causal explanations? Teaching students to understand the advantages and disadvantages of teleological and anthropomorphic statements in biology. Beardsley PM. Middle school student learning in evolution: are current standards achievable? Am Biol Teach. Bell G. The basics of selection.

Selection: the mechanism of evolution. Oxford: Oxford University Press; Evolution and creationism in America's classrooms: a national portrait.

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Sci Educ. The intense population pressure has produced broad immunity to the toxin in the entire species within the span of just a few years [source: EurekAlert ]. Some species of clover developed a mutation that caused the poison cyanide to form in the plant's cells. This gave the clover a bitter taste, making it less likely to be eaten. However, when the temperature drops below freezing, some cells rupture, releasing the cyanide into the plant's tissues and killing the plant.

In warm climates, natural selection acted in favor of the cyanide-producing clover, but where the winters are cold, non-cyanide clover was favored. Each kind exists almost exclusively in each climate area [source: Purves ].

What about humans? Are we subject to natural selection as well? It's certain that we were -- humans only became humans because an assortment of traits larger brains, walking upright conferred advantages to those primates that developed them. But we're capable of influencing the distribution of our genes directly. We can use birth control, so that those of who are "fittest" in terms of natural selection might not pass on our genes at all. We use medicine and science to allow many people to live and reproduce who otherwise wouldn't likely survive past childhood.

Much like domesticated animals, which we breed to specifically favor certain traits, humans are influenced by a sort of unnatural selection. However, we're still evolving. Some humans have more reproductive success than others, and the factors that affect that equation have added a layer of human complexity on top of the already complicated interactions of the animal world. In other words, we don't really know what we're going to evolve into. Change is inevitable, but remember that natural selection doesn't care about making "better" humans, just more of us.

Sign up for our Newsletter! Mobile Newsletter banner close. Mobile Newsletter chat close. Mobile Newsletter chat dots. Mobile Newsletter chat avatar. Mobile Newsletter chat subscribe. Life Science. How Natural Selection Works. Hummingbird Image Gallery Rufous hummingbird Selasphorus rufus.

A hummingbird's long bill and tongue evolved to let the bird reach deep into a flower for nectar. See more hummingbird pictures. Understanding Evolution " ". Scanning electron micrograph SEM of Campylobacter fetus bacteria, magnified 4, times. Basketball players are generally tall, while jockeys are generally short. This is a variation on the trait of height. Tall parents tend to have tall children, so we can see that the trait is inheritable.

Mutations are random changes that occur in genes. They're relatively rare, but over thousands of generations, they can add up to very profound changes. Mutations can introduce traits that are completely new and have never appeared in that species before. Sexual reproduction mixes the genes of each parent by splitting, breaking and blending chromosomes the strands that contain DNA during the creation of each sperm and egg.

When the sperm and the egg combine, some genes from the male parent and some genes from the female parent are blended randomly, creating a unique mix of alleles in their offspring. What's a Population? Fitness " ". Population Pressure " ". Evolutionary Stability. Read More. The Superorganism vs. Vestigial and Atavistic Traits. Case Studies in Natural Selection " ". Sources BBC. Dawkins, Richard. The Selfish Gene. May 25, If no button appears, you cannot download or save the media.

Text on this page is printable and can be used according to our Terms of Service. Any interactives on this page can only be played while you are visiting our website. You cannot download interactives. The theory of natural selection was explored by 19th-century naturalist Charles Darwin.

Natural selection explains how genetic traits of a species may change over time. This may lead to speciation, the formation of a distinct new species. Select from these resources to teach your classroom about this subfield of evolutionary biology. Artificial selection is the identification by humans of desirable traits in plants and animals, and the steps taken to enhance and perpetuate those traits in future generations.

Artificial selection works the same way as natural selection, except that with natural selection it is nature, not human interference, that makes these decisions.

Charles Darwin and Alfred Wallace developed the idea of evolution through natural selection. But this idea was not accepted by scientists until more evidence came along. Use this infographic to explore how Darwinism and genetics came together to explain what we know today about evolution.

Join our community of educators and receive the latest information on National Geographic's resources for you and your students. Skip to content. Photograph by Ian Forsyth via Getty Images.

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