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Curr Opin Genet Dev 10 : — Population genetic variation in genome-wide gene expression. Mol Biol Evol 20 : — Rate tests for selection on quantitative characters during macroevolution and microevolution. Charles Darwin was a famous English naturalist.
During his life he proposed the theory of natural selection and how this drives evolution of new species. Darwin is associated with the term 'survival of the fittest' which describes how natural selection works.
The accepted theory of evolution explains that it happens by natural selection. Bacteria can evolve quickly because they reproduce at a fast rate. Mutations of bacteria produce new strains. Some bacteria have become resistant to certain antibiotics , such as penicillin, and cannot be destroyed by the antibiotic. Differences among organisms are not relevant unless they can be inherited. Genetic variation by itself will not result in natural selection unless it exerts some impact on organism survival and reproduction.
However, any time all of Darwin's postulates hold simultaneously—as they do in most populations—natural selection will occur. The net result in this case is that certain traits or, more precisely, genetic variants that specify those traits will, on average , be passed on from one generation to the next at a higher rate than existing alternatives in the population. Put another way, when one considers who the parents of the current generation were, it will be seen that a disproportionate number of them possessed traits beneficial for survival and reproduction in the particular environment in which they lived.
The important points are that this uneven reproductive success among individuals represents a process that occurs in each generation and that its effects are cumulative over the span of many generations. Over time, beneficial traits will become increasingly prevalent in descendant populations by virtue of the fact that parents with those traits consistently leave more offspring than individuals lacking those traits.
If this process happens to occur in a consistent direction—say, the largest individuals in each generation tend to leave more offspring than smaller individuals—then there can be a gradual, generation-by-generation change in the proportion of traits in the population. This change in proportion and not the modification of organisms themselves is what leads to changes in the average value of a particular trait in the population.
Organisms do not evolve; populations evolve. As the name implies, this is the process by which populations of organisms evolve in such a way as to become better suited to their environments as advantageous traits become predominant. This latter topic is particularly difficult for many to grasp, though of course a crucial first step is to understand the operation of natural selection on smaller scales of time and consequence.
For a detailed discussion of the evolution of complex organs such as eyes, see Gregory b. On first pass, it may be difficult to see how natural selection can ever lead to the evolution of new characteristics if its primary effect is merely to eliminate unfit traits. Indeed, natural selection by itself is incapable of producing new traits, and in fact as many readers will have surmised , most forms of natural selection deplete genetic variation within populations.
How, then, can an eliminative process like natural selection ever lead to creative outcomes? To answer this question, one must recall that evolution by natural selection is a two-step process. The first step involves the generation of new variation by mutation and recombination, whereas the second step determines which randomly generated variants will persist into the next generation.
Most new mutations are neutral with respect to survival and reproduction and therefore are irrelevant in terms of natural selection but not, it must be pointed out, to evolution more broadly. The majority of mutations that have an impact on survival and reproductive output will do so negatively and, as such, will be less likely than existing alternatives to be passed on to subsequent generations.
However, a small percentage of new mutations will turn out to have beneficial effects in a particular environment and will contribute to an elevated rate of reproduction by organisms possessing them.
Even a very slight advantage is sufficient to cause new beneficial mutations to increase in proportion over the span of many generations. Rather, beneficial mutations simply increase in proportion from one generation to the next because, by definition, they happen to contribute to the survival and reproductive success of the organisms carrying them.
Eventually, a beneficial mutation may be the only alternative left as all others have ultimately failed to be passed on. Again, mutation does not occur in order to improve fitness—it merely represents errors in genetic replication. This means that most mutations do not improve fitness: There are many more ways of making things worse than of making them better. It also means that mutations will continue to occur even after previous beneficial mutations have become fixed.
As such, there can be something of a ratcheting effect in which beneficial mutations arise and become fixed by selection, only to be supplemented later by more beneficial mutations which, in turn, become fixed. All the while, neutral and deleterious mutations also occur in the population, the latter being passed on at a lower rate than alternatives and often being lost before reaching any appreciable frequency.
Of course, this is an oversimplification—in species with sexual reproduction, multiple beneficial mutations may be brought together by recombination such that the fixation of beneficial genes need not occur sequentially.
Likewise, recombination can juxtapose deleterious mutations, thereby hastening their loss from the population. Nonetheless, it is useful to imagine the process of adaptation as one in which beneficial mutations arise continually though perhaps very infrequently and with only minor positive impacts and then accumulate in the population over many generations.
The process of adaptation in a population is depicted in very basic form in Fig. Several important points can be drawn from even such an oversimplified rendition:. Mutations are the source of new variation. Natural selection itself does not create new traits; it only changes the proportion of variation that is already present in the population.
The repeated two-step interaction of these processes is what leads to the evolution of novel adaptive features. Mutation is random with respect to fitness. Natural selection is, by definition, non-random with respect to fitness. Mutations occur with all three possible outcomes: neutral, deleterious, and beneficial. Beneficial mutations may be rare and deliver only a minor advantage, but these can nonetheless increase in proportion in the population over many generations by natural selection.
The occurrence of any particular beneficial mutation may be very improbable, but natural selection is very effective at causing these individually unlikely improvements to accumulate. Natural selection is an improbability concentrator.
No organisms change as the population adapts. Rather, this involves changes in the proportion of beneficial traits across multiple generations. The direction in which adaptive change occurs is dependent on the environment.
A change in environment can make previously beneficial traits neutral or detrimental and vice versa. Adaptation does not result in optimal characteristics. It is constrained by historical, genetic, and developmental limitations and by trade-offs among features see Gregory b. As Darwin wrote in a letter to Joseph Hooker 11 Sept.
The process of adaptation by natural selection is not forward-looking, and it cannot produce features on the grounds that they might become beneficial sometime in the future. In fact, adaptations are always to the conditions experienced by generations in the past. A highly simplified depiction of natural selection Correct and a generalized illustration of various common misconceptions about the mechanism Incorrect.
Properly understood, natural selection occurs as follows: A A population of organisms exhibits variation in a particular trait that is relevant to survival in a given environment. In this diagram, darker coloration happens to be beneficial, but in another environment, the opposite could be true.
As a result of their traits, not all individuals in Generation 1 survive equally well, meaning that only a non-random subsample ultimately will succeed in reproducing and passing on their traits B.
Note that no individual organisms in Generation 1 change, rather the proportion of individuals with different traits changes in the population. The individuals who survive from Generation 1 reproduce to produce Generation 2. C Because the trait in question is heritable, this second generation will mostly resemble the parent generation.
However, mutations have also occurred, which are undirected i. In this environment, lighter mutants are less successful and darker mutants are more successful than the parental average. Once again, there is non-random survival among individuals in the population, with darker traits becoming disproportionately common due to the death of lighter individuals D.
This subset of Generation 2 proceeds to reproduce. Again, the traits of the survivors are passed on, but there is also undirected mutation leading to both deleterious and beneficial differences among the offspring E. F This process of undirected mutation and natural selection non-random differences in survival and reproductive success occurs over many generations, each time leading to a concentration of the most beneficial traits in the next generation.
By Generation N , the population is composed almost entirely of very dark individuals. The population can now be said to have become adapted to the environment in which darker traits are the most successful. This contrasts with the intuitive notion of adaptation held by most students and non-biologists.
In the most common version, populations are seen as uniform, with variation being at most an anomalous deviation from the norm X.
It is assumed that all members within a single generation change in response to pressures imposed by the environment Y. When these individuals reproduce, they are thought to pass on their acquired traits. Moreover, any changes that do occur due to mutation are imagined to be exclusively in the direction of improvement Z. Studies have revealed that it can be very difficult for non-experts to abandon this intuitive interpretation in favor of a scientifically valid understanding of the mechanism.
Diagrams based in part on Bishop and Anderson In its most basic form, natural selection is an elegant theory that effectively explains the obviously good fit of living things to their environments. As a mechanism, it is remarkably simple in principle yet incredibly powerful in application. However, the fact that it eluded description until years ago suggests that grasping its workings and implications is far more challenging than is usually assumed.
Three decades of research have produced unambiguous data revealing a strikingly high prevalence of misconceptions about natural selection among members of the public and in students at all levels, from elementary school pupils to university science majors Alters ; Bardapurkar ; Table 2 Footnote 7. It is particularly disconcerting and undoubtedly exacerbating that confusions about natural selection are common even among those responsible for teaching it Footnote 8.
Two obvious hypotheses present themselves for why misunderstandings of natural selection are so widespread. The first is that understanding the mechanism of natural selection requires an acceptance of the historical fact of evolution, the latter being rejected by a large fraction of the population.
While an improved understanding of the process probably would help to increase overall acceptance of evolution, surveys indicate that rates of acceptance already are much higher than levels of understanding. And, whereas levels of understanding and acceptance may be positively correlated among teachers Vlaardingerbroek and Roederer ; Rutledge and Mitchell ; Deniz et al.
The second intuitive hypothesis is that most people simply lack formal education in biology and have learned incorrect versions of evolutionary mechanisms from non-authoritative sources e. Inaccurate portrayals of evolutionary processes in the media, by teachers, and by scientists themselves surely exacerbate the situation e.
However, this alone cannot provide a full explanation, because even direct instruction on natural selection tends to produce only modest improvements in students' understanding e. There also is evidence that levels of understanding do not differ greatly between science majors and non-science majors Sundberg and Dini Misconceptions are well known to be common with many perhaps most aspects of science, including much simpler and more commonly encountered phenomena such as the physics of motion e.
The source of this larger problem seems to be a significant disconnect between the nature of the world as reflected in everyday experience and the one revealed by systematic scientific investigation e.
Intuitive interpretations of the world, though sufficient for navigating daily life, are usually fundamentally at odds with scientific principles. If common sense were more than superficially accurate, scientific explanations would be less counterintuitive, but they also would be largely unnecessary.
It has been suggested by some authors that young students simply are incapable of understanding natural selection because they have not yet developed the formal reasoning abilities necessary to grasp it Lawson and Thompson This could be taken to imply that natural selection should not be taught until later grades; however, those who have studied student understanding directly tend to disagree with any such suggestion e.
Overall, the issue does not seem to be a lack of logic Greene ; Settlage , but a combination of incorrect underlying premises about mechanisms and deep-seated cognitive biases that influence interpretations.
These tend to persist unless replaced with more accurate and equally functional information. In this regard, some experts have argued that the goal of education should be to supplant existing conceptual frameworks with more accurate ones see Sinatra et al. Other authors suggest that students do not actually maintain coherent conceptual frameworks relating to complex phenomena, but instead construct explanations spontaneously using intuitions derived from everyday experience see Southerland et al.
In some cases, students may attempt a more complex explanation but resort to intuitive ideas when they encounter difficulty Deadman and Kelly In either case, it is abundantly clear that simply describing the process of natural selection to students is ineffective and that it is imperative that misconceptions be confronted if they are to be corrected e.
Whereas the causes of cognitive barriers to understanding remain to be determined, their consequences are well documented. As a result, each of the fundamental components of natural selection may be overlooked or misunderstood when it comes time to consider them in combination, even if individually they appear relatively straightforward. The following sections provide an overview of the various, non-mutually exclusive, and often correlated misconceptions that have been found to be most common.
All readers are encouraged to consider these conceptual pitfalls carefully in order that they may be avoided. 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.
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