As Darwin anticipated, extensive variation among individuals has now been well established to exist at the physical, physiological, and behavioral levels. Thanks to the rise of molecular biology and, more recently, of genomics, it also has been possible to document variation at the level of proteins, genes, and even individual DNA nucleotides in humans and many other species.
Darwin saw that overproduction and limited resources create a struggle for existence in which some organisms will succeed and most will not. He also recognized that organisms in populations differ from one another in terms of many traits that tend to be passed on from parent to offspring.
Darwin's brilliant insight was to combine these two factors and to realize that success in the struggle for existence would not be determined by chance, but instead would be biased by some of the heritable differences that exist among organisms. Specifically, he noted that some individuals happen to possess traits that make them slightly better suited to a particular environment, meaning that they are more likely to survive than individuals with less well suited traits.
As a result, organisms with these traits will, on average, leave more offspring than their competitors. Whereas the origin of a new genetic variant occurs at random in terms of its effects on the organism, the probability of it being passed on to the next generation is absolutely non-random if it impacts the survival and reproductive capabilities of that organism. The important point is that this is a two-step process: first, the origin of variation by random mutation, and second, the non-random sorting of variation due to its effects on survival and reproduction Mayr Though definitions of natural selection have been phrased in many ways Table 1 , it is this non-random difference in survival and reproduction that forms the basis of the process.
In order to study the operation and effects of natural selection, it is important to have a means of describing and quantifying the relationships between genotype gene complement , phenotype physical and behavioral features , survival, and reproduction in particular environments.
In the most basic terms, one can state that the more offspring an individual produces, the higher is its fitness. Second, it places undue emphasis on survival: While it is true that dead organisms do not reproduce, survival is only important evolutionarily insofar as it affects the number of offspring produced.
Traits that make life longer or less difficult are evolutionarily irrelevant unless they also influence reproductive output. Indeed, traits that enhance net reproduction may increase in frequency over many generations even if they compromise individual longevity. Conversely, differences in fecundity alone can create differences in fitness, even if survival rates are identical among individuals.
Third, this implies an excessive focus on organisms, when in fact traits or their underlying genes equally can be identified as more or less fit than alternatives. Lastly, this phrase is often misconstrued as being circular or tautological Who survives? The fittest. Who are the fittest? Those who survive. However, again, this misinterprets the modern meaning of fitness, which can be both predicted in terms of which traits are expected to be successful in a specific environment and measured in terms of actual reproductive success in that environment.
Directional natural selection can be understood as a process by which fitter traits or genes increase in proportion within populations over the course of many generations. It must be understood that the relative fitness of different traits depends on the current environment.
Thus, traits that are fit now may become unfit later if the environment changes. Conversely, traits that have now become fit may have been present long before the current environment arose, without having conferred any advantage under previous conditions. Finally, it must be noted that fitness refers to reproductive success relative to alternatives here and now —natural selection cannot increase the proportion of traits solely because they may someday become advantageous.
Careful reflection on how natural selection actually works should make it clear why this is so. Though each has been tested and shown to be accurate, none of the observations and inferences that underlies natural selection is sufficient individually to provide a mechanism for evolutionary change Footnote 6. Overproduction alone will have no evolutionary consequences if all individuals are identical. 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 :. PLoS Comput Biol 8 3 : e PLoS expressly disclaims any and all warranties and liability in connection with the information found in the releases and articles and your use of such information.
PLoS Comput Biol 5 11 : e No prior permission is required from the authors or publisher. This means that sex and recombination can restore variation eliminated by past selection. The surface curvature is not too strong. If too strong, the recombination load is severe. Genetics 78 , — Otto, S.
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It is not possible for an isolated individual to reproduce. Asexual reproduction and evolution In asexual reproduction an exact genetic copy of the parent organism is produced a clone. Advantages of asexual reproduction The population can increase rapidly when the conditions are favourable. Only one parent is needed. It is more time and energy efficient as you don't need a mate. It is faster than sexual reproduction.
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