If the environment changes rapidly, some species may not be able to adapt fast enough through natural selection. Through studying the fossil record, we know that many of the organisms that once lived on Earth are now extinct.
Dinosaurs are one example. An invasive species , a disease organism, a catastrophic environmental change, or a highly successful predator can all contribute to the extinction of species.
Today, human actions such as overhunting and the destruction of habitats are the main cause of extinctions. Extinctions seem to be occurring at a much faster rate today than they did in the past, as shown in the fossil record. The audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit.
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Any force in the environment that favors or disfavors traits is a selective agent. A force can be biological, like a predator, or physical, like temperature. Over time, populations subjected to different selective agents may become so different that they are no longer able to breed with one another.
The biological definition of species is a group of organisms that can successfully breed with each other. Under this definition, when populations can no longer breed with each other, they are considered to be different species. In the s, many people were trying determine why there were so many different kinds of plants and animals in the world. Charles Darwin wondered about the diversity of animals he saw while in the Galapagos Islands.
This led him to develop the theory of natural selection, which is the best explanation we have for the diversity of life. Alfred Russell Wallace also hypothesized that the environment could help to shape the diversity of life by favoring certain traits over others. Wallace noticed that insects in the jungles of Africa and South America were very well adapted to unique environments. These two men, working independently of one another, developed the same basic explanation for the diversity of life: natural selection.
These principles are supported by current scientific research. For natural selection to occur, a population must have a wide variety of individuals with different traits.
For example, natural selection would not influence fish body color if all individuals in a population were exactly the same color. The term phenotype is used to describe these physical traits. The term genotype describes the specific genetic code in the DNA molecular structure that produces a certain phenotype. Variations in genotypes can also produce variations in phenotypes. New genotypes can be produced through the natural process of genetic mutation.
A mutation is an error that is made during the DNA copying process. The mutation results in a change in the genetic code or genotype. Sometimes mutations errors occur within the sections of the DNA strand that do not code for any phenotype or trait. Similarly, some mutations are minor and do not cause any significant change in the appearance, physiology, or behavior of the organism. 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. Teaching biological evolution in higher education. Boston: Jones and Bartlett; Google Scholar. Teaching evolution in higher education. Development and evaluation of the conceptual inventory of natural selection.
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Trends Cogn Sci. Reinventing natural selection. Gould SJ. Shades of Lamarck. In: The Panda's Thumb. New York: Norton; Greene ED. The logic of university students' misunderstanding of natural selection.
Gregory TR. Evolution as fact, theory, and path. A second process called genetic drift describes random fluctuations in allele frequencies in populations, which can eventually cause a population of organisms to be genetically distinct from its original population and result in the formation of a new species.
Further Exploration Concept Links for further exploration evolution genetic drift natural selection mutation allele gene flow Hardy-Weinberg equilibrium species allele frequency divergence reproduction.
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