Natural selection

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In Darwin's comprehensive theory of evolution, there can actually be elucidated at least five major, largely independent theories. The two basic theories, and the ones which I will treat here, are: (1) the theory of evolution by common descent, and (2) the theory of modification through natural selection. The first is a kinematic theory which deals with non-causal relations between things — it deals with the pattern of evolution. The latter is a dynamic theory which deals with mechanisms and causal relationships B it deals with the process. Other theories offered by Darwin deal with (3) evolution as such (the fact of evolution), (4) the gradualness of evolution, and (5) populational speciation.

The "theory of descent with modification" essentially postulates that all organisms have descended from common ancestors by a continuous process of branching. In other words, all life evolved from one kind of organism or from a few simple kinds, and each species arose in a single geographic location, from another species that preceded it in time. Evolutionists have marshaled substantial evidence for the theory of descent with modification. That is, the "pattern of evolution" is well documented by the fossil record, the distribution patterns of existing species, methods of dating fossils, and comparison of homologous structures. Interestingly, all of the classical arguments for evolution are fundamentally arguments for imperfections that reflect history. They fit the pattern of observing that the leg of Reptile B is not the best for walking, because it evolved from Fish A. In other words, why would a rat run, a bat fly, a porpoise swim and a man type all with the same structures utilizing the same bones unless inherited from a common ancestor?

Evidence is so overwhelming for the theory of descent with modification that only religious fundamentalists have attempted to challenge this theory. Among these are the Ascientific creationists.@ Scientific creationists@ are a specific group of creationists who maintain that modern organisms did not descend from common ancestors, and that their only historical connectedness is in the mind of God. Instead, scientific creationists promulgate the view that living organisms are immutable, and were all created by God in a short time period, on a earth whose age is generally measured in 1000s of years. The substantial fossil record is dismissed in various ways, including as a trick of God and as an artifact from the Great Flood (with some organisms sinking faster than others and thus on a lower fossil plane). Although some individual presentations by scientific creationists are quite sophisticated, the overall theory of scientific creationism runs counter to an enormous body of evidence and thus is strongly criticized by most of the scientific community.

The second theory of Darwin, the "theory of modification through natural selection," is one explanation offered for how evolution might have occurred, i.e, the "process" by which evolution took place and arrived at the pattern. This theory of natural selection was the most revolutionary and controversial concept advanced by Darwin. While the theory of descent with modification was accepted soon after its introduction, the theory of natural selection took until the mid-1900s to be accepted by the scientific community. By providing a purely non-teleogical, materialistic explanation for all phenomenon of living nature, it was said it "dethroned God."

According to this theory, natural selection is the directing or creative force of evolution. Natural selection is considered far more than just a minor force for weeding out unfit organisms. Even Paley and other natural theologians accepted natural selection, albeit as a devise for removing unfit organisms, rather than as a directive force for creating new species and new designs. Natural selection had three radical components— (a) purposelessness (no higher purpose, just the struggle of individuals to survive and reproduce); (b) philosophical materialism (matter is seen as the ground of all existence with spirit and mind being produced by or a function of the material brain); and (c) the view that evolution is not progressive from lower to higher, but just an adaptation to local environments; it could form a man with his superior brain or a parasite, but no one could say which is higher or lower.

Concrete evidence for the theory of modification by natural selection is limited to microevolution, such as seen in the systematic color change in the peppered moth, Biston betularia which was observed over a 50-year period in England, or through artificial selection, whereby various breeds of animals and varieties of plants have been produced which are different in some respect from their ancestors. The evidence that natural selection directs the major transitions between species and originates new designs (macroevolution) necessarily involves extrapolation from these evidences on the microevolutionary level. That is, it is inferred that if moths can change their color in 50 years, then new designs or entire new genera can originate over millions of years. If geneticists see population changes for fruit flies in laboratory bottles, then given eons of time, birds can be built from reptiles and fish with jaws from jawless ancestors. One of Darwin's chief purposes in publishing the Origin of Species was to show that natural selection had been the chief agent of the change presented in the theory of descent with modification. The validity of making this extrapolation has recently come under strong challenge from top evolutionists.

Some of the confusion in the dialogue between evolutionists and creationists is what is being referred to by the term “evolution” or “theory of evolution.” For evolutionists, a working definition of the term "evolution" is generally descent with modification or a change of gene frequencies in populations. Since there is considerable experimental and observational evidence of populations systematically changing over time, evolutionists speak of "the fact of evolution." There is evidence on the microevolutionary level (change in gene frequencies within populations), in terms of artificial selection. On a macroevolutionary level (large-scale events such as speciation and origin of new designs), various evidences such as fossil records, biogeography, and studies of homologies have strongly supported the view that all organisms have descended from common ancestors. In fact, renowned evolutionist Mayr contends that “the facts of biogeography posed some of the most insoluble dilemmas for the creationists and were eventually used by Darwin as his most convincing evidence in favor of evolution.” Darwin helped to establish the "fact of evolution." In 1859, most scientists and laymen believed that the world was constant. The massive evidence that Darwin presented was so convincing that within a few years every biologist became an evolutionist, believing that the world was the product of a continuing process of change. For most biologists today, evolution is no longer a theory but simply a fact. They may disagree with the mechanisms, but that evolution takes place — that there is a systematic change in populations — is unquestioned. The statement that “evolution is a fact,” draws the ire of scientific creationists, of course. However, scientific creationists represent only a small body of those individuals that do believe in a creation by a supreme being. Nonetheless, other religious adherents likewise often speak of opposition to evolution, despite having a belief system that allows descent with modification and change in gene frequencies in populations. There are a couple of ready explanations for this. For one, there is the case of terminological confusion. When some individuals and religious adherents use the term “evolution,” they are not referring to simply a systematic change in populations over time — which is a highly established fact — but are instead treating the word “evolution” as synonymous with the specific Darwinian theory of evolution by natural selection — a theory with which even some eminent evolutionists find troublesome as the sole explanation for observed changes. Thus, religious adherents may reject “evolution” since they see the concept of randomness in natural selection as counter to their belief that a Supreme Being directs changes. Furthermore, popular writings often tend to create an artificial dichotomy : either belief in a Creator is correct or evolution is correct — an “either-or dichotomy” which tends to foster an erroneous view of the relationship between evolution and religion. By such means, evolution and religion (specifically creation by a God) are presented as if mutually exclusive alternatives. Thus, many religious adherents reject evolution out of hand, not wishing to reject God. Textbook authors have often confused the dialogue on evolution by treating the term as if it signified one unified whole — not only the fact of evolution having occurred, but also the specific Darwinian and neo-Darwinian theories regarding natural selection, gradualism, speciation, and so forth. Certain textbook authors, in particular, have exacerbated this terminological confusion by lumping “evidences of evolution” into a section placed immediately after a comprehensive presentation on Darwin's overall theory — thereby creating the misleading impression that the evidences are supporting all components of Darwin's theory, including natural selection. In reality, the confirming information is invariably limited to the phenomenon of evolution having occurred (descent from a common ancestor or change of gene frequencies in populations), or perhaps including evidence of natural selection within populations.

The issue has been further complicated by the fact that textbooks have persisted in presenting some proofs for evolution which are false or misleading, as pointed out by Jonathan Wells in his book Icons of Evolution. These widely-known but misleading teachings include the famous Miller –Urey experiment in which sparks are sent through a mixture of gases and yield the building blocks of amino acids, and the drawings by Ernst Haeckel of the early embryonic stages of such vertebrates as fish, chick, rabbits and humans, whereby it is exhibited that the earliest stages in all of these vertebrates are virtually identical. Wells reports that scientists have known for years both that the Miller-Urey experiments did not really approximate conditions of the early earth and that Haeckel had faked his drawings, since in reality the vertebrate embryos never look as similar as he made them look. These errors are well-known, yet textbook authors persist in using these examples. Another interesting case is the classic example of natural selection as seen in the case of the peppered moth (Biston belularia) in England, known as a case of industrial melanism, whereby a shift toward darker melanic forms is seen and attributed to an heightened predation by birds of the light-colored moths, because the lighter forms could more easily be seen on the tree trunks which have been increasingly darkened from pollution. In these cases, individuals have known that peppered moths do not normally alight on tree trunks, and there are even inverse correlations with pollution in many situations. Textbook photos are generally staged by gluing or pinning the moths to tree trunks. Some authors have responded that they knew the peppered moth case had problems, but they were good examples because they were easily grasped by the students. Use of such flawed cases has the unfortunate consequence of causing distrust of science by the students.

Natural selection is a process by which biological populations are altered over time, as a result of the propagation of heritable traits that affect the capacity of individual organisms to survive and reproduce. It is one of several mechanisms that give rise to the evolution of biological species (other mechanisms include genetic drift and gene flow.) However, natural selection has a special significance because it is believed to be the one responsible for organisms being adapted to their environment. The theory of natural selection was proposed by Charles Darwin and Alfred Russel Wallace in 1858, though vaguer and more obscure formulations had been arrived at by earlier workers.

Natural selection can be subdivided into two types: (i) ecological selection, which arises from the portion of an organism's environment not related to direct sexual competition (such as the availability of food, the presence of predators, and so forth); and (ii) sexual selection, which arises from the competition for mates between individuals of the same sex. The reason for this division is that the effects of sexual selection can produce results that seem counterintuitive from the point of view of ecological selection alone (a famous example being the tails of peacocks, which, though cumbersome, serve an important purpose in courtship displays.)

Natural selection is distinguished from artificial selection, which is the alteration of domesticated species resulting from human intervention as opposed the "natural environment". However, the mechanisms of natural and artificial selection are essentially identical, and in fact the observed effects of artificial selection were used by Darwin to illustrate how natural selection works.

The modern theory of natural selection is formulated in terms of genetic differences between individuals, resulting in differences in the frequency of alleles in a population over successive generations. The genetic variation on which natural selection acts are now understood to arise from random mutations.

Overview

The basic concept of natural selection is that conditions (or "nature") determine (or "select") how well particular traits of organisms can serve the survival and reproduction of the organism; organisms lacking these traits might die before reproducing, or be less prolific. As long as environmental conditions remain the same, or similar enough that these traits continue to be adaptive, such traits will become more common within populations. Loss of the species' ecological niche or crowding-out due to population growth can change drastically the adaptive traits required to survive—in such conditions, or in any circumstance where survival is determined by ecology more than by the secondary sexual characteristics, an ecological selection is taking place (this term is used solely to differentiate processes irrelevant to mating, and is of modern usage, having grown up with the field of ecology itself).

Darwin's scientific theory of the evolution of species through natural selection starts from the premise that an organism's traits vary in a nondeterministic way from parent to offspring, a process Darwin called "individuation". This theory does not make any specific claims as to how this process works, although more recent scientific discoveries in genetics explain several mechanisms that occur in the process of reproduction: in the case of both asexual and sexual reproduction, random mutation (including DNA transcription errors); in the case of sexual reproduction (which mixes the DNA of two parents into an offspring), gene flow and genetic drift are also important mechanisms. Competition (typically among males to impregnate females) for mates produces sexual selection—a process which Darwin considered secondary to ecological in most species.

Natural selection does not distinguish between ecological selection and sexual selection, as it is concerned with traits, for example, dexterity of movement, on which both may operate simultaneously. If a particular variation makes the offspring which manifest it better suited to survival or to successful reproduction, that offspring and its descendants will be more likely to survive than those offspring without the variation. The original traits, as well as any maladaptive variations, will disappear as the offspring who carry them are replaced by their more successful relatives.

Therefore, certain traits are preserved due to the selective advantage they provide to their holders, allowing the individual to leave more offspring than individuals without the trait(s). Eventually, through many iterations of this process, organisms will develop more and more complex adaptive traits.

Mechanisms of natural selection

In Chapter 4 of The Origin of Species, Darwin wrote:

It may be said that natural selection is daily and hourly scrutinising, 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 lapses 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.

What makes one trait more likely to succeed is highly dependent on environmental factors, including the species' predators, food sources, abiotic stress, physical environment, and so on. When members of a species become separated, such as geographically, they face different environments and tend to develop in different directions. After a long period of time, their traits will have developed along different paths to such an extent that they can no longer interbreed, at which point they are considered separate species. This is why a species will sometimes separate into multiple species, rather than simply being replaced by a newer form of the species (from this fact Darwin suggested that all species today have evolved from a common ancestor).

Additionally, some scientists have theorized that an adaptation which serves to make the organism more adaptable in the future will also tend to supplant its competitors even though it provides no specific advantage in the near term. Descendants of that organism will be more varied and therefore more resistant to extinction due to environmental catastrophes and extinction events. This has been proposed as one reason for the rise of mammals. While this form of selection is possible, it is more likely to play an important role in cases where selection for adaptation is continuous. For example, the Red Queen hypothesis suggests that sex might have evolved to help organisms adapt to deal with parasites.

Natural selection can be expressed as the following general law (taken from the conclusion of The Origin of Species):

1. IF there are organisms that reproduce, and
2. IF offspring inherit traits from their progenitor(s), and
3. IF there is variability of traits, and
4. IF the environment cannot support all members of a growing population,
5. THEN those members of the population with less-adaptive traits (determined by the environment) will die out, and
6. THEN those members with more-adaptive traits (determined by the environment) will thrive

The result is the evolution of species.

This is a continuing process—it accounts for how species change, and can account for both the extinction of one species and the creation of a new one. The formulation does not rule out selection occurring at all biological levels (e.g., gene, organism, group), and the particular process of introducing new traits does not matter.

Darwin did not maintain that natural selection was the only mechanism of evolution, however, as he pronounced in the introduction to The Origin of Species: "I am convinced that [it] has been the most important, but not the exclusive means of modification."

History of the principle

Charles Darwin's discovery of the principle of natural selection, as his explanation for the origin of species, occurred in about 1838. Over the next twenty years, he shared it with only a very small number of acquaintances, while he amassed evidence in its favor. He first outlined his theory in two unpublished manuscripts, written in 1842 and 1844. In 1858, Alfred Russel Wallace independently discovered the principle, and wrote a letter to Darwin, explaining his hypothesis. This prompted a reading, at the Linnean Society, of tracts from both men describing the principle that year. Darwin published his detailed theory the following year, in The Origin of Species.

Unbeknownst to both Darwin and Wallace, the principle of natural selection had been previously hypothesized by others. Pierre Louis Moreau de Maupertuis in 1745, Erasmus Darwin in 1794–1796, William Charles Wells in 1813, and Patrick Matthew in 1831 were amongst the first to grasp the idea. Maupertuis' discovery is in dispute, but has enough substantial evidence in its favor to warrant mention. Erasmus Darwin was a contemporary and colleague of Wells—not to mention the grandfather of Charles Darwin—and he expressed much of his theory of evolution in poetic verse. His formal exposition of the hypothesis lacks a structured formulation, but has enough merit to be considered a possibility. Wells' hypothesis, applied solely to explain the origin of human races, had been presented in person at the Royal Society. Matthew's hypothesis had appeared in an appendix to his book on arboriculture. Richard Owen also claimed precedence over Darwin. Edward Blyth had also proposed natural selection, as a mechanism of keeping species constant.

Scope and role of natural selection

Natural selection need not apply solely to biological organisms; in theory, it applies to all systems in which entities reproduce in a way that includes both inheritance and variation. Thus, a form of natural selection can occur in the nonbiological realm. Computer-based systems (e.g., artificial life) have shown that natural selection can be highly effective in adapting entities to their environments; whether such systems have demonstrated that natural selection per se can generate complexity is contested.^[1]

Impact of the idea

Perhaps the most radical claim of Darwin's theory of evolution through natural selection is that "elaborately constructed forms, so different from each other, and dependent on each other in so complex a manner" have evolved out of the simplest forms of life and according to a few simple principles. It is this fundamental claim that has inspired some of Darwin's most ardent supporters—and that has provoked the most profound opposition.

In addition, many theories of Artificial selection have been proposed to suggest that economic or social fitness factors assessed by other humans or their built environments are somehow biological or inevitable—Social Darwinism. Others held that there was an evolution of societies analogous to that of species. Many theories of eugenics were created in an attempt to address these issues. Darwin's ideas, along with those of Adam Smith and Karl Marx, are considered by most historians to have had a profound influence on 19th-century thought.

References

ISBN links support NWE through referral fees

• ^  Darwin, Charles (1859). The Origin of Species (First Edition).
• ^  Wolfram, Stephen (2002). A New Kind of Science, p. 1001. Wolfram Media, Inc. ISBN 1579550088

Further reading

• Endler, John A (1986). "Natural Selection in the Wild". Princeton University Press.
• Maynard Smith, John (1993). "The Theory of Evolution. Cambridge University Press.
• Williams, George C (1992). Natural Selection: Domains, Levels and Challenges. Oxford University Press

External links

• Introduction to evolutionary biology (has a section on natural selection in context of evolution)
• Evolution by Natural Selection - An introduction to the logic of the theory of natural selection
• Darwin's Precursors and Influences. Part 4 — Natural selection; by John Wilkins

See also

• adaptation
• artificial selection
• directional selection
• disruptive selection
• ecological selection
• evolution
• Fitness
• genetic drift
• negative selection
• selection
• sexual selection
• stabilizing selection
• survival of the fittest

Basic topics in evolutionary biology	(edit)
Processes of evolution: evidence - macroevolution - microevolution - speciation
Mechanisms: natural selection - genetic drift - gene flow - mutation - phenotypic plasticity
Modes: anagenesis - catagenesis - cladogenesis
History: History of evolutionary thought - Charles Darwin - The Origin of Species - modern evolutionary synthesis
Subfields: population genetics - ecological genetics - human evolution - molecular evolution - phylogenetics - systematics

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