Natural selection

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For the computer game, see Natural Selection (computer game).

Natural selection is the name given to processes that cause individual organisms endowed with favourable traits to reproduce more than those displaying less favourable traits. Natural selection evaluates the performance of organisms in the struggle for existence as determined by each individual's phenotype (traits) and by the way these differ from those of other individuals. When traits favored by natural selection are heritable, their frequency will tend to increase over the generations resulting in adaptive evolution. Natural selection is the force driving the evolution of adaptive organismic characteristics, can let populations become different from each other until they become different species, and is a central organizing concept of modern biological research.

The term was introduced by Charles Darwin in 1859 in his book The Origin of Species, in analogy to how farmers select individuals that display desirable characteristics in the hope of improving crops or live stock (see artificial selection).

Introduction

Organisms can differ from each other in ways that affect their biological function and thus their, or their groups' (see Kin selection), probability of surviving and reproducing. For instance gazelles that run faster than others can survive for additional breeding seasons because their running faster allows them to escape better from predators year after year. The faster gazelles therefore end up reproducing more than slower gazelles.

The processes that causes the enhanced reproduction of some and the reduced reproduction of others were described metaphorically by Darwin as an act of selection by nature, but concretely what really happens is that differential relative function (i.e. one's performance relative to that of others) ultimately results in differential reproduction.

Natural selection is thus the phrase used to refer to the totality of the biological processes that participate in causing such non-random differences in survival and reproduction, differences which in technical biological language are called differences in "fitness".

Variant traits are at times heritable so that the preferential reproduction of individuals that display heritable variant traits which boost reproduction should let such traits become more common over the generations, i.e., natural selection can result in adaptive evolution. Under the pressure of natural selection, populations of the same species that live in different environments can become different from each other and even different species.

Biological research had provided ample support for natural selection being the force driving the evolution of the astounding ways in which organisms cope with their environments and the evolution of the millions of living and extinct species as well as for adaptive divergence being the reason why there is a myriad of species rather than a single one that monopolizes the whole biosphere.

Modern evolutionary biology envisions, and studies, the process of natural selection as a complex causal chain that goes from the genetics and developmental biology of the generation of trait differences between individuals, to the biomechanics and ecology of how such trait differences result in differences in the performance of individual organisms or groups thereof, and ultimately to the reproductive biology of how such differences in organismic performance translate into non-random differences in reproduction among individuals.

In the case of the faster gazelle this chain would include how the fertilized egg develops into a gazelle endowed with bodily structures that enable it to run faster, how the structurally superior gazelle actually uses its body to run faster, how the gazelle's faster running allows it to escape from predators, and how the gazelle's ability to avoid predation allows it to produce more offspring than slower gazelles, e.g., during additional breeding seasons.

Therefore, natural selection cannot be summarized by the circular phrase "preferential reproduction of those who reproduce most", as it is often charged by those who forget, or have never been exposed to, the just mentioned chain of causation that underlies fitness differences.

The reality and pervasive consequences of natural selection throughout the biological world makes the fact that individuals can differ from each other into a crucial property of life. Before Darwin, the prevalent view among naturalists was that differences among individual organisms within a species are uninteresting departures from each species' Platonic ideal or typus. But by the 19th century the Platonic metaphysical straightjacket was already being challenged by emerging evidence that gradual but colossal geological change had occurred throughout the history of the earth; and some evolutionists were embracing the view that adaptive evolution can occur when organisms transmit to their progeny the modifications they acquire while dealing with the environment (which proved to be false; see Lamarckism). In contrast, Darwin argued that adaptive evolution results from the fact that heritable differences between individuals which arise without directionality can allow some individuals to perform better than others in the struggle for existence and thus to reproduce more, so that over the generations the functionally superior traits will increase in frequency.

Overview

The basic concept of natural selection is that "nature" (the physical and biological environment) "selects" variations in characteristics or traits which improve individual survival and reproduction (adaptive traits) and selects against unfavourable traits which burden individuals (maladaptive traits), described as follows by Darwin in Chapter 4 of The Origin of Species:

Can it, then, be thought improbable, seeing that variations useful to man have undoubtedly occurred, that other variations useful in some way to each being in the great and complex battle of life, should sometimes occur in the course of thousands of generations? If such do occur, can we doubt (remembering that many more individuals are born than can possibly survive) that individuals having any advantage, however slight, over others, would have the best chance of surviving and of procreating their kind? On the other hand, we may feel sure that any variation in the least degree injurious would be rigidly destroyed. This preservation of favourable variations and the rejection of injurious variations, I call Natural Selection.

Individuals disadvantaged by maladaptive traits might not survive until reproduction and/or reach reproduction in bad condition and only be able to produce fewer and/or lower-quality progeny, while individuals carrying favorable traits might be more likely to survive until reproduction and/or be able to produce more and/or higher-quality progeny. As long as environmental conditions remain the same, or similar enough, the traits' adaptive values will remain unchanged, and when the traits are heritable, adaptive traits will become more common and maladaptive ones rarer over the generations. Sudden or gradual changes in the physical and biological environment, where the latter includes changes brought about by the activities of the very population of interest, can change the adaptive value of a trait regardless of the trait's previous evolutionary history.

Darwin's theory of natural selection starts from the premise that traits in organisms vary in a non-preordained way among individuals. Darwin called "individuation" the process by which variation between individuals is generated but did not make any specific claims as to how such differences arise. In general, phenotypic (trait) differences between individuals can result from environmental effects (e.g. bad nutrition) as well as from genetic differences. Although differences caused by environmental factors can be conspicuous, they are mostly not heritable in a lasting way and thus the fitness differences they may cause do not alter gene frequencies and hence cannot result in adaptive evolution. Phenotypic differences triggered by heritable genetic factors can also be striking and are necessary for natural selection to result in adaptive evolutionary change. Modern genetics has characterized several mechanisms that generate heritable genetic differences between individuals: Permanent alterations of the genetic material (DNA), e.g., can result from errors during DNA replication, as well as from damage during the transcription of genes and genetic recombination, or caused by chemicals and physical agents (e.g. X rays, see mutagen); the natural mutation rate of humans is on the order of 10 per nucleotide per generation. In sexual populations, genetic recombination and segregation/syngamy mix the DNA of two parents into that of offspring so that the latter are guaranteed to differ genetically from each other and from their parents.

Although darwinian fitness is often thought to be partitionable into an ecological ability (viability) component and a fecundity component (which is often the component most affected by sexual selection), many traits can be involved in determining more than one fitness component. For example, motor skills not only influence foraging success and survival but often make one attractive to mates. Sexual selection, therefore, can but need not lead to ecologically maladaptative traits. Recent modelling work, moreover, suggests that even sexual selection for maladaptative traits can have beneficial overall fitness consequences, e.g., when it leads to positive assortative mating according to overall genetic quality, which can reduce strongly the genetic load burdening a population .

Both the viability and fecundity components of fitness can have an ecological component and a sexual-selection component. The ecological component is determined by a variant's ability to negotiate environmental challenges not related directly to sexual competition (such as the ability to gather food, to fend off or avoid predators, and so forth). The sexual-selection component is determined by a variant's ability to perform in the at times highly elaborated rituals that determine an individual's success at attracting mates and prevailing at such against other individuals of the same sex, which can be a major factor influencing fecundity (and more rarely viability). Because of sexual selection's dire potential to affect total fitness, it is not surprising that evolution by sexual selection has led to traits that are clearly maladaptive from the point of view of ecological performance (a famous example being the tails of male peacocks, which are very important in wooing females during courtship but are obviously detrimental to locomotion).

Natural selection is distinguished from artificial selection which refers to the evolution of domesticated species as a result of human culling rather than culling by the "natural environment". However, the mechanisms of natural and artificial selection are similar, and in fact outstanding cases of evolution by artificial selection like the diversity of dog and pigeon breeds were used by Darwin to illustrate how the process of selection can result in evolution.

Mechanisms of natural selection

Differential reproduction due to natural selection can result from differences in functional performance at many levels of biological organization, not only at the level of individual organisms (see unit of selection). Historically, because of the focus on evolution by natural selection, the emphasis has been on the selection of individual organisms that differ in some trait(s) which affect individual performance and result in a higher or lower reproductive output (so called positive and negative selection). Elliott Sober in his book "The Nature of Selection" has stressed that natural selection can entail the differential reproduction of many things ("selection of") but that what is most essential to natural selection as a natural process is what causes the differences in reproductive output ("selection for"), i.e., that the workings of natural selection do not depend on the factors that allow gene frequencies to react or not to eventual fitness differences.

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.

Whether a trait is likely to result in higher or lower fitness for its carrier depends on the environment, including predators, food sources, challenges from the physical environment, etc. When populations of a species become separated, e.g. by a geographic barrier, they may have to negotiate different environments and thus may be selected in different ways and may start evolving in different directions, and if enough time goes by for the traits of the separated populations to become very different, the populations can become different species. For this reason Darwin suggested that all species today have evolved from a common ancestor, but he also stressed that a species can evolve into a new form without splitting. Modern evolutionary biology stresses the lack of interbreeding as critical criterion for speciation but, as R.C. Lewontin has recently stressed, what allows sexual and asexual species to be around is enough ecological divergence for competitive exclusion not to take place.

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):

If there are organisms that reproduce, and
If offspring inherit traits from their parents(s), and
If there is variability of traits, and
If the environment limits the size of natural populations,
Then those members of the population with maladaptive traits (as determined by the environment) will die out or reproduce less, and
Then those members with adaptive traits (as determined by the environment) will survive to reproduction or reproduce more

The result is the evolutionary change of populations and eventually of species.

This is a continuing process that accounts for how species change and can account for both the extinction of species and the origin of new ones. Since the formulation is not explicit about how the environment determines whether traits are more or less adaptive, the formulation does not rule out selection occurring at biological levels other than the individual level (e.g., gene, group). Finally the formulation does not invoke specific mechanisms that generate new traits but their continuous action is postulated.

Darwin did not maintain that natural selection is the only mechanism of evolution. In fact he wrote explicitly in the introduction to The Origin of Species:

I am convinced that has been the most important, but not the exclusive means of modification.

Although natural selection is often called the mechanism of evolution, the generation of heritable phenotypic diversity is also crucial since without it selection cannot result in adaptive evolution. Such variation is now understood to be generated by the shuffling of genetic material (crossing over) that occurs during meiosis and syngamy, by random alterations of the genetic material like point mutations, insertions, and deletions, and by the insertion and deletion of self-replicating genetic elements like transposons as well as of viruses that integrate their genomes in that of their hosts.

History of the principle

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Charles Darwin's discovery of the principle of natural selection, as his explanation for the origin of species, occurred around 1837 or 1838. Over the next twenty years, he shared it with only a very small number of acquaintances, while he amassed evidence in its favour. 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. Darwin, moreover, postulated that adaptive evolution by natural selection can let populations diverge ecologically until they become different species.

Similar ideas go back to ancient times. For example, the Ionian physician Empedocles said that many races of beings "must have been unable to beget and continue their kind. For in the case of every species that exists, either craft or courage or speed has from the beginning of its existence protected and preserved it". A number of eighteenth-century thinkers had previously written about similar theories (most prominently, Pierre Louis Moreau de Maupertuis in 1745, and Darwin's grandfather Erasmus Darwin in 1794–1796), though none had formulated it in quite the same terms as Darwin nor had developed it along with compelling evidence, and were not taken seriously as possible progenitors until well after Darwin's publication. As Darwin fully acknowledged in his introduction to the 6th edition of The Origin of Species, a few other people had proposed similar theories earlier — notably William Charles Wells in 1813, and Patrick Matthew in 1831 — but had not presented them fully or in general scientific publications. 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. Edward Blyth has also been suggested as a having proposed a method of natural selection as a mechanism of keeping species constant.

In any case, almost none of the above "precursors" had any effect on the history of evolutionary thought, with perhaps the exception of Erasmus Darwin who was certainly an influence on Charles, though the former's theory of evolution was not formulated in a scientific fashion. Most modern historians of science do not consider any of the above to have any sense of historical priority over Darwin himself, as they did not work to develop the theory nor combine it in a rigorous sense with an argument for evolution. The historian of biology Peter J. Bowler has gone so far as to say that efforts to fine "precursors" of this sort "misunderstand the whole point of the history of science."

Darwin's Origin of Species succeeded in elevating the idea of evolution to the level of real scientific discourse, and within a decade most scientists and educated lay-people had begun to believe that evolution had occurred in some form or another. Natural selection, however, was not a popular notion. Of the many ideas of evolution which surfaced in the years following Darwin, only the neo-Darwinism of August Weismann contained selectionism as the main driver of the theory, and this was considered as unfounded by his contemporaries. Most evolutionary theorists — even Darwin's supporter Thomas Henry Huxley — believed that evolution had occurred with more "purpose" in it than natural selection afforded, and neo-Lamarckism was also very popular.

During what has been termed as the "eclipse of Darwinism" from the late-nineteenth-century through the first decades of the twentieth century, evolution was largely accepted by scientists though natural selection was not. Only after the integration of a theory of evolution with a complex statistical appreciation of Gregor Mendel's "re-discovered" laws of inheritance did natural selection become highly favored by the scientific community. The work by Ronald Fisher, J.B.S. Haldane, Sewall Wright, Theodosius Dobzhansky, and others, to form the modern evolutionary synthesis propelled Darwinism into the forefront of evolutionary theories, where it remains to a large extent today.

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. The mathematician and science fiction writer Rudy Rucker explored the use of natural selection to create artificial intelligence in his best-known work, the Ware Tetralogy, as well as in his novel The Hacker and the Ants.

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.

Classification

By effect on phenotypic composition of the population

Balancing selection - acts to maintain allelic diversity
Directional selection - selects for phenotypes that depart from the mean in one direction
Disruptive selection - selects for phenotypes that depart from the mean in either direction
Stabilizing selection or purifying selection - selects against extreme phenotypes, keeps phenotype constant

By aspect of fitness affected

Ecological selection - viability and female fecundity
Sexual selection - usually male mating success (but males can also, rarely, be choosy, especially where they engage in brood care)

Notes

Nachman, Michael W. & Crowell, Susan L. 2000. Estimate of the Mutation Rate per Nucleotide in Humans. Genetics 156, 297-304.
Siller, S. (2001). Sexual selection and the maintenance of sex. Nature 411: 689-692
Lewontin, R. C. (1997). Dobzhansky's genetics and the origin of species: is it still relevant? Genetics. 147(2): 351-355.

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

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