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Natural selection

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

Natural selection is the process by which biological individuals that are endowed with favorable or deleterious traits end up reproducing more or less than other individuals that do not possess such traits. Differential reproduction imposed by random factors is normally not considered to be an instance of natural selection because natural selection requires that differential reproduction be caused by differences between individuals. When a superior trait is heritable so that in the next generation one observes more individuals displaying the trait, one speaks of adaptive evolution by natural selection. Evolution, therefore, can involve changes not driven by natural selection; and natural selection is not sufficient for evolutionary change to take place, let alone for adaptive evolutionary change (since the latter requires that the selected traits be heritable). In general, however, adaptive evolution requires natural selection because the possibility that favorable traits become more frequent across generations due to random fluctuations in trait occurrence, is negligible (see genetic drift). Favorable traits that owe their occurrence in a population to the fact that the genes encoding them were enriched in the population through evolution by natural selection are called adaptations.

Charles Darwin and Alfred Russel Wallace proposed in 1858 that adaptive evolution is driven by natural selection and that such evolution can let populations diverge ecologically until they become different species. Natural selection therefore has a special significance because it explains the evolution of the astounding ways in which organisms are adapted to their environments, and why there are millions of species rather than a single one that does everything best as well as how these millions of species came into being.

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), but historically 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 it is the cause of the differences in reproductive output what points to the target of selection and thus defines what the level and the unit of selection are.

As stated above, functional biological performance is what determines the fitness of individual variants, i.e., whether a variant produces more or fewer progeny (and/or better- or lower-quality progeny) than others in a population. The major fitness components when talking about individual selection are viability (survival) and fecundity (progeny output).

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 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 essentially identical, 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 natural selection can result in evolution.

The modern theory of evolution by natural selection states that genetic differences between individuals can result in differences in functionally relevant traits, in higher reproduction of the individuals endowed with the better traits, in preferential transmission to the next generation of the genes encoding the traits that result in higher reproduction, and thus in changes in the frequency in successive generations of these genes and of the traits that individuals display. The heritable genetic variation that is necessary for natural selection to result in evolution is now understood to arise from random mutations.

Overview

The basic concept of natural selection is that "nature" (the physical and biological environment) "selects" individuals that are endowed with variant traits which improve survival and reproduction (adaptive traits) and selects against individuals burdened by traits that are unfavorable (maladaptive traits). 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 organismic traits 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 or caused by chemicals and physical agents (e.g. X rays); and 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 fitnesss 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 (Siller S, 2001; see below).

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.

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 (Lewontin R.C. 1997; see below).

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 parents(s), and
  3. If there is variability of traits, and
  4. If the environment limits the size of natural populations,
  5. Then those members of the population with maladaptive traits (as determined by the environment) will die out or reproduce less, and
  6. 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 [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 1837. 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. Darwin, moreover, postulated that adaptive evolution by natural selection can let populations diverge ecologically until they become different 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.

The largest single advance in the theory of evolution since Darwin was the incorporation of Gregor Mendel's genetic theory of inheritance into the theory natural selection by Ronald Fisher, J.B.S. Haldane, Sewall Wright et al. to form the Modern synthesis.

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.


Further reading

  • Endler, John A (1986). "Natural Selection in the Wild". Princeton University Press.
  • Lewontin RC. (1997). "Dobzhansky's genetics and the origin of species: is it still relevant?"

Genetics. 147(2): 351-355.

  • Maynard Smith, John (1993). "The Theory of Evolution". Cambridge University Press.
  • Sober, Elliott (1984; 1993) "The Nature of Selection: Evolutionary Theory in Philosophical Focus". The University of Chicago Press.
  • Williams, George C (1992). "Natural Selection: Domains, Levels and Challenges". Oxford University Press
  • Siller S (2001). Sexual selection and the maintenance of sex. Nature 411: 689-692

See also

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