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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 variants displaying favorable or deleterious traits end up producing more or fewer progeny relative to other individuals of the same population. Biological variants within a population tend to produce more or less progeny relative to other variants in the same population, if they happen to perform/function better or worse than the other variants. When the causes of the superior performances are heritable and become enriched in the next generation so that more variants of the superior type are observed in the next generation, one speaks of adaptive evolution by natural selection. The possibility of evolution by 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. Darwin, moreover, postulated that adaptive evolution by natural selection can drive populations to diverge ecologically until they become different species. Evolutionary biology has continued to refine these two fundamental insights ever since, e.g., by incorporating mendelian and quantitative genetics into the description of evolutionary change once such knowledge became available (see Modern synthesis).

From the above it follows that evolution can involve changes not driven by natural selection (as Darwin himself observed) and that natural selection is not sufficient for evolutionary change to take place, let alone for adaptive evolutionary change (since the selected traits must be heritable). In general, however, adaptive evolution requires natural selection as the possibility that favorable traits become more frequent across generations due to random fluctuations in trait occurrence, is negligible (see genetic drift).

Natural selection therefore has a special significance because it is responsible for the evolution of the astounding ways in which organisms appear to be adapted to their environment.

Natural selection can involve variants 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 such that the individuals end up having a higher or lower reproductive outpur (so called positive and negative selection). Elliott Sober in his book "The Nature of Selection" has stressed that natural selection can change the frequencies across generations of many things ("selection of") but that it is the cause of the fitness differences ("selection for") driving the process each generation 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 displays but are obviously detrimental for 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 differential reproduction of individuals, in differential transmission to the next generation of the genes encoding the traits that lead to the differential reproduction, and thus in changes in the frequency of these genes and of the traits these genes generate in individuals of the next generation(s). The 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 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 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.

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.


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

See also

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