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The theory of evolution by natural selection , as developed by Darwin, holds that natural selection results in favorable, heritable traits becoming more common in subsequent populations and, over time, is the creative force even in macroevolutionary changes, such as the development of new species, higher taxa, and major new designs. The existence of the mechanism of natural selection is nearly universally recognized, as is its ability to impact gene frequencies in populations microevolution and remove unfit phenotypes. However, the ability of natural selection to be the main creative force of changes on the macroevolutionary level, such as the development of higher taxa and major new designs, remains controversial.

Evolutionist Ernst Mayr defines natural selection as "the process by which in every generation individuals of lower fitness are removed from the population. Natural selection generally is defined independently of whether or not there is actually an effect on the gene -frequency of a population. That is, it is limited to the selection process itself, whereby individuals in a population experience differential survival and reproduction based on a particular phenotypic variation s.

If the phenotypic variation has a genetic basis, then natural selection will impact the gene-frequency of the population. If the variation does not have a genetic basis, then natural selection will not impact the gene-frequency. More inclusive definitions define natural selection as the process and a resulting change in the gene-frequency of the population. In these cases, only if there is a change in the gene-frequency can the mechanism be called natural selection.

The theory of evolution by natural selection encompasses both minor changes in gene frequency in populations, brought about by the creative force of natural selection, and major evolutionary changes brought about through natural selection, such as the origin of new designs. For Darwin, the term natural selection was synonymous with evolution by natural selection. Other mechanisms of evolution, such as evolution by genetic drift, were not explicitly formulated at that time, but Darwin realized there may be other mechanisms at work: "I am convinced that it [natural selection] has been the main, but not exclusive means of modification" Darwin Now, scientists use natural selection mainly to describe the mechanism, not the theory of evolution by natural selection.

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In this sense, natural selection includes any selection by a natural agent, including sexual selection discussed below. Selection targets specific traits of an individual, and if such a trait has a heritable component, the frequency of that trait will increase in the next generation. So selection for a specific trait results in selection of certain individuals Sober This distinction is important, because an individual is more than the trait selected for. For example, sometimes two or more traits are genetically linked through mechanisms such as pleiotropy a single gene that affects multiple traits and linkage disequilibrium non-random association of two genes.

Sometimes, selection of a trait relates to a specific function of that trait, while that trait also has other functions that are not affected by natural selection. In either case, direct selection for specific traits or functions results in indirect selection of other traits or functions. Directionality of selection. Selection can be divided into diverse classes, on the basis of how it drives an allele a specific version of a gene to fixation or toward removal of the allele from the population. Positive or directional selection occurs when a certain allele confers a higher fitness than others, resulting in that allele increasing in frequency until it is fixed and the entire population expresses the more fit phenotype.

Essentially, it causes the population distribution, with its range of phenotypes, to move in one direction, such as selecting for birds with larger beaks. Far more common is purifying or stabilizing selection, which lowers the frequency of alleles with deleterious phenotypes that is, a lower fitness until they are fixed out of the population entirely.

Natural selection - New World Encyclopedia

It may remove phenotypes on both extremes of the population distribution, such as selecting for a certain birth size for newborns versus very large or very small offspring. Disruptive or diversifying selection favors genotypes that depart from the average in either direction that is, the opposite of overdominance , and can result in a bimodal distribution of trait values, such as birds with small beaks and with large beaks, but not intermediate size beaks. This type of selection maintains variation in the population. Balancing selection refers to those selections that maintain an allele at intermediate frequencies in a population, rather than fixing the allele in the population.

For example, this can occur in diploid species with two pairs of chromosomes when individuals with a combination of two different alleles at a single position on the chromosome heterozygote have a higher fitness than individuals that have two of the same alleles homozygote. This is called heterozygote advantage or overdominance.

For example, there is documented evidence of a heterozygote advantage of sickle cell anemia in humans in central African countries because of resistance conferred against malaria. Ecological selection and sexual selection. It is useful to make a distinction between ecological selection and sexual selection. Ecological selection covers any mechanism of selection as a result of the environment, such as temperature, predation, humidity, competition, and so forth. Sexual selection refers specifically to competition between organisms for mates. Sexual selection includes mechanisms such as mate choice and male-male competition, although the two forms can act in combination in some species, when females choose the winners of the male-male competition.

Mate choice typically involves "female choice," but in some species it is the males that choose. Some features that are confined to one sex only of a particular species often are explained by selection exercised by the other sex in the choice of a mate, such as the extravagant plumage of some male birds. Aggression between members of the same sex intrasexual selection is typically referred to as "male-male competition," and is sometimes associated with very distinctive features, such as the antlers of stags , which are used in combat with other stags.

Sometimes, sexual selection is distinguished from natural selection, but it may also be considered one category of natural selection with ecological selection being another category. Natural selection acts on the outward form of an individual, the phenotype.

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The phenotype is the overall result of an individual's genetic make-up genotype , the environment , and the interactions between genes and between genes and the environment. A key element in understanding natural selection is the concept of fitness. Natural selection acts on individuals, but its average effect on all individuals with a particular genotype is the fitness of that genotype.

Fitness is measured as the proportion of progeny that survives, multiplied by the average fecundity potential reproductive capacity , and it is equivalent to the reproductive success of a genotype. A fitness value of greater than one indicates that the frequency of that genotype in the population increases, while a value of less than one indicates that it decreases.

Natural selection can act on any phenotypic trait, and any aspect of the environment, including mates and competitors, can result in a selective pressure. However, this does not imply that natural selection is always directional and results in adaptive evolution; natural selection is considered to often result in the maintenance of the situation. Natural selection is often discussed in terms of a struggle among individual organisms for reproductive success. However, other objects of natural selection have been suggested on levels both below and above the individual.

Some have proposed the gene as the principal object of selection. Dawkins argued that "the fundamental unit of selection, and therefore of self-interest, is not the species , nor the group, nor even, strictly, the individual. It is the gene, the unit of heredity…. Selection occurs at only one lowest level—the gene. While a number of evolutionists support this view, Mayr , for one, considers gene selection as invalid, both because a gene is only one part of the genotype and natural selection acts on the phenotype , and because it fails to recognize that genes do not act independently of other genes.

Likewise, Gould insists that only individuals can reproduce or die, and hence genes could not be the unit of selection. Symbiotic DNA in eukaryotic genomes.

Natural Selection vs Artificial Selection - Mechanisms of Evolution

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Parasitology Potvin, C.

Selection: The Mechanism of Evolution

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Evolution and natural selection

Powerful short-term selection for recombination as the result of negative species interactions. Gillis, D. Journal of Theoretical Biology Stewart, M. Partitioning the transplant site effect in reciprocal transplant experiments with Impatiens capensis and Impatiens pallida. The cost of reproduction. Oxford Surveys in Evolutionary Biology Vol.

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    The real problem when assigning this classification to viruses comes from the fact that they do not have a cellular structure and they are obligate intracellular parasites. Viruses do not possess a cell membrane in the sense of a phospholipid bilayer or metabolize on their own and they need to hijack the machinery of the cell to reproduce.

    Viruses cannot reproduce outside a living cell.

    Evolution myths: Natural selection is the only means of evolution

    Their real position in the tree of life continues to generate controversy in the scientific community as they move in a thin boundary line between the living and the nonliving. Influenza Virus. On the other hand, Antigenic shift is the process according to which at least two different strains of a flu virus are combined, giving rise to a new subtype with a new genetic material.