Posted by Ali Reda | Posted in | Posted on 1/20/2013

Evolution is the change in the inherited characteristics of biological populations over successive generations. Population genetics is the study of allele frequency distribution and change under the influence of the four main evolutionary processes: natural selection, genetic drift, mutation and gene flow. It also takes into account the factors of recombination, population subdivision and population structure. It attempts to explain such phenomena as adaptation and speciation.

There were four major alternatives to natural selection in the late 19th century. Theistic evolution was the belief that God directly guided evolution. The idea that evolution was driven by the inheritance of characteristics acquired during the life of the organism was called neo-Lamarckism. Orthogenesis was the belief that organisms were affected by internal forces or laws of development that drove evolution in particular directions, and saltationism was the idea that evolution was largely the product of large mutations that created new species in a single step.

Natural selection
is a process in which traits vary among individuals, leading to different rates of survival and reproduction, trait differences are heritable.Thus, when members of a population die they are replaced by the progeny of parents that were better adapted to survive and reproduce in the environment in which natural selection took place (Fitness). The concept was simple but powerful: individuals best adapted to their environments are more likely to survive and reproduce. As long as there is some variation between them, there will be an inevitable selection of individuals with the most advantageous variations. If the variations are inherited, then differential reproductive success will lead to a progressive evolution of particular populations of a species, and populations that evolve to be sufficiently different eventually become different species. This process creates and preserves traits that are seemingly fitted for the functional roles they perform. Darwin presented a body of evidence that the diversity of life arose by common descent through a branching pattern of evolution.
Natural selection is the only known cause of adaptation, but not the only known cause of evolution. Other, nonadaptive causes of evolution include mutation and genetic drift.

When Charles Darwin proposed his theory of evolution in 1859, one of its major problems was the lack of an underlying mechanism for heredity. Darwin believed in a mix of blending inheritance and the inheritance of acquired traits (pangenesis).

Blending inheritance would lead to uniformity across populations in only a few generations and thus would remove variation from a population on which natural selection could act. This led to Darwin adopting some Lamarckian ideas "use and disuse inheritance" in later editions of On the Origin of Species.Lamarck's theory was that physical changes to an individual to adapt to its environment were passed on to its offspring. The example always quoted in textbooks is that a proto-giraffe who developed a longer neck by constantly stretching it would have offspring with a long neck.The discovery of how DNA works completely refutes this idea. Altering the muscles in your neck does not affect the DNA in your gametes. Before that, though, people could simply observe that the process wasn't happening. Children do not inherit traits that their parents acquired during their lifetimes, only traits that the parents were born with.

This led to the Eclipse of darwinism until genetics was integrated with Darwin's theory of evolution by natural selection through the discipline of population genetics. The work of Ronald Fisher (who developed the required mathematical language and The Genetical Theory of Natural Selection) J.B.S. Haldane (who introduced the concept of the "cost" of natural selection), Sewall Wright (who elucidated the nature of selection and adaptation), Theodosius Dobzhansky (who established the idea that mutation, by creating genetic diversity, supplied the raw material for natural selection: see Genetics and the Origin of Species), William Hamilton (who conceived of kin selection), Ernst Mayr (who recognised the key importance of reproductive isolation for speciation: see Systematics and the Origin of Species) and many others formed the modern evolutionary synthesis. This synthesis cemented natural selection as the foundation of evolutionary theory, where it remains today.

Genetic drift
is a change in allele frequencies caused by random sampling. That is, the alleles in the offspring are a random sample of those in the parents. Genetic drift may cause gene variants to disappear completely, and thereby reduce genetic variability. In contrast to natural selection, which makes gene variants more common or less common depending on their reproductive success, the changes due to genetic drift are not driven by environmental or adaptive pressures, and may be beneficial, neutral, or detrimental to reproductive success and chance has a role in determining whether a given individual survives and reproduces.

is the ultimate source of genetic variation in the form of new alleles. Mutation can result in several different types of change in DNA sequences; these can either have no effect, alter the product of a gene, or prevent the gene from functioning.

Gene flow
is the transfer of alleles or genes from one population to another, Maintained gene flow between two populations can also lead to a combination of the two gene pools, reducing the genetic variation between the two groups. It is for this reason that gene flow strongly acts against speciation, by recombining the gene pools of the groups, and thus, repairing the developing differences in genetic variation that would have led to full speciation and creation of daughter species.

is the evolutionary process by which new biological species arise, Ecologists refer to speciation in terms of ecological niches. A niche must be available in order for a new species to be successful. In ecology, a niche is a term describing the way of life of a species. Each species is thought to have a separate, unique niche. The ecological niche describes how an organism or population responds to the distribution of resources and competitors (e.g., by growing when resources are abundant, and when predators, parasites and pathogens are scarce) and how it in turn alters those same factors (e.g., limiting access to resources by other organisms, acting as a food source for predators and a consumer of prey).

Unit of Selection
is a biological entity within the hierarchy of biological organisation (e.g. self-reproducing molecules, genes, cells, individuals, groups, species) that is subject to natural selection."The generality of the principles of natural selection means that any entities in nature that have variation, reproduction, and heritability may evolve. ...the principles can be applied equally to genes, organisms, populations, species, and at opposite ends of the scale, prebiotic molecules and ecosystems."

Selection at the level of the gene
The contention is that the genes that get passed on are the ones whose consequences serve their own implicit interests (to continue being replicated), Dawkins writes that gene combinations which help an organism to survive and reproduce tend to also improve the gene's own chances of being passed on and, as a result, frequently "successful" genes will also be beneficial to the organism. An example of this might be a gene that protects the organism against a disease, which helps the gene spread and also helps the organism..Proponents of the gene-centered viewpoint argue that it permits understanding of diverse phenomena such as altruism and intragenomic conflict that are otherwise difficult to explain from an organism-focused perspective, where organisms act altruistically, against their individual interests (in the sense of health, safety or personal reproduction) to help related organisms reproduce, can be explained as gene sets "helping" copies of themselves (or sequences with the same phenotypic effect) in other bodies to replicate. Interestingly, the "selfish" actions of genes lead to unselfish actions by organisms.

Selection at the level of individual organism
it can be described as Darwinism, and is well understood and considered common. If a relatively faster gazelle manages to survive and reproduce more, the causation of the higher fitness of this gazelle can be fully accounted for if one looks at how individual gazelles fare under predation.

Selection at the level of the group
If a group of organisms, owing to their interactions or division of labor, provides superior fitness compared to other groups, where the fitness of the group is higher or lower than the mean fitness of the constituent individuals, group selection can be declared to occur.
Specific syndromes of selective factors can create situations in which groups are selected because they display group properties which are selected-for. Many common examples of group traits are reducible to individual traits, however. Selection of these traits is thus more simply explained as selection of individual traits.

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