Molecular Evolution Exceptions To The General Description

Genomic imprinting (which is " epigenetic") constitutes heritability that is not coded in DNA. Evolution is prevalent also in viruses, although these are not considered to be organisms. The genetic material in viruses may consist of DNA or RNA.

2 Principles of molecular evolution

2.1 Mutations

Mutations are permanent, transmissible changes to the genetic material (usually DNA or RNA) of a cell. Mutations can be caused by copying errors in the genetic material during cell division and by exposure to radiation, chemicals, or viruses, or can occur deliberately under cellular control during the processes such as meiosisMeiosis is a cellular process that forms the basis for sexual reproduction, together with syngamy. It is a form of nuclear division by which a diploid parent produces four haploid daughter cells. This includes two stages of nuclear division (meiosis I and or hypermutation . Mutations are considered the driving force of evolution, where less favorable (or deleterious) mutations are removed from the gene pool by natural selectionAlternative meaning Natural Selection (computer game . Natural selection is the primary mechanism within the scientific theory of evolution, i. it alters the frequency of alleles within a population. It was first proposed as the main mechanism of evolutio, while more favorable (or beneficial) ones tend to accumulate. Neutral mutationsThe neutral theory of molecular evolution (also, simply the neutral theory of evolution is an influential theory that was introduced with provocative effect by Motoo Kimura in the late 1960s and early 1970s. Although the theory was received by some as an do not affect the organism's chances of survival in its natural environment and can accumulate over time, which might result in what is known as punctuated equilibriumPunctuated equilibrium is a theory of evolution which postulates that changes such as speciation can occur very quickly, with long periods of little change ( equilibria) in between. This theory explains the evolutionary patterns of species as observed in; the modern interpretation of classic evolutionary theory.

2.2 Causes of change in allele frequency

Main article: Population geneticsPopulation genetics is the study of the distribution of and change in allele frequencies under the influence of the four evolutionary forces: natural selection, genetic drift, mutation and migration. It also takes account of population subdivision and pop

There are four known processes that affect the survival of a characteristic; or, more specifically, the frequency of an allele (variant of a gene):

The production and redistribution of variation is produced by three of the four agents of evolution: mutation, genetic drift, and gene flow. Natural selection, in turn, acts on the variation produced by these agents.

2.3 Molecular study of phylogeny

Main articles: Molecular systematics, Phylogenetics

Molecular systematics is a product of the traditional field of systematics and molecular genetics. It is the process of using data on the molecular constitution of biological organisms' DNA, RNA, or both, in order to resolve questions in systematics, i.e. about their correct scientific classification or taxonomy from the point of view of evolutionary biology.

Molecular systematics has been made possible by the availability of techniques for gene sequencing, which allow the determination of the exact sequence of nucleotidess or bases in either DNA or RNA. At present it is still a long and expensive process to sequence the entire genome of an organism, and this has been done for only a few species. However it is quite feasible to determine the sequence of a defined area of a particular chromosome. Typical molecular systematic analyses require the sequencing of around 1000 base pairs.

2.4 The neutral theory

Main article: Neutral theory of molecular evolution

One of the questions concerning molecular evolution is what proportion of mutations are neutral with respect to natural selection, meaning mutations that do not convey a selective advantage or disadvantage to the individual that inherits them. Answering such questions is an aim of population genetics.

Rare spontaneous errors in DNA replication cause the mutations that drive molecular evolution. The molecular clock technique, which researchers use to date when two species diverged by comparing their DNA, deduces elapsed time from the number of differences. The technique was inspired by the once common assumption that the DNA error rate is constant--not just over time, but across all species and every part of a genome that you might want to compare. Because the enzymes that replicate DNA differ only very slightly between species, the assumption seemed reasonable a priori. But as molecular evidence has accumulated, the constant-rate assumption has proven false--or at least overly general. Molecular clock users are developing workaround solutions.

2.5 Infinite alleles model

The Japanese geneticist Motoo Kimura and American geneticist James Crow (1964) introduced the infinite alleles model, an attempt to determine for a finite population what proportion of loci would be homozygous. This was, in part, motivated by assertions by other geneticists that more than 50 percent of Drosophila loci were heterozygous, a claim they initially doubted. In order to answer this question they assumed first, that there were a large enough number of alleles so that any mutation would lead to a different allele (that is the probability of back mutation to the original allele would be low enough to be negligible); and second, that the mutations would result in a number of different outcomes from neutral to deleterious.

They determined that in the neutral case, the probability that an individual would be homozygous, F, was:

F = 1/(4N_eu + 1)

where u is the mutation rate, and N_e is the effective population size . From this it is possible to determine an upper limit to the number of possible alleles in a population, n as the inverse of the homozygosity:

n = 1/F = 4N_eu + 1

If the effective population is large, then a large number of alleles can be maintained. However, this result only holds for the neutral case, and is not necessarily true for the case when some alleles are more or less fit than others, for example when the fittest genotype is a heterozygote (a situation often referred to as overdominance or heterosis).

In the case of overdominance, because Mendel's second law (the law of segregation) necessarily results in the production of homozygotes (which are by definition in this case, less fit), this means that population will always harbor a number of less fit individuals, which leads to a decrease in the average fitness of the population. This is sometimes referred to as genetic load , in this case it is a special kind of load known as segregational load. Crow and Kimura showed that at equilibrium conditions, for a given strength of selection (s), that there would be an upper limit to the number of fitter alleles (polymorphisms) that a population could harbor for a particular locus. Beyond this number of alleles, the selective advantage of presence of those alleles in heterozygous genotypes would be cancelled out by continual generation of less fit homozygous genotypes.

These results became important in the formation of the neutral theory, because neutral (or nearly neutral) alleles create no such segregational load, and allow for the accumulation of a great deal of polymorphism. When Richard Lewontin and J. Hubby published their groundbreaking results in 1966 which showed high levels of genetic variation in Drosophila via protein electrophoresis, the theoretical results from the infinite alleles model were used by Kimura and others to support the idea that this variation would have to be neutral (or result in excess segregational load).

2.6 Infinite sites model

3 Related fields

An important area within the study of molecular evolution is the use of molecular data to determine the correct scientific classification of organisms. This is called molecular systematics.

4 See also

5 References

Wen-Hsiung Li Molecular Evolution, Sinauer, 1997 BooksEnthsiast.com
Motoo Kimura and James Crow ( 1964), "The Number of Alleles that Can Be Maintained in a Finite Population," Genetics 49:725-738.
Roderic D.M. Page and Edward C. Holmes Molecular Evoluion: A Phylogenetic Approach, Blackwell Science, 1998 BooksEnthsiast.com

6 External links

MIT History of Science

Basic topics in evolutionary biology
Processes of evolution: macroevolution - microevolution - speciation
Mechanisms: selection - genetic drift - gene flow - mutation
History: Charles Darwin - The Origin of Species - modern evolutionary synthesis
Subfields: population genetics - ecological genetics - molecular evolution - phylogenetics - systematics - evo-devo
List of evolutionary biology topics \| Timeline of evolution

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1 Exceptions to the general description