Contents
Evolution
From Wikipedia, the free encyclopedia
Evolution
Introduction
Mechanisms and processes
Adaptation
Genetic drift
Gene flow
Mutation
Natural selection
Speciation
Research and history
Evidence
Evolutionary history of life
History
Modern synthesis
Social effect
Theory and fact
Objections / Controversy
Evolutionary biology fields
Cladistics
Ecological genetics
Evolutionary development
Human evolution
Molecular evolution
Phylogenetics
Population genetics
Biology Portal ·
In biology, evolution is change in the inherited traits of a population of organisms from one generation to the next. These changes are caused by a combination of three main processes: variation, reproduction, and selection. Genes that are passed on to an organism's offspring produce the inherited traits that are the basis of evolution. These traits vary within populations, with organisms showing heritable differences in their traits. When organisms reproduce, their offspring may have new or altered traits. These new traits arise in two main ways: either from mutations in genes, or from the transfer of genes between populations and between species. In species that reproduce sexually, new combinations of genes are also produced by genetic recombination, which can increase variation between organisms. Evolution occurs when these heritable differences become more common or rare in a population.
Two major mechanisms drive evolution. The first is natural selection, a process causing heritable traits that are helpful for survival and reproduction to become more common in a population, and harmful traits to become more rare. This occurs because individuals with advantageous traits are more likely to reproduce, so that more individuals in the next generation inherit these traits.[1][2] Over many generations, adaptations occur through a combination of successive, small, random changes in traits, and natural selection of those variants best-suited for their environment.[3] The second major mechanism is genetic drift, an independent process that produces random changes in the frequency of traits in a population. Genetic drift results from the role probability plays in whether a given trait will be passed on as individuals survive and reproduce. Though the changes produced in any one generation by drift and selection are small, differences accumulate with each subsequent generation and can, over time, cause substantial changes in the organisms. This process can culminate in the emergence of new species.[4] Indeed, the similarities between organisms suggest that all known species are descended from a common ancestor (or ancestral gene pool) through this process of gradual divergence.[1]
Evolutionary biology documents the fact that evolution occurs, and also develops and tests theories that explain its causes. Studies of the fossil record and the diversity of living organisms had convinced most scientists by the mid-nineteenth century that species changed over time.[5][6] However, the mechanism driving these changes remained unclear until the 1859 publication of Charles Darwin's On the Origin of Species, detailing the theory of evolution by natural selection.[7] Darwin's work soon led to overwhelming acceptance of evolution within the scientific community.[8][9][10][11] In the 1930s, Darwinian natural selection was combined with Mendelian inheritance to form the modern evolutionary synthesis,[12] in which the connection between the units of evolution (genes) and the mechanism of evolution (natural selection) was made. This powerful explanatory and predictive theory directs research by constantly raising new questions, and it has become the central organizing principle of modern biology, providing a unifying explanation for the diversity of life on Earth.[9][10][13]
Heredity
Evolution in organisms occurs through changes in heritable traits – particular characteristics of an organism. In humans, for example, eye color is an inherited characteristic, which individuals can inherit from one of their parents.[14] Inherited traits are controlled by genes and the complete set of genes within an organism's genome is called its genotype.[15]
The complete set of observable traits that make up the structure and behavior of an organism is called its phenotype. These traits come from the interaction of its genotype with the environment.[16] As a result, not every aspect of an organism's phenotype is inherited. Suntanned skin results from the interaction between a person's genotype and sunlight; thus, suntans are not passed on to people's children. However, people have different responses to sunlight, arising from differences in their genotype; a striking example is individuals with the inherited trait of albinism, who do not tan and are highly sensitive to sunburn.[17]
Heritable traits are propagated between generations via DNA, a molecule which is capable of encoding genetic information.[15] DNA is a polymer composed of four types of bases. The sequence of bases along a particular DNA molecule specify the genetic information, in a manner akin to a sequence of letters specifying a text or a sequence of bits specifying a computer program. Portions of a DNA molecule that specify a single functional unit are called genes; different genes have different sequences of bases. Within cells, the long strands of DNA associate with proteins to form condensed structures called chromosomes. A specific location within a chromosome is known as a locus. If the DNA sequence at a locus varies between individuals, the different forms of this sequence are called alleles. DNA sequences can change through mutations, producing new alleles. If a mutation occurs within a gene, the new allele may affect the trait that the gene controls, altering the phenotype of the organism. However, while this simple correspondence between an allele and a trait works in some cases, most traits are more complex and are controlled by multiple interacting genes.[18][19]
Variation
An individual organism's phenotype results from both its genotype and the influence from the environment it has lived in. A substantial part of the variation in phenotypes in a population is caused by the differences between their genotypes.[19] The modern evolutionary synthesis defines evolution as the change over time in this genetic variation. The frequency of one particular allele will fluctuate, becoming more or less prevalent relative to other forms of that gene. Evolutionary forces act by driving these changes in allele frequency in one direction or another. Variation disappears when an allele reaches the point of fixation — when it either disappears from the population or replaces the ancestral allele entirely.[20]
Variation comes from mutations in genetic material, migration between populations (gene flow), and the reshuffling of genes through sexual reproduction. Variation also comes from exchanges of genes between different species; for example, through horizontal gene transfer in bacteria, and hybridization in plants.[21] Despite the constant introduction of variation through these processes, most of the genome of a species is identical in all individuals of that species.[22] However, even relatively small changes in genotype can lead to dramatic changes in phenotype: chimpanzees and humans differ in only about 5% of their genomes.[23]
Mutation
Genetic variation comes from random mutations that occur in the genomes of organisms. Mutations are changes in the DNA sequence of a cell's genome and are caused by radiation, viruses, transposons and mutagenic chemicals, as well as errors that occur during meiosis or DNA replication.[24][25][26] These mutagens produce 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. Studies in the fly Drosophila melanogaster suggest that if a mutation changes a protein produced by a gene, this will probably be harmful, with about 70 percent of these mutations having damaging effects, and the remainder being either neutral or weakly beneficial.[27] Due to the damaging effects that mutations can have on cells, organisms have evolved mechanisms such as DNA repair to remove mutations.[24] Therefore, the optimal mutation rate for a species is a trade-off between costs of a high mutation rate, such as deleterious mutations, and the metabolic costs of maintaining systems to reduce the mutation rate, such as DNA repair enzymes.[28] Some species such as retroviruses have such high mutation rates that most of their offspring will possess a mutated gene.[29] Such rapid mutation may have been selected so that these viruses can constantly and rapidly evolve, and thus evade the responses of the human immune system.[30]