What Is Evolution?
Jul 14, · The theory of evolution is a scientific theory that essentially states that species change over time. There are many different ways species change, but most of them can be described by the idea of natural selection. Evolution, theory in biology postulating that the various types of plants, animals, and other living things on Earth have their origin in other preexisting types and that the distinguishable differences are due to modifications in successive generations. The theory of evolution is one of the fundamental keystones of modern biological theory.
Evolutiontheory in biology postulating that the various types of plants, animals, and other living things on Earth have their origin in other preexisting types and that the distinguishable differences are due to modifications in successive generations.
The theory of evolution is one of the fundamental keystones of modern biological theory. The diversity of the living world is staggering. More than 2 million existing what is the concept of evolution of organisms have been named and described; many more remain to be discovered—from 10 million to 30 million, according to some estimates.
The virtually infinite variations on life are the fruit of the evolutionary process. All living creatures are related by descent from common ancestors. Humans and other mammals descend from shrewlike creatures that lived more than million years ago; mammals, birds, reptiles, amphibians, and fishes share as ancestors aquatic worms that lived million years ago; and all plants and animals derive from bacteria-like microorganisms that originated more than 3 billion years ago.
Biological evolution is a process of descent with modification. Lineages of organisms change through generations; diversity arises because the lineages that descend from common ancestors diverge through time. The 19th-century English naturalist Charles Darwin argued that organisms come about by evolution, and he provided a scientific explanationessentially correct but incomplete, of how evolution occurs and why it is that organisms have features—such as wings, eyes, and kidneys—clearly structured to what is the concept of evolution specific functions.
Natural selection was the fundamental concept in his explanation. Natural selection occurs because individuals having more-useful traits, such as more-acute vision or swifter legs, survive better and produce more progeny than individuals with less-favourable traits. Geneticsa science born in the 20th century, reveals in detail how natural selection works and led to the development of the modern theory of evolution.
Beginning in the s, a related scientific disciplinemolecular biologyenormously advanced knowledge of biological evolution and made it possible to investigate detailed problems that had seemed completely out of reach only a short time previously—for example, how similar the gene s of humans and chimpanzees might be they differ in about 1—2 percent of the units that make up the genes.
This article discusses evolution as it applies generally to living things. How to do a factory reset on droid 4 a discussion of human evolution, see the article human evolution. For a more complete treatment of a discipline that has proved essential to the study of evolution, see the articles genetics, human and heredity. Specific aspects of evolution are discussed in the articles coloration and mimicry.
Applications of evolutionary theory to plant and animal breeding are discussed in the articles plant breeding and animal breeding. A detailed discussion of the life and thought of Charles Darwin is found in the article Darwin, Charles.
Darwin and other 19th-century biologists found compelling evidence for biological evolution in the comparative study of living organisms, in what is the concept of evolution geographic distribution, and in the fossil remains of extinct organisms. The amount of information about evolutionary history stored in the DNA and proteins of living things is virtually unlimited; scientists can reconstruct any detail of the evolutionary history of life by investing sufficient time and laboratory resources.
Evolutionists no longer are concerned with obtaining evidence to support the fact of evolution but rather are concerned with what sorts of knowledge can be obtained from different sources of evidence. The following sections identify the most productive of these sources and illustrate the types of information they have provided. Evolution Article Media Additional Info.
Article Contents. Print print Print. Table What is the fastest i7 processor for laptop Contents. While every effort has been made to follow citation style rules, there may be some discrepancies.
Please refer to the appropriate style manual or other sources if you have any questions. Facebook Twitter. Give Feedback External Websites. Let us know if you have suggestions to improve this article requires login. External Websites. Your Genome - What is evolution? Articles from Britannica Encyclopedias for elementary and high school students.
Francisco Jose Ayala Francisco J. See Article History. The geologic time scale from million years ago to the present, showing major evolutionary what is an abstract in writing. Get a Britannica Premium subscription what is the concept of evolution gain access to exclusive content.
Subscribe Now. Overview of Charles Darwin's life, with a focus on his work involving evolution. Load Next Page.
2. Modes of Evolution
THE CONCEPT OF EVOLUTION' A. R. MANSER THERE appears to be a wide measure of agreement, both amongst biologists and others, that Darwin's theory of evolution marks a major breakthrough in the science of biology; Darwin has even been called 'Biology's Newton', the highest term of praise that could be bestowed on a scientist. A. G. N. What is the hologenome concept of evolution? All multicellular organisms are colonized by microbes, but a gestalt study of the composition of microbiome communities and their influence on the ecology and evolution of their macroscopic hosts has only recently become possible. One approach to thinking about the topic is to view the host-microbio Cited by: The underlying thesis of the present work is, therefore, that the path that leads to the development of the concept of evolution is the path that studies the possibilities of the evolution of.
Evolution is change in the heritable characteristics of biological populations over successive generations. Different characteristics tend to exist within any given population as a result of mutation , genetic recombination and other sources of genetic variation. The scientific theory of evolution by natural selection was conceived independently by Charles Darwin and Alfred Russel Wallace in the midth century and was set out in detail in Darwin's book On the Origin of Species.
This is followed by three observable facts about living organisms: 1 traits vary among individuals with respect to their morphology, physiology and behaviour phenotypic variation , 2 different traits confer different rates of survival and reproduction differential fitness and 3 traits can be passed from generation to generation heritability of fitness. In the early 20th century, other competing ideas of evolution such as mutationism and orthogenesis were refuted as the modern synthesis reconciled Darwinian evolution with classical genetics , which established adaptive evolution as being caused by natural selection acting on Mendelian genetic variation.
All life on Earth shares a last universal common ancestor LUCA    that lived approximately 3. Existing patterns of biodiversity have been shaped by repeated formations of new species speciation , changes within species anagenesis and loss of species extinction throughout the evolutionary history of life on Earth.
Evolutionary biologists have continued to study various aspects of evolution by forming and testing hypotheses as well as constructing theories based on evidence from the field or laboratory and on data generated by the methods of mathematical and theoretical biology.
Their discoveries have influenced not just the development of biology but numerous other scientific and industrial fields, including agriculture , medicine and computer science. The proposal that one type of organism could descend from another type goes back to some of the first pre-Socratic Greek philosophers , such as Anaximander and Empedocles. In contrast to these materialistic views, Aristotelianism considered all natural things as actualisations of fixed natural possibilities, known as forms.
Variations of this idea became the standard understanding of the Middle Ages and were integrated into Christian learning, but Aristotle did not demand that real types of organisms always correspond one-for-one with exact metaphysical forms and specifically gave examples of how new types of living things could come to be. In the 17th century, the new method of modern science rejected the Aristotelian approach. It sought explanations of natural phenomena in terms of physical laws that were the same for all visible things and that did not require the existence of any fixed natural categories or divine cosmic order.
However, this new approach was slow to take root in the biological sciences, the last bastion of the concept of fixed natural types. John Ray applied one of the previously more general terms for fixed natural types, "species", to plant and animal types, but he strictly identified each type of living thing as a species and proposed that each species could be defined by the features that perpetuated themselves generation after generation.
Other naturalists of this time speculated on the evolutionary change of species over time according to natural laws. In , Pierre Louis Maupertuis wrote of natural modifications occurring during reproduction and accumulating over many generations to produce new species. In particular, Georges Cuvier insisted that species were unrelated and fixed, their similarities reflecting divine design for functional needs.
In the meantime, Ray's ideas of benevolent design had been developed by William Paley into the Natural Theology or Evidences of the Existence and Attributes of the Deity , which proposed complex adaptations as evidence of divine design and which was admired by Charles Darwin. The crucial break from the concept of constant typological classes or types in biology came with the theory of evolution through natural selection, which was formulated by Charles Darwin in terms of variable populations.
Darwin used the expression " descent with modification " rather than "evolution". In each generation, many offspring fail to survive to an age of reproduction because of limited resources. This could explain the diversity of plants and animals from a common ancestry through the working of natural laws in the same way for all types of organism.
Their separate papers were presented together at an meeting of the Linnean Society of London. Thomas Henry Huxley applied Darwin's ideas to humans, using paleontology and comparative anatomy to provide strong evidence that humans and apes shared a common ancestry. Some were disturbed by this since it implied that humans did not have a special place in the universe. The mechanisms of reproductive heritability and the origin of new traits remained a mystery.
Towards this end, Darwin developed his provisional theory of pangenesis. Mendel's laws of inheritance eventually supplanted most of Darwin's pangenesis theory.
De Vries was also one of the researchers who made Mendel's work well known, believing that Mendelian traits corresponded to the transfer of heritable variations along the germline. Haldane set the foundations of evolution onto a robust statistical philosophy. The false contradiction between Darwin's theory, genetic mutations, and Mendelian inheritance was thus reconciled.
In the s and s, the so-called modern synthesis connected natural selection and population genetics, based on Mendelian inheritance, into a unified theory that applied generally to any branch of biology. The modern synthesis explained patterns observed across species in populations, through fossil transitions in palaeontology. Since then, the modern synthesis has been further extended in the light of numerous discoveries, to explain biological phenomena across the full and integrative scale of the biological hierarchy , from genes to populations.
The publication of the structure of DNA by James Watson and Francis Crick with contribution of Rosalind Franklin in demonstrated a physical mechanism for inheritance. Advances were also made in phylogenetic systematics , mapping the transition of traits into a comparative and testable framework through the publication and use of evolutionary trees.
One extension, known as evolutionary developmental biology and informally called "evo-devo," emphasises how changes between generations evolution acts on patterns of change within individual organisms development. Evolution in organisms occurs through changes in heritable traits—the inherited characteristics of an organism. In humans, for example, eye colour is an inherited characteristic and an individual might inherit the "brown-eye trait" from one of their parents.
The complete set of observable traits that make up the structure and behaviour of an organism is called its phenotype. These traits come from the interaction of its genotype with the environment. For example, suntanned skin comes from the interaction between a person's genotype and sunlight; thus, suntans are not passed on to people's children. However, some people tan more easily than others, due to differences in genotypic variation; a striking example are people with the inherited trait of albinism , who do not tan at all and are very sensitive to sunburn.
Heritable traits are passed from one generation to the next via DNA, a molecule that encodes genetic information. The sequence of bases along a particular DNA molecule specify the genetic information, in a manner similar to a sequence of letters spelling out a sentence. 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 form condensed structures called chromosomes.
The specific location of a DNA sequence 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. Recent findings have confirmed important examples of heritable changes that cannot be explained by changes to the sequence of nucleotides in the DNA.
These phenomena are classed as epigenetic inheritance systems. For example, ecological inheritance through the process of niche construction is defined by the regular and repeated activities of organisms in their environment. This generates a legacy of effects that modify and feed back into the selection regime of subsequent generations.
Descendants inherit genes plus environmental characteristics generated by the ecological actions of ancestors. Evolution can occur if there is enough genetic variation within a population. Variation comes from mutations in the genome, reshuffling of genes through sexual reproduction and migration between populations gene flow.
Despite the constant introduction of new variation through mutation and gene flow, most of the genome of a species is identical in all individuals of that species.
An individual organism's phenotype results from both its genotype and the influence of the environment it has lived in. A substantial part of the phenotypic variation in a population is caused by genotypic variation. The frequency of one particular allele will become more or less prevalent relative to other forms of that gene. Variation disappears when a new allele reaches the point of fixation —when it either disappears from the population or replaces the ancestral allele entirely.
Before the discovery of Mendelian genetics, one common hypothesis was blending inheritance. But with blending inheritance, genetic variation would be rapidly lost, making evolution by natural selection implausible. The Hardy—Weinberg principle provides the solution to how variation is maintained in a population with Mendelian inheritance. The frequencies of alleles variations in a gene will remain constant in the absence of selection, mutation, migration and genetic drift.
Mutations are changes in the DNA sequence of a cell's genome. When mutations occur, they may alter the product of a gene , or prevent the gene from functioning, or have no effect. Mutations can involve large sections of a chromosome becoming duplicated usually by genetic recombination , which can introduce extra copies of a gene into a genome.
New genes can be generated from an ancestral gene when a duplicate copy mutates and acquires a new function.
This process is easier once a gene has been duplicated because it increases the redundancy of the system; one gene in the pair can acquire a new function while the other copy continues to perform its original function. The generation of new genes can also involve small parts of several genes being duplicated, with these fragments then recombining to form new combinations with new functions.
In asexual organisms, genes are inherited together, or linked , as they cannot mix with genes of other organisms during reproduction. In contrast, the offspring of sexual organisms contain random mixtures of their parents' chromosomes that are produced through independent assortment. In a related process called homologous recombination , sexual organisms exchange DNA between two matching chromosomes. The two-fold cost of sex was first described by John Maynard Smith.
This cost does not apply to hermaphroditic species, like most plants and many invertebrates. The Red Queen hypothesis has been used to explain the significance of sexual reproduction as a means to enable continual evolution and adaptation in response to coevolution with other species in an ever-changing environment. Gene flow is the exchange of genes between populations and between species. Gene flow can be caused by the movement of individuals between separate populations of organisms, as might be caused by the movement of mice between inland and coastal populations, or the movement of pollen between heavy-metal-tolerant and heavy-metal-sensitive populations of grasses.
Gene transfer between species includes the formation of hybrid organisms and horizontal gene transfer. Horizontal gene transfer is the transfer of genetic material from one organism to another organism that is not its offspring; this is most common among bacteria. Large-scale gene transfer has also occurred between the ancestors of eukaryotic cells and bacteria, during the acquisition of chloroplasts and mitochondria.
It is possible that eukaryotes themselves originated from horizontal gene transfers between bacteria and archaea. From a neo-Darwinian perspective, evolution occurs when there are changes in the frequencies of alleles within a population of interbreeding organisms,  for example, the allele for black colour in a population of moths becoming more common. Mechanisms that can lead to changes in allele frequencies include natural selection, genetic drift, gene flow and mutation bias.
Evolution by means of natural selection is the process by which traits that enhance survival and reproduction become more common in successive generations of a population. It has often been called a "self-evident" mechanism because it necessarily follows from three simple facts: . More offspring are produced than can possibly survive, and these conditions produce competition between organisms for survival and reproduction.
Consequently, organisms with traits that give them an advantage over their competitors are more likely to pass on their traits to the next generation than those with traits that do not confer an advantage.
The central concept of natural selection is the evolutionary fitness of an organism. If an allele increases fitness more than the other alleles of that gene, then with each generation this allele will become more common within the population. These traits are said to be "selected for. Conversely, the lower fitness caused by having a less beneficial or deleterious allele results in this allele becoming rarer—they are "selected against. Natural selection within a population for a trait that can vary across a range of values, such as height, can be categorised into three different types.
The first is directional selection , which is a shift in the average value of a trait over time—for example, organisms slowly getting taller.