Evolution

In biology, the concept of evolution applies to a multi-causal natural process whereby populations of inbreeding living organisms change in their biological characteristics over generations. Evolutionary biologist Douglas Futuyma defined it this way:

"In its broadest sense, evolution simply means "change"...Usually..we don't apply the term to the changes an individual entity undergoes during its lifetime. Rather, an evolving system is ordinarily one in which there is descent of entities [viz., populations of similar entities], one generation from another, over time, and in which characteristics of the entities differ across generations. Thus evolution in a broad sense is descent with modification, and often with diversification."

Technically, evolution involves change in the heritable traits of a population over successive generations, as determined by shifts in the allele frequencies of genes. Over time, this process can result in speciation, the development of new species from existing ones. A main assumption in evolutionary theory is that contemporary organisms are related to each other through common descent from a single, or small number, of distantly common ancestors - the first life forms on earth that succeeded in giving rise to viable offspring.

The concept of evolution is attractive to scientists because it can account for the vast biodiversity on Earth, including the many extinct species attested in the fossil record. Biologists distinguish between the fact of evolution and theories of its mechanism.

The basic mechanisms that produce evolutionary change are natural selection and genetic drift. Natural selection was first described in 1858 paper by Charles Darwin and Alfred Russel Wallace, and then popularized in Darwin's 1859 book The Origin of Species. It is the process by which individual organisms with favorable traits are more likely to survive and reproduce. If those traits are heritable, they are passed to succeeding generations, with the result that beneficial heritable traits become more common in the next generation. Given enough time, this passive process can result in varied adaptations to changing environmental conditions.

Although there is overwhelming scientific consensus supporting the validity of evolution by natural selection, because of its potential implications for the origins of humankind, evolutionary theory has been at the center of many social and religious controversies since its inception.

Brief history of evolutionary thought

The concept that plants and animals change form over generations has existed since ancient times, notably among Greek philosophers such as Anaximander and Epicurus and Indian philosophers such as Patañjali.

Jean-Baptiste Lamarck theorized that living things passed on their traits to offspring, but his theories differed from modern evolutionary theory in that he believed that acquired traits were passed on. Thus, Lamarckian evolution posits that the giraffe acquired its long neck from stretching to reach higher foliage. An individual giraffes lengthened its neck from a lifetime of stretching, and passed on the longer neck to its offspring, which themselves stretched to reach even-higher branches, creating a cycle that concluded with the long necks of today's giraffes.

The transmutation of species was accepted by many scientists before 1859, but Charles Darwin's On The Origin of Species by Means of Natural Selection provided the first convincing exposition of a mechanism by which evolutionary change could occur: natural selection. After many years of working in private on his theory, Darwin was motivated to publish his work on evolution when he received a letter from Alfred Russell Wallace in which Wallace revealed his own, independent discovery of natural selection. Accordingly, Wallace is sometimes given shared credit for originating the theory.

Darwin's book sparked a great deal of scientific and social debate. Darwin's work relied on many different fields of scientific inquiry for its evidence, and debates over the theory took place in many different arenas. Although the occurrence of biological evolution was generally accepted by scientists, Darwin's specific ideas about evolution—that it occurred gradually, through natural selection—were contested. Additionally, while Darwin was able to observe variation, and infer natural selection and thereby adaptation, he was unable to explain how variation might arise, or be altered over generations. Darwin's proposal of a hereditary mechanism (pangenesis) lacked evidence and was ultimately rejected, being replaced by genetics.

From the end of the 19th century through the early 20th century, forms of neo-Lamarckism, "progressive" evolution (orthogenesis), and an evolution which worked by "jumps" (saltationism, as opposed to gradualism) became popular, although a form of neo-Darwinism, led by August Weismann, also enjoyed some minor success. The biometric school of evolutionary theory, resulting from the work of Darwin's cousin, Francis Galton, emerged as well, using statistical approaches to biology which emphasized gradualism and some aspects of natural selection.

Mendel's work was "rediscovered" in 1901, and

Debates over various aspects of how evolution occurs have continued. One prominent debate was over the theory of punctuated equilibrium, proposed in 1972 by paleontologists Niles Eldredge and Stephen Jay Gould to explain the paucity of gradual transitions between species in the fossil record, as well as the absence of change or stasis that is observed over significant intervals of time.

Academic disciplines

Evolutionary biology is concerned with the origin and descent of species, as well as their changes over time. It is an interdisciplinary field including scientists from many traditional taxonomically-oriented disciplines, including scientists with specialist training in particular organisms, such as mammalogy, ornithology, or herpetology, but who use those organisms to answer general questions in evolution. Evolutionary biology as an academic discipline in its own right emerged as a result of the modern evolutionary synthesis in the 1930s and 1940s.

Evolutionary developmental biology (informally, evo-devo) is a field of biology that compares the developmental processes of different animals in an attempt to determine the ancestral relationship between organisms and how developmental processes evolved. The discovery of genes regulating development in model organisms allowed for comparisons to be made with genes and genetic networks of related organisms.

Physical anthropology emerged in the late 19th century as the study of human osteology, and the fossilized skeletal remains of other hominids. At that time, anthropologists debated whether their evidence supported Darwin's claims, because skeletal remains revealed temporal and spatial variation among hominids, but Darwin had not offered an explanation of the specific mechanisms that produce variation. With the recognition of Mendelian genetics and the rise of the modern synthesis, however, evolution became both the fundamental conceptual framework for, and the object of study of, physical anthropologists. In addition to studying skeletal remains, they began to study genetic variation among human populations (population genetics); thus, some physical anthropologists began calling themselves biological anthropologists.

Fossil Record

Fossils are critical evidence for estimating when various lineages originated. Since fossilization of an organism is an uncommon occurrence, usually requiring hard parts (like teeth, bone or pollen), the fossil record is traditionally thought to provide only sparse and intermittent information about ancestral lineages. Fossilization of organisms without hard body parts is rare, but does happen under unusual circumstance; very rapid burial, and both low oxygen environment and sparse microbial action.

The fossil record contains the earliest known examples of life itself, as well as the earliest occurrences of individual lineages. The first complex animals date from the early Cambrian period, approximately 520 million years ago. The records of individual species yield information regarding the patterns and rates of evolution, showing for example if species evolve into new species (speciation) gradually and incrementally, or in relatively brief intervals of geologic time. The fossil record is a document of large scale patterns and events in the history of life, many of which have influenced the evolutionary history of numerous lineages. Mass extinctions frequently resulted in the loss of entire groups of species, such as the non-avian dinosaurs, while leaving others relatively unscathed. Recently, molecular biologists have used the time since divergence of related lineages to calibrate the rate at which mutations accumulate, and at which the genomes of different lineages evolve.

Phylogenetics, the study of the ancestry of species, has revealed that structures with similar internal organization may perform divergent functions. Vertebrate limbs are a common example of such homologous structures. The appendages on bat wings, for example, are very structurally similar to human hands, and may constitute a vestigial structure. Other examples include the presence of hip bones in whales and snakes. Such structures may exist with little or no function in a more current organism, yet have a clear function in an ancestral species of the same. Examples of vestigial structures in humans include wisdom teeth, the coccyx and the vermiform appendix.

Molecular evidence

Comparison of DNA sequences allows organisms to be grouped by sequence similarity, and the resulting phylogenetic trees are typically congruent with traditional taxonomy, and are often used to strengthen or correct taxonomic classifications. Sequence comparison is considered a measure robust enough to be used to correct erroneous assumptions in the phylogenetic tree in instances where other evidence is scarce. For example, neutral human DNA sequences are approximately 1.2% divergent (based on substitutions) from those of their nearest genetic relative, the chimpanzee, 1.6% from gorillas, and 6.6% from baboons. Genetic sequence evidence thus allows inference and quantification of genetic relatedness between humans and other apes. The sequence of the 16S rRNA gene, a vital gene encoding a part of the ribosome, was used to find the broad phylogenetic relationships between all extant life. The analysis, originally done by Carl Woese, resulted in the three-domain system, arguing for two major splits in the early evolution of life. The first split led to modern Bacteria and the subsequent split led to modern Archaea and Eukaryote.

The proteomic evidence also supports the universal ancestry of life. Vital proteins, such as the ribosome, DNA polymerase, and RNA polymerase are found in the most primitive bacteria to the most complex mammals. The core part of the protein is conserved across all lineages of life, serving similar functions. Higher organisms have evolved additional protein subunits, largely affecting the regulation and protein-protein interaction of the core. Other overarching similarities between all lineages of extant organisms, such as DNA, RNA, amino acids, and the lipid bilayer, give support to the theory of common descent. The chirality of DNA, RNA, and amino acids is conserved across all known life. As there is no functional advantage to right or left handed molecular chirality, the simplest hypothesis is that the choice was made randomly in the early beginnings of life and passed on to all extant life through common descent.

Molecular evidence also offers a mechanism for large evolutionary leaps and macroevolution. Horizontal gene transfer, the process in which an organism transfers genetic material (i.e. DNA) to another cell that is not its offspring, allows for large sudden evolutionary leaps in a species by incorporating beneficial genes evolved in another species. The Endosymbiotic theory explains the origin of mitochondria and plastids (e.g. chloroplasts), which are organelles of eukaryotic cells, as the incorporation of an ancient prokaryotic cell into ancient eukaryotic cell. Rather than evolving eukaryotic organelles slowly, this theory offers a mechanism for a sudden evolutionary leap by incorporating the genetic material and biochemical composition of a separate species. This evolutionary mechanism has been observed. Heneta, a protist, is an extant organism that is undergoing endosymbiotic evolution.

Further evidence for reconstructing ancestral lineages comes from so-called junk DNA such as pseudogenes, i.e., 'dead' genes, which steadily accumulate mutations.

Mutation

What accounts for genetic variation? Mutations are permanent, transmissible changes to the genetic material of a cell. These changes can be caused by: "copying errors" in the genetic material during cell division. (Explain frame shift) Exposure to radiation, certain chemicals, and some viruses can destabilize these genetic molecules and cause biochemical changes that are permanent. When mutations are made in the genes of a cell they ordinarily will be passed on to daughter cells when the cell divides. In multicellular organisms, the mutations will be passed on to progeny if germline mutations, that is- if the genes in the gametes are changed. Mutations that are not affected by natural selection are called neutral mutations. Their frequency in the population is governed by mutation rate, genetic drift and selective pressure on linked alleles. It is understood that most of a species' genome, in the absence of selection, undergoes a steady accumulation of neutral mutations.

Mobile elements, transposons, make up a major fraction of the genomes of plants and animals and appear to have played a significant role in the evolution of genomes. These mobile insertional elements can jump within a genome and alter existing genes and gene networks to produce evolutionary change and diversity.

Selection and adaptation

Natural selection can be subdivided into two categories:

• Ecological selection occurs when organisms that survive and reproduce increase the frequency of their genes in the gene pool over those that do not survive.
• Sexual selection occurs when organisms which are more attractive to the opposite sex because of their features reproduce more and thus increase the frequency of those features in the gene pool.

Natural selection also operates on mutations in several different ways:

• Positive or directional selection increases the frequency of a beneficial mutation, or pushes the mean in either direction.
• Purifying or stabilizing selection maintains a common trait in the population by decreasing the frequency of harmful mutations and weeding them out of the population. "Living fossils" are arguably the product of stabilizing selection, as their form and traits have remained virtually identical over a long period of time. It is argued that stabilizing selection is the most common form of natural selection.
• Artificial selection refers to purposeful breeding of a species to produce a more desirable and “perfect” breed. Humans have directed artificial selection in the breeding of both animals and plants, with examples ranging from agriculture (crops and livestock) to pets and horticulture. However, because humans are only part of the environment, the fractions of change in a species due to natural or artificial means can be difficult to determine. Artificial selection within human populations is a controversial enterprise known as eugenics.
• Balancing selection maintains variation within a population through a number of mechanisms, including:
  • Heterozygote advantage or overdominance, where the heterozygote is more fit than either of the homozygous forms (exemplified by human sickle cell anemia conferring resistance to malaria)
  • Frequency-dependent selection, where rare variants either have increased fitness or decreased fitness, because of their rarity.
• Disruptive selection favors both extremes, and results in a bimodal distribution of gene frequency. The mean may or may not shift.
• Selective sweeps describe the affect of selection acting on linked alleles. It comes in two forms:
  • Background selection occurs when a deleterious mutation is selected against, and linked mutations are eliminated along with the deleterious variant, resulting in lower genetic polymorphism in the surrounding region.
  • Genetic hitchhiking occurs when a beneficial allele is selected for, and linked alleles, which can be neutral or beneficial, are pushed towards fixation along with the beneficial allele.

Most biologists believe that adaptation occurs through the accumulation of many mutations of small effect. However, macromutation is an alternative process for adaptation that involves a single, very large scale mutation.

Recombination

In asexual organisms, variants in genes on the same chromosome will always be inherited together - they are linked, by virtue of being on the same DNA molecule. However, sexual organisms, in the production of gametes, shuffle linked alleles on homologous chromosomes inherited from the parents via meiotic recombination. This shuffling allows independent assortment of alleles (mutations) in genes to be propagated in the population independently. This allows bad mutations to be purged and beneficial mutations to be retained more efficiently than in asexual populations.

However, the meitoic recombination rate is not very high - on the order of one crossover (recombination event between homomolgous chromosomes) per chromosome arm per generation. Therefore, linked alleles are not perfectly shuffled away from each other, but tend to be inherited together. This tendency may be measured by comparing the co-occurrence of two alleles, usually quantified as linkage disequilibrium (LD). A set of alleles that are often co-propagated is called a haplotype. Strong haplotype blocks can be a product of strong positive selection.

Gene flow and population structure

Gene flow (also called gene admixture or simply migration) is the exchange of genetic variation between populations, when geography and culture are not obstacles. Ernst Mayr thought that gene flow is likely to be homogenising, and therefore counteract selective adaptation. Where there are obstacles to gene flow, the situation is termed reproductive isolation and is considered to be necessary for speciation.

The free movement of alleles through a population may also be impeded by population structure. For example, most real-world populations are not actually fully interbreeding; geographic proximity has a strong influence on the movement of alleles within the population.

An example of the effect of population structure is the so-called founder effect, resulting from a migration or population bottleneck, in which a population temporarily has very few individuals, and therefore loses a lot of genetic variation. In this case, a single, rare allele may suddenly increase very rapidly in frequency within a specific population if it happened to be prevalent in a small number of "founder" individuals. The frequency of the allele in the resulting population can be much higher than otherwise expected, especially for deleterious, disease-causing alleles. Since population size has a profound effect on the relative strengths of genetic drift and natural selection, changes in population size can alter the dynamics of these processes considerably.

Drift

Genetic drift describes changes in allele frequency from one generation to the next due to sampling variance. The frequency of an allele in the offspring generation will vary according to a probability distribution of the frequency of the allele in the parent generation. Thus, over time even in the absence of selection upon the alleles, allele frequencies will tend to "drift" upward or downward, eventually becoming "fixed" - that is, going to 0% or 100% frequency. Thus, fluctuations in allele frequency between successive generations may result in some alleles disappearing from the population due to chance alone. Two separate populations that begin with the same allele frequencies therefore might drift apart by random fluctuation into two divergent populations with different allele sets (for example, alleles present in one population could be absent in the other, or vice versa).

The impact of genetic drift depends strongly on the size of the population (generally abbreviated as N): drift is important in small mating populations (see Founder effect and Population bottleneck), where chance fluctuations from generation to generation can be large. The relative importance of natural selection and genetic drift in determining the fate of new mutations also depends on the population size and the strength of selection: when N times s (population size times strength of selection) is small, genetic drift predominates. When N times s is large, selection predominates. Thus, natural selection is predominant in large populations, or equivalently, genetic drift is stronger in small populations. Finally, the time for an allele to become fixed in the population by genetic drift (that is, for all individuals in the population to carry that allele) depends on population size, with smaller populations requiring a shorter time to fixation.

Speciation and extinction

Speciation is the process by which new biological species arise. This may take place by various mechanisms. Allopatric speciation occurs in populations that become isolated geographically, such as by habitat fragmentation or migration.. Sympatric speciation occurs when new species emerge in the same geographic area Ernst Mayr's peripatric speciation is a type of speciation that exists in between the extremes of allopatry and sympatry. Peripatric speciation is a critical underpinning of the theory of punctuated equilibrium. An example of rapid sympatric speciation can be eloquently represented in the triangle of U; where new species of Brassica sp. have been made by the fusing of separate genomes from related plants.

Extinction is the disappearance of species (i.e. gene pools). The moment of extinction generally occurs at the death of the last individual of that species. Extinction is not an unusual event in geological time — species are created by speciation, and disappear through extinction. The Permian-Triassic extinction event was the Earth's most severe extinction event, rendering extinct 90% of all marine species and 70% of terrestrial vertebrate species. In the Cretaceous-Tertiary extinction event many forms of life perished (including approximately 50% of all genera), the most often mentioned among them being the extinction of the non-avian dinosaurs.

Current research

Evolution is still an active field of research in the scientific community. Improvements in sequencing methods have resulted in a large increase of sequenced genomes, allowing for the testing and refining of the theory of evolution with respect to whole genome data. Advances in computational hardware and software have allowed for the testing and extrapolation of increasingly advanced evolutionary models. Discoveries in biotechnology have produced methods for the ‘’de novo’’ synthesis of proteins and, potentially, entire genomes, driving evolutionary studies at the molecular level.

Common misunderstandings about evolutionary theory

One of the most common misunderstandings of evolution is that one species can be "more highly evolved" than another, that evolution is necessarily progressive and/or leads to greater "complexity", or that its converse is "devolution". Evolution provides no assurance that later generations are more intelligent or complex than earlier generations. The claim that evolution results in progress is not part of modern evolutionary theory; it derives from earlier belief systems which were held around the time Darwin devised his theory of evolution.

Evolution has involved "progression" towards more complexity, since all of the earliest lifeforms were extremely simple and there was nowhere to go but up. However, there is no guarantee that any particular organism existing today will become more intelligent, more complex, bigger, or stronger in the future. In fact, natural selection will only favor this kind of "progression" if it increases chance of survival, i.e. the ability to live long enough to raise offspring to sexual maturity. The same mechanism can actually favor lower intelligence, lower complexity, and so on if those traits become a selective advantage in the organism's environment. One way of understanding the apparent "progression" of lifeforms over time is to remember that the earliest life began as components of cells. Evolution caused life to become more complex, since becoming simpler wasn't advantageous. Once individual lineages have attained sufficient complexity, simplifications (specialization) become more likely.

Speciation

It is sometimes claimed that speciation – the origin of new species – has never been directly observed, and thus evolution cannot be called sound science. This is a misunderstanding of both science and evolution. First, scientific discovery does not occur solely through reproducible experiments; the principle of uniformitarianism allows natural scientists to infer causes through their empirical effects. Moreover, since the publication of On the Origin of Species scientists have confirmed Darwin's hypothesis by data gathered from sources that did not exist in his day, such as DNA similarity among species and new fossil discoveries. Finally, speciation has actually been directly observed. (See the hawthorn fly example.)

A variation of this assertion that microevolution has been observed and macroevolution has not been observed is subject to the same counterarguments. While there is debate over the details of how evolution happened, it is generally accepted that macroevolution uses the same mechanisms of change as those already observed in microevolution.

Social and religious controversies

Starting with the publication of The Origin of Species in 1859, the modern science of evolution has been a source of nearly constant controversy. In general, controversy has centered on the philosophical, cosmological, social, and religious implications of evolution, not on the science of evolution itself. The proposition that biological evolution occurs through the mechanism of natural selection has been almost completely uncontested within the scientific community for much of the 20th century.

As Darwin recognized early on, perhaps the most controversial aspect of evolutionary thought is its applicability to human beings. The idea that all diversity in life, including human beings, arose through natural processes without a need for supernatural intervention poses difficulties for the belief in purpose inherent in most religious faiths — and especially for the Abrahamic religions. Many religious people are able to reconcile the science of evolution with their faith, or see no real conflict; Judaism and Catholicism are notable as major faith traditions whose adherents generally see no conflict between evolutionary theory and religious belief. The idea that faith and evolution are compatible has been called theistic evolution. Another group of religious people, generally referred to as creationists, consider evolutionary origin beliefs to be incompatible with their faith, their religious texts and their perception of design in nature, and so cannot accept what they call "unguided evolution".

One particularly contentious topic evoked by evolution is the biological status of humanity. Whereas the classical religious view can broadly be characterized as a belief in the great chain of being (in which people are "above" the animals but slightly "below" the angels), the science of evolution is clear both that humans are animals and that they share common ancestry with chimpanzees, gibbons, gorillas, and orangutans. Some people find the idea of common ancestry repellent, as, in their opinion, it "degrades" humankind. A related conflict arises when critics combine the religious view of people's superior status with the mistaken notion that evolution is necessarily "progressive". If human beings are superior to animals yet evolved from them, these critics claim, "inferior" animals would not still exist. Because animals that are (in their view) "inferior" creatures do demonstrably exist, those criticising evolution sometimes incorrectly take this as supporting their claim that evolution is false.

In some countries — notably the USA — these and other tensions between religion and science have fueled what has been called the creation-evolution controversy, which, among other things, has generated struggles over the teaching curriculum. While many other fields of science, such as cosmology and earth science, also conflict with a literal interpretation of many religious texts, evolutionary studies have borne the brunt of these debates.

Evolution has been used to support philosophical and ethical choices which most modern scientists argue are neither mandated by evolution nor supported by science. For example, the eugenic ideas of Francis Galton were developed into arguments that the human gene pool should be improved by selective breeding policies, including incentives for reproduction for those of "good stock" and disincentives, such as compulsory sterilization, "euthanasia", and later, prenatal testing, birth control, and genetic engineering, for those of "bad". Another example of an extension of evolutionary theory that is widely regarded as unwarranted is "Social Darwinism"; a term given to the 19th century Whig Malthusian theory developed by Herbert Spencer into ideas about "survival of the fittest" in commerce and human societies as a whole, and by others into claims that social inequality, racism, and imperialism were justified.