Evolution

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.

Partly influenced by An Essay on the Principle of Population by Thomas Robert Malthus , Darwin noted that population growth would lead to a "struggle for existence" in which favourable variations prevailed as others perished. 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, and complex cellular mechanisms in developmental biology. Advancements 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. Since then, the modern synthesis has been further extended to explain biological phenomena across the full and integrative scale of the biological hierarchy , from genes to species.

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.

The evidence for evolution

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. 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 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. Natural selection will only cause evolution if there is enough genetic variation in a population. Before the discovery of Mendelian genetics, one common hypothesis was blending inheritance.

But with blending inheritance, genetic variance 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.

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. 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, [78] 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, genetic hitchhiking, mutation and gene flow. 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. This would be when either short or tall organisms had an advantage, but not those of medium height. Finally, in stabilising selection there is selection against extreme trait values on both ends, which causes a decrease in variance around the average value and less diversity. A special case of natural selection is sexual selection, which is selection for any trait that increases mating success by increasing the attractiveness of an organism to potential mates.

Although sexually favoured, traits such as cumbersome antlers, mating calls, large body size and bright colours often attract predation, which compromises the survival of individual males. Natural selection most generally makes nature the measure against which individuals and individual traits, are more or less likely to survive. Eugene Odum , a founder of ecology , defined an ecosystem as: These relationships involve the life history of the organism, its position in the food chain and its geographic range. This broad understanding of nature enables scientists to delineate specific forces which, together, comprise natural selection.

Natural selection can act at different levels of organisation , such as genes, cells, individual organisms, groups of organisms and species. In addition to being a major source of variation, mutation may also function as a mechanism of evolution when there are different probabilities at the molecular level for different mutations to occur, a process known as mutation bias. Mutation bias effects are superimposed on other processes. If selection would favour either one out of two mutations, but there is no extra advantage to having both, then the mutation that occurs the most frequently is the one that is most likely to become fixed in a population.

Most loss of function mutations are selected against. But when selection is weak, mutation bias towards loss of function can affect evolution. Loss of sporulation ability in Bacillus subtilis during laboratory evolution appears to have been caused by mutation bias, rather than natural selection against the cost of maintaining sporulation ability. In parasitic organisms, mutation bias leads to selection pressures as seen in Ehrlichia. Mutations are biased towards antigenic variants in outer-membrane proteins.

Genetic drift is the random fluctuations of allele frequencies within a population from one generation to the next. Genetic drift may therefore eliminate some alleles from a population due to chance alone. Even in the absence of selective forces, genetic drift can cause two separate populations that began with the same genetic structure to drift apart into two divergent populations with different sets of alleles.

The neutral theory of molecular evolution proposed that most evolutionary changes are the result of the fixation of neutral mutations by genetic drift. The time for a neutral allele to become fixed by genetic drift depends on population size, with fixation occurring more rapidly in smaller populations. It is usually difficult to measure the relative importance of selection and neutral processes, including drift.

Recombination allows alleles on the same strand of DNA to become separated. However, the rate of recombination is low approximately two events per chromosome per generation. As a result, genes close together on a chromosome may not always be shuffled away from each other and genes that are close together tend to be inherited together, a phenomenon known as linkage. A set of alleles that is usually inherited in a group is called a haplotype. This can be important when one allele in a particular haplotype is strongly beneficial: Gene flow involves the exchange of genes between populations and between species.

Due to the complexity of organisms, any two completely isolated populations will eventually evolve genetic incompatibilities through neutral processes, as in the Bateson-Dobzhansky-Muller model , even if both populations remain essentially identical in terms of their adaptation to the environment. If genetic differentiation between populations develops, gene flow between populations can introduce traits or alleles which are disadvantageous in the local population and this may lead to organisms within these populations evolving mechanisms that prevent mating with genetically distant populations, eventually resulting in the appearance of new species.

Thus, exchange of genetic information between individuals is fundamentally important for the development of the Biological Species Concept BSC. During the development of the modern synthesis, Sewall Wright developed his shifting balance theory , which regarded gene flow between partially isolated populations as an important aspect of adaptive evolution. Evolution influences every aspect of the form and behaviour of organisms.

What is evolution?

Most prominent are the specific behavioural and physical adaptations that are the outcome of natural selection. These adaptations increase fitness by aiding activities such as finding food, avoiding predators or attracting mates. Organisms can also respond to selection by cooperating with each other, usually by aiding their relatives or engaging in mutually beneficial symbiosis.

In the longer term, evolution produces new species through splitting ancestral populations of organisms into new groups that cannot or will not interbreed. These outcomes of evolution are distinguished based on time scale as macroevolution versus microevolution. Macroevolution refers to evolution that occurs at or above the level of species, in particular speciation and extinction; whereas microevolution refers to smaller evolutionary changes within a species or population, in particular shifts in allele frequency and adaptation. For instance, a large amount of variation among individuals allows a species to rapidly adapt to new habitats , lessening the chance of it going extinct, while a wide geographic range increases the chance of speciation, by making it more likely that part of the population will become isolated.

In this sense, microevolution and macroevolution might involve selection at different levels—with microevolution acting on genes and organisms, versus macroevolutionary processes such as species selection acting on entire species and affecting their rates of speciation and extinction. A common misconception is that evolution has goals, long-term plans, or an innate tendency for "progress", as expressed in beliefs such as orthogenesis and evolutionism; realistically however, evolution has no long-term goal and does not necessarily produce greater complexity.

Adaptation is the process that makes organisms better suited to their habitat. For example, the adaptation of horses ' teeth to the grinding of grass. By using the term adaptation for the evolutionary process and adaptive trait for the product the bodily part or function , the two senses of the word may be distinguished. Adaptations are produced by natural selection. Adaptation may cause either the gain of a new feature, or the loss of an ancestral feature. An example that shows both types of change is bacterial adaptation to antibiotic selection, with genetic changes causing antibiotic resistance by both modifying the target of the drug, or increasing the activity of transporters that pump the drug out of the cell.

Adaptation occurs through the gradual modification of existing structures. Consequently, structures with similar internal organisation may have different functions in related organisms. This is the result of a single ancestral structure being adapted to function in different ways. The bones within bat wings, for example, are very similar to those in mice feet and primate hands, due to the descent of all these structures from a common mammalian ancestor.

During evolution, some structures may lose their original function and become vestigial structures. Examples include pseudogenes , [] the non-functional remains of eyes in blind cave-dwelling fish, [] wings in flightless birds, [] the presence of hip bones in whales and snakes, [] and sexual traits in organisms that reproduce via asexual reproduction. However, many traits that appear to be simple adaptations are in fact exaptations: However, in this species, the head has become so flattened that it assists in gliding from tree to tree—an exaptation.

An area of current investigation in evolutionary developmental biology is the developmental basis of adaptations and exaptations. Interactions between organisms can produce both conflict and cooperation. When the interaction is between pairs of species, such as a pathogen and a host , or a predator and its prey, these species can develop matched sets of adaptations. Here, the evolution of one species causes adaptations in a second species.

These changes in the second species then, in turn, cause new adaptations in the first species. This cycle of selection and response is called coevolution. In this predator-prey pair, an evolutionary arms race has produced high levels of toxin in the newt and correspondingly high levels of toxin resistance in the snake. Not all co-evolved interactions between species involve conflict. For instance, an extreme cooperation exists between plants and the mycorrhizal fungi that grow on their roots and aid the plant in absorbing nutrients from the soil.

Here, the fungi actually grow inside plant cells, allowing them to exchange nutrients with their hosts, while sending signals that suppress the plant immune system. Coalitions between organisms of the same species have also evolved. An extreme case is the eusociality found in social insects, such as bees , termites and ants , where sterile insects feed and guard the small number of organisms in a colony that are able to reproduce.

On an even smaller scale, the somatic cells that make up the body of an animal limit their reproduction so they can maintain a stable organism, which then supports a small number of the animal's germ cells to produce offspring. Here, somatic cells respond to specific signals that instruct them whether to grow, remain as they are, or die. If cells ignore these signals and multiply inappropriately, their uncontrolled growth causes cancer.

Such cooperation within species may have evolved through the process of kin selection , which is where one organism acts to help raise a relative's offspring. Speciation is the process where a species diverges into two or more descendant species. There are multiple ways to define the concept of "species. Despite the diversity of various species concepts, these various concepts can be placed into one of three broad philosophical approaches: Defined by evolutionary biologist Ernst Mayr in , the BSC states that "species are groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups.

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Barriers to reproduction between two diverging sexual populations are required for the populations to become new species. Gene flow may slow this process by spreading the new genetic variants also to the other populations. Depending on how far two species have diverged since their most recent common ancestor , it may still be possible for them to produce offspring, as with horses and donkeys mating to produce mules. In this case, closely related species may regularly interbreed, but hybrids will be selected against and the species will remain distinct.

However, viable hybrids are occasionally formed and these new species can either have properties intermediate between their parent species, or possess a totally new phenotype. Speciation has been observed multiple times under both controlled laboratory conditions see laboratory experiments of speciation and in nature. There are four primary geographic modes of speciation. The most common in animals is allopatric speciation , which occurs in populations initially isolated geographically, such as by habitat fragmentation or migration. Selection under these conditions can produce very rapid changes in the appearance and behaviour of organisms.

The second mode of speciation is peripatric speciation , which occurs when small populations of organisms become isolated in a new environment. This differs from allopatric speciation in that the isolated populations are numerically much smaller than the parental population. Here, the founder effect causes rapid speciation after an increase in inbreeding increases selection on homozygotes, leading to rapid genetic change.

The third mode is parapatric speciation. This is similar to peripatric speciation in that a small population enters a new habitat, but differs in that there is no physical separation between these two populations. Instead, speciation results from the evolution of mechanisms that reduce gene flow between the two populations. One example is the grass Anthoxanthum odoratum , which can undergo parapatric speciation in response to localised metal pollution from mines. Microbial mat fossils in 3. Evolution does not attempt to explain the origin of life covered instead by abiogenesis , but it does explain how early lifeforms evolved into the complex ecosystem that we see today.

Among offspring there are variations of genes due to the introduction of new genes via random changes called mutations or via reshuffling of existing genes during sexual reproduction. If those differences are helpful, the offspring is more likely to survive and reproduce.

This means that more offspring in the next generation will have that helpful difference and individuals will not have equal chances of reproductive success. In this way, traits that result in organisms being better adapted to their living conditions become more common in descendant populations. This process is responsible for the many diverse life forms in the world. The forces of evolution are most evident when populations become isolated, either through geographic distance or by other mechanisms that prevent genetic exchange. Over time, isolated populations can branch off into new species.

The majority of genetic mutations neither assist, change the appearance of, nor bring harm to individuals. Through the process of genetic drift, these mutated genes are neutrally sorted among populations and survive across generations by chance alone. In contrast to genetic drift, natural selection is not a random process because it acts on traits that are necessary for survival and reproduction. The modern understanding of evolution began with the publication of Charles Darwin 's On the Origin of Species.

In addition, Gregor Mendel 's work with plants helped to explain the hereditary patterns of genetics. Scientists now have a good understanding of the origin of new species speciation and have observed the speciation process in the laboratory and in the wild. Evolution is the principal scientific theory that biologists use to understand life and is used in many disciplines, including medicine , psychology , conservation biology , anthropology , forensics , agriculture and other social-cultural applications.

The main ideas of evolution may be summarized as follows:. In the 19th century, natural history collections and museums were popular. The European expansion and naval expeditions employed naturalists , while curators of grand museums showcased preserved and live specimens of the varieties of life.

Charles Darwin was an English graduate educated and trained in the disciplines of natural history. Such natural historians would collect, catalogue, describe and study the vast collections of specimens stored and managed by curators at these museums. Darwin served as a ship's naturalist on board HMS Beagle , assigned to a five-year research expedition around the world. Darwin gained extensive experience as he collected and studied the natural history of life forms from distant places.

Through his studies, he formulated the idea that each species had developed from ancestors with similar features. In , he described how a process he called natural selection would make this happen. The size of a population depends on how much and how many resources are able to support it. For the population to remain the same size year after year, there must be an equilibrium, or balance between the population size and available resources. Since organisms produce more offspring than their environment can support, not all individuals can survive out of each generation.

There must be a competitive struggle for resources that aid in survival. As a result, Darwin realised that it was not chance alone that determined survival. Instead, survival of an organism depends on the differences of each individual organism, or "traits," that aid or hinder survival and reproduction. Well-adapted individuals are likely to leave more offspring than their less well-adapted competitors. Traits that hinder survival and reproduction would disappear over generations. Traits that help an organism survive and reproduce would accumulate over generations.

Darwin realised that the unequal ability of individuals to survive and reproduce could cause gradual changes in the population and used the term natural selection to describe this process. Observations of variations in animals and plants formed the basis of the theory of natural selection. For example, Darwin observed that orchids and insects have a close relationship that allows the pollination of the plants. He noted that orchids have a variety of structures that attract insects, so that pollen from the flowers gets stuck to the insects' bodies. In this way, insects transport the pollen from a male to a female orchid.

In spite of the elaborate appearance of orchids, these specialised parts are made from the same basic structures that make up other flowers. In his book, Fertilisation of Orchids , Darwin proposed that the orchid flowers were adapted from pre-existing parts, through natural selection. Darwin was still researching and experimenting with his ideas on natural selection when he received a letter from Alfred Russel Wallace describing a theory very similar to his own. This led to an immediate joint publication of both theories.

The tips of the limbs represented modern species and the branches represented the common ancestors that are shared amongst many different species. To explain these relationships, Darwin said that all living things were related, and this meant that all life must be descended from a few forms, or even from a single common ancestor. He called this process descent with modification. Darwin published his theory of evolution by natural selection in On the Origin of Species in The implication that all life on Earth has a common ancestor has met with objections from some religious groups.

Their objections are in contrast to the level of support for the theory by more than 99 percent of those within the scientific community today. Natural selection is commonly equated with survival of the fittest , but this expression originated in Herbert Spencer 's Principles of Biology in , five years after Charles Darwin published his original works.

Survival of the fittest describes the process of natural selection incorrectly, because natural selection is not only about survival and it is not always the fittest that survives. Darwin's theory of natural selection laid the groundwork for modern evolutionary theory, and his experiments and observations showed that the organisms in populations varied from each other, that some of these variations were inherited, and that these differences could be acted on by natural selection. However, he could not explain the source of these variations. Like many of his predecessors, Darwin mistakenly thought that heritable traits were a product of use and disuse, and that features acquired during an organism's lifetime could be passed on to its offspring.

He looked for examples, such as large ground feeding birds getting stronger legs through exercise, and weaker wings from not flying until, like the ostrich , they could not fly at all. In the late 19th century this theory became known as Lamarckism. Darwin produced an unsuccessful theory he called pangenesis to try to explain how acquired characteristics could be inherited.

What is Darwin's Theory of Evolution?

In the s August Weismann 's experiments indicated that changes from use and disuse could not be inherited, and Lamarckism gradually fell from favor. The missing information needed to help explain how new features could pass from a parent to its offspring was provided by the pioneering genetics work of Gregor Mendel. Mendel's experiments with several generations of pea plants demonstrated that inheritance works by separating and reshuffling hereditary information during the formation of sex cells and recombining that information during fertilisation. This is like mixing different hands of playing cards , with an organism getting a random mix of half of the cards from one parent, and half of the cards from the other.

Mendel called the information factors ; however, they later became known as genes. Genes are the basic units of heredity in living organisms.

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They contain the information that directs the physical development and behavior of organisms. Genes are made of DNA. DNA is a long molecule made up of individual molecules called nucleotides. Genetic information is encoded in the sequence of nucleotides, that make up the DNA, just as the sequence of the letters in words carries information on a page. The genes are like short instructions built up of the "letters" of the DNA alphabet.

Put together, the entire set of these genes gives enough information to serve as an "instruction manual" of how to build and run an organism. The instructions spelled out by this DNA alphabet can be changed, however, by mutations, and this may alter the instructions carried within the genes. Within the cell , the genes are carried in chromosomes , which are packages for carrying the DNA. It is the reshuffling of the chromosomes that results in unique combinations of genes in offspring.

Since genes interact with one another during the development of an organism, novel combinations of genes produced by sexual reproduction can increase the genetic variability of the population even without new mutations. This can introduce genes into a population that were not present before. Evolution is not a random process. Although mutations in DNA are random, natural selection is not a process of chance: Evolution is an inevitable result of imperfectly copying, self-replicating organisms reproducing over billions of years under the selective pressure of the environment.

The outcome of evolution is not a perfectly designed organism. The end products of natural selection are organisms that are adapted to their present environments. Natural selection does not involve progress towards an ultimate goal. Evolution does not strive for more advanced , more intelligent, or more sophisticated life forms. Rapid environmental changes typically cause extinctions.

Genetic drift is a cause of allelic frequency change within populations of a species. Alleles are different variations of specific genes. They determine things like hair color , skin tone , eye color and blood type ; in other words, all the genetic traits that vary between individuals. Genetic drift does not introduce new alleles to a population, but it can reduce variation within a population by removing an allele from the gene pool.

Genetic drift is caused by random sampling of alleles. A truly random sample is a sample in which no outside forces affect what is selected. It is like pulling marbles of the same size and weight but of different colors from a brown paper bag. In any offspring, the alleles present are samples of the previous generations alleles, and chance plays a role in whether an individual survives to reproduce and to pass a sample of their generation onward to the next.

The allelic frequency of a population is the ratio of the copies of one specific allele that share the same form compared to the number of all forms of the allele present in the population. Genetic drift affects smaller populations more than it affects larger populations. The Hardy—Weinberg principle states that a large population in Hardy—Weinberg equilibrium will have no change in the frequency of alleles as generations pass. A population must be infinite in size. There must be a zero percent mutation rate between generations, because mutations can alter existing alleles or create new ones.

There can be no immigration or emigration in the population, because individuals arriving and leaving directly change allelic frequencies. There can be no selective pressures of any kind on the population, meaning that no individual is more likely than any other to survive and reproduce. Finally, mating must be totally random, with all males or females in some cases being equally desirable mates. This ensures a true random mixing of alleles. A population that is in Hardy—Weinberg equilibrium is analogous to a deck of cards ; no matter how many times the deck is shuffled, no new cards are added and no old ones are taken away.

A population bottleneck occurs when the population of a species is reduced drastically over a short period of time due to external forces. A bottleneck can reduce or eliminate genetic variation from a population. Further drift events after the bottleneck event can also reduce the population's genetic diversity. The lack of diversity created can make the population at risk to other selective pressures. A common example of a population bottleneck is the Northern elephant seal. Due to excessive hunting throughout the 19th century, the population of the northern elephant seal was reduced to 30 individuals or less.

They have made a full recovery, with the total number of individuals at around , and growing.

Introduction to evolution - Wikipedia

The effects of the bottleneck are visible, however. The seals are more likely to have serious problems with disease or genetic disorders, because there is almost no diversity in the population. The founder effect occurs when a small group from one population splits off and forms a new population, often through geographic isolation. This new population's allelic frequency is probably different from the original population's, and will change how common certain alleles are in the populations.

The founders of the population will determine the genetic makeup, and potentially the survival, of the new population for generations. One example of the founder effect is found in the Amish migration to Pennsylvania in Two of the founders of the colony in Pennsylvania carried the recessive allele for Ellis—van Creveld syndrome. Because the Amish tend to be religious isolates, they interbreed, and through generations of this practice the frequency of Ellis—van Creveld syndrome in the Amish people is much higher than the frequency in the general population.

The modern evolutionary synthesis is based on the concept that populations of organisms have significant genetic variation caused by mutation and by the recombination of genes during sexual reproduction. It defines evolution as the change in allelic frequencies within a population caused by genetic drift, gene flow between sub populations, and natural selection. Natural selection is emphasised as the most important mechanism of evolution; large changes are the result of the gradual accumulation of small changes over long periods of time.

The modern evolutionary synthesis is the outcome of a merger of several different scientific fields to produce a more cohesive understanding of evolutionary theory. In the s, Ronald Fisher , J. Haldane and Sewall Wright combined Darwin's theory of natural selection with statistical models of Mendelian genetics , founding the discipline of population genetics. In the s and s, efforts were made to merge population genetics, the observations of field naturalists on the distribution of species and sub species, and analysis of the fossil record into a unified explanatory model.

This colouring enabled them to hide from potential predators on trees with pale-coloured bark, such as birch trees. The rarer dark-coloured peppered moths were easily seen against the pale bark of trees and therefore more easily seen by predators. A pale peppered moth on an oak tree.

A pale peppered moth on a dark tree. Evolution of modern humans. Evolution of the human brain. Are humans still evolving? What is genetic variation? Fruit flies in the laboratory. What is selective breeding? Is this page helpful? Anything else you'd like to see? What were you looking for?