Natural selection

Modern biology began in the nineteenth century with Charles Darwin's work on evolution by natural selection.
Aristotle considered whether different forms could have appeared, only the useful ones surviving.
Part of Thomas Malthus's table of population growth in England 1780–1810, from his Essay on the Principle of Population, 6th edition, 1826
Charles Darwin noted that pigeon fanciers had created many kinds of pigeon, such as Tumblers (1, 12), Fantails (13), and Pouters (14) by selective breeding.
Evolutionary developmental biology relates the evolution of form to the precise pattern of gene activity, here gap genes in the fruit fly, during embryonic development.
During the industrial revolution, pollution killed many lichens, leaving tree trunks dark. A dark (melanic) morph of the peppered moth largely replaced the formerly usual light morph (both shown here). Since the moths are subject to predation by birds hunting by sight, the colour change offers better camouflage against the changed background, suggesting natural selection at work.
1: directional selection: a single extreme phenotype favoured. 2, stabilizing selection: intermediate favoured over extremes. 3: disruptive selection: extremes favoured over intermediate. X-axis: phenotypic trait Y-axis: number of organisms Group A: original population Group B: after selection
Different types of selection act at each life cycle stage of a sexually reproducing organism.
The peacock's elaborate plumage is mentioned by Darwin as an example of sexual selection, and is a classic example of Fisherian runaway, driven to its conspicuous size and coloration through mate choice by females over many generations.
Selection in action: resistance to antibiotics grows though the survival of individuals less affected by the antibiotic. Their offspring inherit the resistance.

Differential survival and reproduction of individuals due to differences in phenotype.

- Natural selection
Modern biology began in the nineteenth century with Charles Darwin's work on evolution by natural selection.

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Wallace in 1895

Alfred Russel Wallace

British naturalist, explorer, geographer, anthropologist, biologist and illustrator.

British naturalist, explorer, geographer, anthropologist, biologist and illustrator.

Wallace in 1895
Arenga pinnata sketched by Wallace on a visit to Celebes and later reworked by Walter Hood Fitch
A photograph from Wallace's autobiography shows the building Wallace and his brother John designed and built for the Neath Mechanics' Institute.
A map from The Malay Archipelago shows the physical geography of the archipelago and Wallace's travels around the area. The thin black lines indicate where Wallace travelled, and the red lines indicate chains of volcanoes.
An illustration from The Malay Archipelago depicts the flying frog Wallace discovered.
A photograph of Wallace taken in Singapore in 1862
Wallace's grave in Broadstone Cemetery, Broadstone, Dorset, which was restored by the A. R. Wallace Memorial Fund in 2000. It features a 7 ft tall fossil tree trunk from Portland mounted on a block of Purbeck limestone.
The Darwin–Wallace Medal was issued by the Linnean Society on the 50th anniversary of the reading of Darwin and Wallace's papers on natural selection. Wallace received the only gold example.
An illustration from the chapter on the application of natural selection to humans in Wallace's 1889 book Darwinism shows a chimpanzee.
A map of the world from The Geographical Distribution of Animals shows Wallace's six biogeographical regions.
The line separating the Indo-Malayan and the Austro-Malayan region in Wallace's On the Physical Geography of the Malay Archipelago (1863)
Spirit photograph taken by Frederick Hudson of Wallace and his late mother; he may have used double exposure.
Wallace and his signature on the frontispiece of Darwinism (1889)
Anthony Smith's statue of Wallace, looking up at a bronze model of a Wallace's golden birdwing butterfly. Natural History Museum, London, unveiled 7 November 2013
Alfred Russel Wallace, attributed to John William Beaufort (1864–1943), a portrait in the Central Hall of the Natural History Museum, London.
Corvus enca celebensis, Sula Islands, registered in 1861 at a forerunner of Naturalis Biodiversity Center
Toxorhamphus novaeguineae novaeguineae, Misool, Raja Ampat Islands, 1865
Pitohui ferrugineus leucorhynchus, Waigeo, West-Papua, no year
Nectarinia jugularis clementiae, Seram Island, 1865
Mino anais anais, South West Papua, 1863

He is best known for independently conceiving the theory of evolution through natural selection.

African pygmy kingfisher, showing coloration shared by all adults of that species to a high degree of fidelity.

Speciation

Evolutionary process by which populations evolve to become distinct species.

Evolutionary process by which populations evolve to become distinct species.

African pygmy kingfisher, showing coloration shared by all adults of that species to a high degree of fidelity.
Comparison of allopatric, peripatric, parapatric and sympatric speciation
Cichlids such as Haplochromis nyererei diversified by sympatric speciation in the Rift Valley lakes.
Rhagoletis pomonella, the hawthorn fly, appears to be in the process of sympatric speciation.
Reinforcement assists speciation by selecting against hybrids.
Gaur (Indian bison) can interbreed with domestic cattle.
Male Drosophila pseudoobscura
Speciation via polyploidy: A diploid cell undergoes failed meiosis, producing diploid gametes, which self-fertilize to produce a tetraploid zygote. In plants, this can effectively be a new species, reproductively isolated from its parents, and able to reproduce.
Phyletic gradualism, above, consists of relatively slow change over geological time. Punctuated equilibrium, bottom, consists of morphological stability and rare, relatively rapid bursts of evolutionary change.

Charles Darwin was the first to describe the role of natural selection in speciation in his 1859 book On the Origin of Species.

The title page of the 1859 edition
of On the Origin of Species

On the Origin of Species

Work of scientific literature by Charles Darwin that is considered to be the foundation of evolutionary biology.

Work of scientific literature by Charles Darwin that is considered to be the foundation of evolutionary biology.

The title page of the 1859 edition
of On the Origin of Species
Darwin pictured shortly before publication
Cuvier's 1799 paper on living and fossil elephants helped establish the reality of extinction.
In mid-July 1837 Darwin started his "B" notebook on Transmutation of Species, and on page 36 wrote "I think" above his first evolutionary tree.
Darwin researched how the skulls of different pigeon breeds varied, as shown in his Variation of Plants and Animals Under Domestication of 1868.
A photograph of Alfred Russel Wallace (1823–1913) taken in Singapore in 1862
On the Origin of Species by means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life, 2nd edition. By Charles Darwin, John Murray, London, 1860. National Museum of Scotland
American botanist Asa Gray (1810–1888)
John Gould's illustration of Darwin's rhea was published in 1841. The existence of two rhea species with overlapping ranges influenced Darwin.
This tree diagram, used to show the divergence of species, is the only illustration in the Origin of Species.
In the 1870s, British caricatures of Darwin with a non-human ape body contributed to the identification of evolutionism with Darwinism.
Huxley used illustrations to show that humans and apes had the same basic skeletal structure.
Haeckel showed a main trunk leading to mankind with minor branches to various animals, unlike Darwin's branching evolutionary tree.
The liberal theologian Baden Powell defended evolutionary ideas by arguing that the introduction of new species should be considered a natural rather than a miraculous process.
A modern phylogenetic tree based on genome analysis shows the three-domain system.

Darwin's book introduced the scientific theory that populations evolve over the course of generations through a process of natural selection.

Genetic diversity

Total number of genetic characteristics in the genetic makeup of a species, it ranges widely from the number of species to differences within species and can be attributed to the span of survival for a species.

Total number of genetic characteristics in the genetic makeup of a species, it ranges widely from the number of species to differences within species and can be attributed to the span of survival for a species.

A graphical representation of the typical human karyotype.
Varieties of maize in the office of the Russian plant geneticist Nikolai Vavilov
Photomontage of planktonic organisms.
A Tanzanian cheetah.

Variation in the populations gene pool allows natural selection to act upon traits that allow the population to adapt to changing environments.

Charles Darwin in 1868

Darwinism

Charles Darwin in 1868
As evolution became widely accepted in the 1870s, caricatures of Charles Darwin with the body of an ape or monkey symbolised evolution.

Darwinism is a theory of biological evolution developed by the English naturalist Charles Darwin (1809–1882) and others, stating that all species of organisms arise and develop through the natural selection of small, inherited variations that increase the individual's ability to compete, survive, and reproduce.

The shells of individuals within the bivalve mollusk species Donax variabilis show diverse coloration and  patterning in their phenotypes.

Phenotype

Set of observable characteristics or traits of an organism.

Set of observable characteristics or traits of an organism.

The shells of individuals within the bivalve mollusk species Donax variabilis show diverse coloration and  patterning in their phenotypes.
Here the relation between genotype and phenotype is illustrated, using a Punnett square, for the character of petal color in pea plants. The letters B and b represent genes for color, and the pictures show the resultant phenotypes. This shows how multiple genotypes (BB and Bb) may yield the same phenotype (purple petals).
ABO blood groups determined through a Punnett square and displaying phenotypes and genotypes
Biston betularia morpha typica, the standard light-colored peppered moth
B.betularia morpha carbonaria, the melanic form, illustrating discontinuous variation

Phenotypic variation (due to underlying heritable genetic variation) is a fundamental prerequisite for evolution by natural selection.

Several major ideas about evolution came together in the population genetics of the early 20th century to form the modern synthesis, including genetic variation, natural selection, and particulate (Mendelian) inheritance. This ended the eclipse of Darwinism and supplanted a variety of non-Darwinian theories of evolution.

Modern synthesis (20th century)

The early 20th-century synthesis reconciling Charles Darwin's theory of evolution and Gregor Mendel's ideas on heredity in a joint mathematical framework.

The early 20th-century synthesis reconciling Charles Darwin's theory of evolution and Gregor Mendel's ideas on heredity in a joint mathematical framework.

Several major ideas about evolution came together in the population genetics of the early 20th century to form the modern synthesis, including genetic variation, natural selection, and particulate (Mendelian) inheritance. This ended the eclipse of Darwinism and supplanted a variety of non-Darwinian theories of evolution.
Darwin's pangenesis theory. Every part of the body emits tiny gemmules which migrate to the gonads and contribute to the next generation via the fertilised egg. Changes to the body during an organism's life would be inherited, as in Lamarckism.
Blending inheritance, implied by pangenesis, causes the averaging out of every characteristic, which as the engineer Fleeming Jenkin pointed out, would make evolution by natural selection impossible.
August Weismann's germ plasm theory. The hereditary material, the germplasm, is confined to the gonads and the gametes. Somatic cells (of the body) develop afresh in each generation from the germplasm.
William Bateson championed Mendelism.
Karl Pearson led the biometric school.
Sewall Wright introduced the idea of a fitness landscape with local optima.
Drosophila pseudoobscura, the fruit fly which served as Theodosius Dobzhansky's model organism
E. B. Ford studied polymorphism in the scarlet tiger moth for many years.
Julian Huxley presented a serious but popularising version of the theory in his 1942 book Evolution: The Modern Synthesis.
Ernst Mayr argued that geographic isolation was needed to provide sufficient reproductive isolation for new species to form.
George Gaylord Simpson argued against the naive view that evolution such as of the horse took place in a "straight-line". He noted that any chosen line is one path in a complex branching tree, natural selection having no imposed direction.
Speciation via polyploidy: a diploid cell may fail to separate during meiosis, producing diploid gametes which self-fertilize to produce a fertile tetraploid zygote that cannot interbreed with its parent species.
Ant societies have evolved elaborate caste structures, widely different in size and function.
Evolutionary developmental biology has formed a synthesis of evolutionary and developmental biology, discovering deep homology between the embryogenesis of such different animals as insects and vertebrates.
A 21st century tree of life showing horizontal gene transfers among prokaryotes and the saltational endosymbiosis events that created the eukaryotes, neither fitting into the 20th century's modern synthesis
Inputs to the modern synthesis, with other topics (inverted colours) such as developmental biology that were not joined with evolutionary biology until the turn of the 21st century

The 19th-century ideas of natural selection and Mendelian genetics were put together with population genetics, early in the twentieth century.

A red tulip exhibiting a partially yellow petal due to a mutation in its genes

Mutation

Alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA.

Alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA.

A red tulip exhibiting a partially yellow petal due to a mutation in its genes
Mutation with double bloom in the Langheck Nature Reserve near Nittel, Germany
Prodryas persephone, a Late Eocene butterfly
A covalent adduct between the metabolite of benzo[a]pyrene, the major mutagen in tobacco smoke, and DNA
Five types of chromosomal mutations
Selection of disease-causing mutations, in a standard table of the genetic code of amino acids
The structure of a eukaryotic protein-coding gene. A mutation in the protein coding region (red) can result in a change in the amino acid sequence. Mutations in other areas of the gene can have diverse effects. Changes within regulatory sequences (yellow and blue) can effect transcriptional and translational regulation of gene expression.
The distribution of fitness effects (DFE) of mutations in vesicular stomatitis virus. In this experiment, random mutations were introduced into the virus by site-directed mutagenesis, and the fitness of each mutant was compared with the ancestral type. A fitness of zero, less than one, one, more than one, respectively, indicates that mutations are lethal, deleterious, neutral, and advantageous.
A mutation has caused this moss rose plant to produce flowers of different colors. This is a somatic mutation that may also be passed on in the germline.
Dutch botanist Hugo de Vries making a painting of an evening primrose, the plant which had apparently produced new forms by large mutations in his experiments, by Thérèse Schwartze, 1918

Mutation is the ultimate source of all genetic variation, providing the raw material on which evolutionary forces such as natural selection can act.

In this simulation, each black dot on a marble signifies that it has been chosen for copying (reproduction) one time. fixation in the blue "allele" occurs within five generations.

Genetic drift

Change in the frequency of an existing gene variant (allele) in a population due to random chance.

Change in the frequency of an existing gene variant (allele) in a population due to random chance.

In this simulation, each black dot on a marble signifies that it has been chosen for copying (reproduction) one time. fixation in the blue "allele" occurs within five generations.
Ten simulations of random genetic drift of a single given allele with an initial frequency distribution 0.5 measured over the course of 50 generations, repeated in three reproductively synchronous populations of different sizes. In these simulations, alleles drift to loss or fixation (frequency of 0.0 or 1.0) only in the smallest population.
Changes in a population's allele frequency following a population bottleneck: the rapid and radical decline in population size has reduced the population's genetic variation.
When very few members of a population migrate to form a separate new population, the founder effect occurs. For a period after the foundation, the small population experiences intensive drift. In the figure this results in fixation of the red allele.

In the middle of the 20th century, vigorous debates occurred over the relative importance of natural selection versus neutral processes, including genetic drift.

The second of Jean-Baptiste Lamarck's two factors (the first being a complexifying force) was an adaptive force that causes animals with a given body plan to adapt to circumstances by inheritance of acquired characteristics, creating a diversity of species and genera.

Adaptation

In biology, adaptation has three related meanings.

In biology, adaptation has three related meanings.

The second of Jean-Baptiste Lamarck's two factors (the first being a complexifying force) was an adaptive force that causes animals with a given body plan to adapt to circumstances by inheritance of acquired characteristics, creating a diversity of species and genera.
Some generalists, such as birds, have the flexibility to adapt to urban areas.
In this sketch of a fitness landscape, a population can evolve by following the arrows to the adaptive peak at point B, and the points A and C are local optima where a population could become trapped.
Pollinating insects are co-adapted with flowering plants.
Images A and B show real wasps; the others show Batesian mimics: three hoverflies and one beetle.
An Indian peacock's train
in full display
The feathers of Sinosauropteryx, a dinosaur with feathers, were used for insulation, making them an exaptation for flight.
"Behaviour with a purpose": a young springbok stotting. A biologist might argue that this has the function of signalling to predators, helping the springbok to survive and allowing it to reproduce.

Thirdly, it is a phenotypic trait or adaptive trait, with a functional role in each individual organism, that is maintained and has evolved through natural selection.