The logarithm of fitness as a function of the number of deleterious mutations. Synergistic epistasis is represented by the red line - each subsequent deleterious mutation has a larger proportionate effect on the organism's fitness. Antagonistic epistasis is in blue. The black line shows the non-epistatic case, where fitness is the product of the contributions from each of its loci.
Gregor Mendel, the Moravian Augustinian monk who founded the modern science of genetics
Drosophila melanogaster
Characteristics Mendel used in his experiments
Gene flow is the transfer of alleles from one population to another population through immigration of individuals. In this example, one of the birds from population A immigrates to population B, which has fewer of the dominant alleles, and through mating incorporates its alleles into the other population.
P-Generation and F1-Generation: The dominant allele for purple-red flower hides the phenotypic effect of the recessive allele for white flowers. F2-Generation: The recessive trait from the P-Generation phenotypically reappears in the individuals that are homozygous with the recessive genetic trait.
The Great Wall of China is an obstacle to gene flow of some terrestrial species.
Myosotis: Colour and distribution of colours are inherited independently.
Current tree of life showing vertical and horizontal gene transfers.
F1 generation: All individuals have the same genotype and same phenotype expressing the dominant trait ( red ).
F2 generation: The phenotypes in the second generation show a 3 : 1 ratio.
In the genotype 25 % are homozygous with the dominant trait, 50 % are heterozygous genetic carriers of the recessive trait, 25 % are homozygous with the recessive genetic trait and expressing the recessive character.
In Mirabilis jalapa and Antirrhinum majus are examples for intermediate inheritance. As seen in the F1-generation, heterozygous plants have " light pink " flowers—a mix of " red " and "white". The F2-generation shows a 1:2:1 ratio of red : light pink : white
A Punnett square for one of Mendel's pea plant experiments – self-fertilization of the F1 generation
Segregation and independent assortment are consistent with the chromosome theory of inheritance.
When the parents are homozygous for two different genetic traits (llSS and LL sP sP), their children in the F1 generation are heterozygous at both loci and only show the dominant phenotypes (Ll S sP). P-Generation: Each parent possesses one dominant and one recessive trait purebred (homozygous). In this example, solid coat color is indicated by S (dominant), Piebald spotting by sP (recessive), while fur length is indicated by L (short, dominant) or l (long, recessive). All individuals are equal in genotype and phenotype. In the F2 generation all combinations of coat color and fur length occur: 9 are short haired with solid colour, 3 are short haired with spotting, 3 are long haired with solid colour and 1 is long haired with spotting. The traits are inherited independently, so that new combinations can occur. Average number ratio of phenotypes 9:3:3:1
For example 3 pairs of homologous chromosomes allow 8 possible combinations, all equally likely to move into the gamete during meiosis. This is the main reason for independent assortment. The equation to determine the number of possible combinations given the number of homologous pairs = 2x (x = number of homologous pairs)

Ronald Fisher combined these ideas with the theory of natural selection in his 1930 book The Genetical Theory of Natural Selection, putting evolution onto a mathematical footing and forming the basis for population genetics within the modern evolutionary synthesis.

- Mendelian inheritance

Population genetics began as a reconciliation of Mendelian inheritance and biostatistics models.

- Population genetics
The logarithm of fitness as a function of the number of deleterious mutations. Synergistic epistasis is represented by the red line - each subsequent deleterious mutation has a larger proportionate effect on the organism's fitness. Antagonistic epistasis is in blue. The black line shows the non-epistatic case, where fitness is the product of the contributions from each of its loci.

5 related topics

Alpha

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.

Fisher in 1913

Ronald Fisher

British polymath who was active as a mathematician, statistician, biologist, geneticist, and academic.

British polymath who was active as a mathematician, statistician, biologist, geneticist, and academic.

Fisher in 1913
As a child
Inverforth House, North End Way NW3, where Fisher lived from 1896 to 1904
On graduating from Cambridge University, 1912
The peacock tail in flight, the classic example of a Fisherian runaway
Rothamsted Research
Memorial plaque over his remains, lectern-side aisle of St Peter's Cathedral, Adelaide
Stained glass window (now removed) in the dining hall of Caius College, in Cambridge, commemorating Ronald Fisher and representing a Latin square, discussed by him in The Design of Experiments
As a steward at the First International Eugenics Conference, 1912

In genetics, his work used mathematics to combine Mendelian genetics and natural selection; this contributed to the revival of Darwinism in the early 20th-century revision of the theory of evolution known as the modern synthesis.

Together with J. B. S. Haldane and Sewall Wright, Fisher is known as one of the three principal founders of population genetics.

Haldane in 1914

J. B. S. Haldane

British scientist who worked in physiology, genetics, evolutionary biology, and mathematics.

British scientist who worked in physiology, genetics, evolutionary biology, and mathematics.

Haldane in 1914
Marcello Siniscalco (standing) and Haldane in Andhra Pradesh, India, 1964
J.B.S. Haldane Avenue in Kolkata, the busy connecting road from Eastern Metropolitan Bypass to Park Circus area containing Science City
A Low cartoon featuring Haldane – "Prophesies for 1949"
Lysenko speaking at the Kremlin in 1935. Behind him are (left to right) Stanislav Kosior, Anastas Mikoyan, Andrei Andreev and Joseph Stalin.
Oxford University Museum of Natural History display dedicated to Haldane and his reply when asked to comment on the mind of the Creator.

Subsequent works established a unification of Mendelian genetics and Darwinian evolution by natural selection whilst laying the groundwork for modern evolutionary synthesis and thus helped to create population genetics.

First edition title page

The Genetical Theory of Natural Selection

First edition title page
The peacock plumage is a classic example of the hypothesized Fisherian runaway.

The Genetical Theory of Natural Selection is a book by Ronald Fisher which combines Mendelian genetics with Charles Darwin's theory of natural selection, with Fisher being the first to argue that "Mendelism therefore validates Darwinism" and stating with regard to mutations that "The vast majority of large mutations are deleterious; small mutations are both far more frequent and more likely to be useful", thus refuting orthogenesis.

First published in 1930 by The Clarendon Press, it is one of the most important books of the modern synthesis, and helped define population genetics.

Blending inheritance leads to the averaging out of every characteristic, which as the engineer Fleeming Jenkin pointed out, makes evolution by natural selection impossible.

Genetics

Branch of biology concerned with the study of genes, genetic variation, and heredity in organisms.

Branch of biology concerned with the study of genes, genetic variation, and heredity in organisms.

Blending inheritance leads to the averaging out of every characteristic, which as the engineer Fleeming Jenkin pointed out, makes evolution by natural selection impossible.
Morgan's observation of sex-linked inheritance of a mutation causing white eyes in Drosophila led him to the hypothesis that genes are located upon chromosomes.
DNA, the molecular basis for biological inheritance. Each strand of DNA is a chain of nucleotides, matching each other in the center to form what look like rungs on a twisted ladder.
A Punnett square depicting a cross between two pea plants heterozygous for purple (B) and white (b) blossoms.
Genetic pedigree charts help track the inheritance patterns of traits.
Human height is a trait with complex genetic causes. Francis Galton's data from 1889 shows the relationship between offspring height as a function of mean parent height.
The molecular structure of DNA. Bases pair through the arrangement of hydrogen bonding between the strands.
DNA sequence
Walther Flemming's 1882 diagram of eukaryotic cell division. Chromosomes are copied, condensed, and organized. Then, as the cell divides, chromosome copies separate into the daughter cells.
Thomas Hunt Morgan's 1916 illustration of a double crossover between chromosomes.
The genetic code: Using a triplet code, DNA, through a messenger RNA intermediary, specifies a protein.
Siamese cats have a temperature-sensitive pigment-production mutation.
Transcription factors bind to DNA, influencing the transcription of associated genes.
Gene duplication allows diversification by providing redundancy: one gene can mutate and lose its original function without harming the organism.
This is a diagram showing mutations in an RNA sequence. Figure (1) is a normal RNA sequence, consisting of 4 codons. Figure (2) shows a missense, single point, non silent mutation. Figures (3 and 4) both show frameshift mutations, which is why they are grouped together. Figure 3 shows a deletion of the second base pair in the second codon. Figure 4 shows an insertion in the third base pair of the second codon. Figure (5) shows a repeat expansion, where an entire codon is duplicated.
An evolutionary tree of eukaryotic organisms, constructed by the comparison of several orthologous gene sequences.
The common fruit fly (Drosophila melanogaster) is a popular model organism in genetics research.
Schematic relationship between biochemistry, genetics and molecular biology.
Colonies of E. coli produced by cellular cloning. A similar methodology is often used in molecular cloning.

Genetics has given rise to a number of subfields, including molecular genetics, epigenetics and population genetics.

These observations of discrete inheritance and the segregation of alleles are collectively known as Mendel's first law or the Law of Segregation.