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Evolution, dispersal of genetics and Fisher’s equation. Charles Darwin (12 February 1809 – 19 April 1882). On the Origin of Species (1859) Many individuals of s species are destined to die before reaching reproduction age.

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Charles darwin 12 february 1809 19 april 1882 l.jpg
Charles Darwin (12 February 1809 – 19 April 1882)

  • On the Origin of Species (1859)

  • Many individuals of s species are destined to die before reaching reproduction age.

  • Advantageous gene tends to be persevered, thus change the characteristics of the species

  • Evolution by natural selection (slight modifications are passed on through generations)

  • But how does it work?

Gregor mendel 1822 1884 l.jpg
Gregor Mendel (1822-1884)

  • Study of the inheritance of traits in pea plants. Over a period of 7 years he bred and counted about 28,000 pea plants.

  • Traits are passed through generations unchanged, a child can inherit the trait from either its mother or its father

  • The trait is determined by two factors, one inherited from each parent, and it comes with probability from either parent

  • For pairs of contrasting traits, one of the two is dominant and always overrules the other (recessive) factor

  • Trait - a variant for a character

  • Character - a heritable feature

  • Factor – Mendel’s factor is gene today

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Experiments on Plant Hybridization, Gregor Mendel (1865, Proceedings of the Natural History Society)http://www.mendelweb.org/Mendel.html (original paper)

Mendel compared seven discrete traits:

  • Smoothness of the seeds.

  • Color of the seeds.

  • Color of the seed coats.

  • Shape of the pods.

  • Color of unripe pods.

  • Position of flowers.

  • Length of the stems.

    Through experimentation, Mendel discovered that one inheritable trait would invariably be dominant to its recessive alternative. This model, later known as Mendelian inheritance or Mendelian genetics, provided an alternative to blending inheritance, which was the prevailing theory at the time. Unfortunately, Mendel's work received little attention from the scientific community and was largely forgotten. It was not until the early 20th century that Mendel's work was rediscovered and his ideas used to help form the modern synthesis.

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Examples of Mendel traits

Difference in the color of the seed albumen

Yellow (I) and green (i) cotyledons

Difference in the form of the ripe seeds

Round (R) and wrinkled (r) cotyledons

Difference in the color of the seed coat

Colored (A) and white (a) flowers

Difference in the color of the unripe pods

Green (Gp) or yellow (gp) pod wall

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Genetic Terms

  • Phenotype - the outward, physical appearance of a particular trait (Pea: round or wrinkled seed phenotype; yellow or green seed phenotype)

  • Genotype - genetic make-up of a particular trait, the specific allelic combination of a certain gene (AA, BB, or AB)

  • Allele - one alternative form of a given allelic pair(A or B)

  • Homozygote - an individual which contains only one allele at the allelic pair (AA or BB)

  • Heterozygote - an individual which contains one of each member of the gene pair (AB)

  • Dominant - the allele that expresses itself at the expense of an alternate allele; the phenotype that is expressed in the F1 generation from the cross of two pure lines

  • Recessive - an allele whose expression is suppressed in the presence of a dominant allele; the phenotype that disappears in the F1 generation from the cross of two pure lines and reappears in the F2 generation

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More general, the probabilities (frequencies) of allele A or a in a population can be p and q, where p+q=1. Then the probability of each genotype in F2 is

AA: p^2 Aa: 2pq aa: q^2

Hardy-Weinberg’s law: the frequency remain the same for each genotype

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Wilhelm Weinberg(1862 — 1937)

G. H. Hardy


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Assumptions in Hardy-Weinberg’s Law

  • Expected sex ratio is independent of genotype

  • Mating is random

  • Fertility is independent of genotype

  • Survivorship is independent of genotype

  • There is no mutation or migration

    Different survival rate or fertility rate for the two genotypes could break Hardy-Weinberg’s law

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Evolution of genes under selection:Fisher-Haldane-Wright equation

Ronald fisher 1890 1962 j b s haldane 1892 1964 sewall wright 1889 1988 l.jpg
Ronald Fisher (1890-1962)J. B. S. Haldane (1892-1964)Sewall Wright (1889-1988)

Darwin-Wallace Medalis a medal awarded by the Linnean Society of London every 50 years, beginning in 1908, 50 years after the joint presentation by Charles Darwin and Alfred Russel Wallace of two scientific papers - On the Tendency of Species to form Varieties; and on the Perpetuation of Varieties and Species by Natural Means of Selection - to the Linnean Society of London on 1 July 1858. It is awarded for "major advances in evolutionary biology"

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Evolution to advantageous gene: numerical solutions by Matlab

(upper): dominant and advantageous, left: s=0.002, right: s=0.2

(lower): dominant and recessive, left: s=0.002, right: s=0.2