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Evolution, dispersal of genetics and Fisher’s equation

Evolution, dispersal of genetics and Fisher’s equation. Charles Darwin (1809-1882). 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

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Evolution, dispersal of genetics and Fisher’s equation

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  1. Evolution, dispersal of genetics and Fisher’s equation

  2. Charles Darwin (1809-1882) • 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?

  3. 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

  4. 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.

  5. Plant material Mendel used: sweet pea

  6. 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

  7. 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

  8. 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 (The equation at page 120 of Britton’s book is wrong.)

  9. 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

  10. Example: blood type • The ABO blood group in humans are determined by a system of three alleles, A, B and O. Genotype AA and AO are group A, BB and BO are group B, AB is group AB, and OO is group O. The frequencies of blood groups in England are 32.1% A, 22.4% B, 7.1% AB and 38.4% O. Are these proportions consistent with the assumptions of random mating?

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

  12. Some special cases

  13. 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

  14. Finally, Fisher’s equation with diffusion !

  15. Traveling wave of Fisher’s equation

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