Evolution

I. Cuvier and the fixism theory (1769-1832)

• Fixism and Creationism.
  Until the 19th century in Occident, the most largely believed theory was fixism. The species always are what they have been since their creation. They are fixe and never change because the world has been created by God. This theory is similar to creationism.

II. Lamarck and the transformism theory (1744-1829)

• In contact with nature the organisms acquire capacities to become more and more complex.
  
• Nature generates circumstances that forces organisms to change to be adapted to their environment (these circumstances are called the Lamarckian factors).
  
• The transformism or the assertion of a principle of change.

1. The compared morpho-anatomy and the palaeontology show that the organisation of the living organisms follows a scale of complexity.
   
2. « Time and circumstances » are at the origin of the gradually change observed in the fossil series.
   
3. This gradual transformation indicates a relationship between related species.

III. Darwin and the Theory of Evolution (1809-1882)

III.1. The inferences of Darwin theory

1. Environmental resources are limited for an increasing number of individuals.
   
2. The survival is not due to random, but depends on the hereditary constitution of the individuals.
   
3. The natural selection that takes effect on the countless successive generations is the beginning of a slow and continuous change of populations.

III.2. The implications of the Darwin theory

1. Refutation of Fixism.
   
2. Refutation of Essentialism.
   
3. Refutation of Creationism.
   
4. Refutation of Anthropocentrism.
   
5. A classification based on genealogy.

III.3. After Darwin

1. The non-inheritance of acquired characters (Weismann 1883).
   
2. The genetic foundations of heredity.
   
3. The population genetics.

IV. Population Genetics

IV.1. Hypothesis

• Does a genetic polymorphism exist in natural populations?
  
• Which models are able to describe the allele frequencies in these populations?
  
• Which natural selections modify these allele frequencies?

IV.2. Role of diverse factors (evolutionary forces) on the allele frequencies variation through generations

IV.2.1. Hardy-Weinberg Model (basic model)
• Random mating (gametes and individuals): the panmictic hypothesis.
  
• No mutation or migration.
  
• No selection.
  
• Population of infinite size.

  Under these conditions the allele frequencies do not vary.

IV.2.2. Impact of Consanguinity

• Mating occurs according to the lineage with for consequence the existence of common ancestor(s).
  
• Mating occurs between related. In this case, their descendants are consanguineous.
  
• The consanguinity increases the probability to constitute homozygous genotypes.
  
• Repeated consanguine mating leads to the homogenisation of populations and therfore to the no maintenance of the genetic polymorphism.

IV.2.3. Impact of Genetic drift


• The genetic drift leads to a homogenisation of populations and therefore not to the maintenance of the genetic polymorphism.

IV.2.4. Impact of selection

The differential selection between phenotypes (and thus between genotypes) leads to the fixation of an advantaged allele, if the selective value of the homozygous for this allele is higher than any of the others genotypes or it leads to the maintenance of a genetic polymorphism, if the selective value of the heterozygous is higher than the others genotypes.


IV.2.5. Combine impact of the genetic drift and mutations: The neutral theory of evolution

• Mr Motoo Kimura (1924 - 1994) proposes a model in which the different mutations have no differential impact on the survival of the bearer. The mutations are selectively neutral.
  
• These mutations occur randomly and disappear more or less quickly depending on the action of genetic drift, according to the population size which is always constant (N individuals, 2N gametes).
  
• No selection --> the alleles are selectively neutral.
  
• Mutations: the neutral mutation rate (µ) (for a locus and by generation, it is 10^-5- 10^-6).
  
• Model: with an infinite number of alleles.

• These results are based on several hypothesis:
  
  * Constant accumulation of mutations (molecular clock hypothesis).
  * There are as many alleles lost by genetic drift as new alleles produce by mutation (equilibrium mutation drift).
  * The process of fixation can be extremely long (4N generations).
  * 4N = the average time for a new neutral allele to replace the former. Then, the time for this allele to be fixed is of 4N generations (N the population size) (coalescence time).
  * 1/µ = the substitution time for a new neutral allele to replace the former (in number of generations).
  * The fixations are probably concentrated when the specie is constituted of few individuals (as in its beginning) during the process of speciation.

IV.2.6. The new synthesis theory of evolution or the evolutionary synthesis
It was elaborated in the 40-50’s and it modifies and improves Darwin theory.

It is Theodosius Dobzhansky (1900-1975), (naturalist then geneticist) who revised the evolutionism. In his book "Genetics and the Origin of Species", he considers that under the action of natural selection, all the evolutionary phenomena are the result of change in the gene frequency within the line.

Gradual evolution is explained by the interactions between mutations and recombination through the screen of natural selection.

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