ARCHIVE - Evolution 3

Doing Science

More about evolution . . .

Evolution 3 Mendel and the Neodarwinian Synthesis 1Nov03

Review: Evolution by natural selection is a consequence of these conditions

potential for rapid reproduction — most organisms can produce more offspring than the environment is capable of supporting

therefore, the availability of resources (food, water, space, light, etc.) will limit the size of the population

where there is heritable variation in traits that influence reproductive success, individuals with those traits should become increasingly predominant in the population.

After Darwin and Wallace, some resistance due to

literal interpretation of the Bible.
Teleological thinking: "final causes" toward even greater perfection (orthogenesis)

Populations as the unit of evolution: "variational" evolution (Mayr)

No two individuals are alike
Known to taxonomists and animal breeders

The better adapted forms increase in frequency in a population

Major points of Darwin's theory

Species are not constant
All organisms have descended from common ancestors
Evolution is gradual
Diversity is generated over time
Natural selection as mechanism

"Origin" published 1859, theory not universally accepted until 1940s.

Problem - Darwin had no idea of the mechanism of inheritance. Consequently

The relationship between natural selection and the effects of "use and disuse" were unclear.
Lack of particulate inheritance: Under Darwin's concept of inheritance, the effects of selection would be wiped out at the first mating.

Mendel - worked on peas, published in 1866, practically ignored

Superlative example of the hypothetico-deductive method.
Hypotheses stated in probablistic terms
Major points
- Particulate inheritance (not blending)
- Each individual carries a pair of hereditary factors for a given trait
- Gametes carry only one hereditary factor per trait
- Segregation and independent assortment

Mendelian genetics, rediscovered by Bateson, translated and published in 1902

Terms:

genome

genes

alleles

genotype

phenotype

ploidy - diploid, haploid

homozygous, heterozygous

dominant, recessive

F₁, F₂

Punnet square

Examples e.g.

Smooth, wrinkled (S, s): cross SS with ss

Cross F₁ from the above

Cross tall, smooth (TTSS) with short, wrinkled (ttss)

Cross F₁ from the above

Proportions of homozygotes (dominant and recessive) and heterozygotes in a population.

Hardy-Weinberg equation: p² + 2pq + q² gives proportions if no evolution is occurring

Factors that can distort the distribution away from Hardy-Weinberg equilbrium

Assortative mating
Mutation
Drift
Linkage
Selection

Mendelian concepts at first seen as alternative to natural selection - Bateson et al. not naturalists

T.H. Morgan (1932) possible mechanisms for evolution

Inheritance of acquired characters - rejected
Natural selection - to clean the genome of harmful mutations
Mutation - most important

Fundamentals worked out late 1920s-early 1930s by Chetverikov, Fisher, Haldane, Wright,

further developed by many others in 1940s - 1950s.

Next: Molecular genetics.

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Sources: Evolution (1977). Dobzhansky, Ayala, Stebbins, Valentine.

Mystery of Mysteries (1999) Ruse

What Evolution Is (2001) Mayr

and any good general biology book on Mendelian genetics.