Notes for Lecture #3:

Darwin's theory of evolution by natural selection

Natural Selection

• Populations show variation for almost all traits.
• Certain variations improve survivorship and the number of offspring left (improve fitness ).
• If these variations are heritable, their frequency increases each generation.
• These three steps are often called Survival of the Fittest.

Darwin's mechanism of evolution

• Modification occurs by natural selection.
• Hence, Darwin's theory is called the theory of evolution by natural selection.

Key figures influencing Darwin

• Erasmus Darwin
• Hutton and Lyell
• Malthus

Thomas Malthus

• In 1798 wrote Essay on the Principles of Population
• Populations tend to outgrow their resources, thus strong competition among individuals is expected.

Lyell and Hutton

• Charles Lyell's Principles of Geology (1830) was read by Darwin during the voyage of the Beagle .
• Lyell's expanded upon Hutton's notion of Uniformitarianism.
• Processes shaping the Earth today are the same as those that have shaped it in the past.
• Evidence that the earth was very old.

Wallace and Darwin

• It took Darwin 20-25 years to come up with his theory.
• Alfred Russell Wallace (during a two-week bout with malaria) independently came up with the same theory
• Both Wallace and Darwin presented joint papers on natural selection on 1 July 1858 at the Linnean Society of London.
• in 1859 Darwin published the first edition of The Origin of Species by Means of Natural Selection

Examples of the explanatory powers of Darwin's theory

• Drug resistance in bacteria.
• Pesticide resistance in insects.
• Sickle cell anemia
• Rabbits and myxomatosis.
• Industrial melanism

Sickle cell anemia and malaria --- how can something bad be good?

• Sickle-cell allele (S ) is a defect in the beta hemoglobin chain resulting from a single animo acid change.

• hemoglobins have two pairs of chains , alpha and beta
  • Sickle allele, S, is a change in the beta chain (146 aa)
  • glutamic acid at position 6 in normal beta replaced by valine
• S S homozygotes die, yet the sickle-cell allele is widespread throughout the world

• Key: the sickle-cell heterozygote is resistant to malaria
  • The frequency of the S allele closely matches the world-wide distribution of malaria.

• In 1859, 12 cute and cuddly rabbits brought to Australia.
• by 1886, the army of rabbits was advancing at over 66 miles a year, smashing through Òrabbit-proofÓ barriers. They spanned Australia by 1907.
• Myxomatosis introduced in 1950, with an initial mortality rate of 99.9%
• Mortality rate today is about 40%

• Dark (melanic) forms of the peppered moth increased in frequency in Southern England as air pollution (soot) darkened trees.
• As pollution controls were put in place, the light form of the moth increased in frequency.