Darwinian Concepts and Natural Selection

Species definition

"A species are a group of interbreeading natural populations that are reproductively isolated from other such groups."

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William Paley, 1802

  • All living things must have been designed.
  • Complicated objects must have been designed and cannot arise by accident - watchmaker analogy. 

Erasmus Darwin - Evolution was life 'striving for improvement'. 

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  • An ancestral giraffe stretched its neck, as it reached more food it left more young who inherited stretched neck.
  • Cells GAIN information in lifetime which is passed onto offspring.
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Darwins Findings

  • Individuals within a population show variation.
  • This variation can be inherited.
  • Organisms produce more offspring than can survive.
  • Those organisms with variations that are favourable are more likely to leave offspring.
  • Differential survival results in a shift of characters.
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  • Variation - shape, size, colour, behaviour ect. 
  • Vary in both qualitative and quantitative ways.
  • Quantative variation often follows a normal distribution.
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  • Used studies of embryology to illustrate common descent. 
  • Solution to the problem of inheritance - Mendel showed that traits were inherited as discrete characters that do not blend between generations.
  • August Weismann - The factors controlling inheritance lay in special 'germ cells' (germ plasm theory). Thus inheritance lies within the egg and sperm cells only - other cells do not function as agents of heredity. 
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Marie Curie

  • Discovered radioactive elements 
  • Solution to variation - Thomas Hunt Morgan - fruit flie - began to understand the nature of genetic inheritance - he suggested that genes mixed during recombination in meiosis. 
  • Muller (a student of Morgans) showed that new mutations could be made to arise at a fast rate with the application of radiation. 
  • THis explains natural selection as changes in the frequency of genes within a population - population genetics.
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Macroevolution - Large effects over a long period of time. eg. evolution of the vertebrate eye or horse hoof.

Microevolution - Small effects on a species over a short period of time e.g prevalence of sickle cell trait in people from West africa. 

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Differential survival results in a shift of charac

The Peppered Moth

  • Pollution levels in industrial revolution reduced the amounts of lichen on tree trunks and darkened them with soot.
  • Moths began to turn black 'morphs' arising due to melanism.
  • Natural selection due to environment - increased fitness.
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The ability of an individual to contribute to the next generation.

The individuals with traits that are favoured by natural selection will leave more offspring so they are said to have a high degree of fitness. 

REMEMBER - compare fitness of individuals within a species to another e.g gazelle to gazelle, not gazelle to cheetah.

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Mosquitos, Malaria and DDT 

  • No individual insect becomes resistant
  • Number of resistant insects increases within a population.
  • Use of DDT creates a different selection pressure.
  • The population shows adaptation to this selective force.
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What is adaptation?

The process of how a population fits the environment in which it exsists. 

Environment e.g temperature, rainfall, competitors, predators and prey.

Speciation - adaptation to a changing environment.

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