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NATURAL SELECTION

NATURAL SELECTION. The great biological paradigm. © 2008 Paul Billiet ODWS. Observation 1: Exponential growth. Populations tend to produce more offspring than the environment can support. © 2008 Paul Billiet ODWS. Thomas Malthus. Essay on the Principle of Population (1798)

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NATURAL SELECTION

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  1. NATURAL SELECTION The great biological paradigm © 2008 Paul Billiet ODWS

  2. Observation 1: Exponential growth Populations tend to produce more offspring than the environment can support © 2008 Paul Billiet ODWS

  3. Thomas Malthus • Essay on the Principle of Population (1798) • Populations in nature cannot continually increase sooner or later food supply is insufficient and famine stops further growth • Both Charles Darwin and Alfred Russel Wallace had read Malthus and understood the idea of exponential population growth © 2008 Paul Billiet ODWS

  4. Population Growth • 1 pair of cockroaches could produce 164 000 million in 7 months Cockroach population © 2008 Paul Billiet ODWS

  5. Observation 2: Zero growth The numbers of individuals in a population remain stable In terms of population growth the population at its carrying capacity has zero growth © 2008 Paul Billiet ODWS

  6. Numbers Time Population growth K 3 2 1 © 2008 Paul Billiet ODWS

  7. Deduction 1 Competition There must be a struggle for survival Some of the offspring produced in a generation do not survive. Darwin identified competition as a major factor limiting population sizes © 2008 Paul Billiet ODWS

  8. Observation 3: Variation • Some of these variations are inherited by the offspring • The mechanism of the inheritance of genes was being worked out at this time remained undiscovered by biologists until 1900 • Darwin was however aware that sexual reproduction mixes variations to produce new combinations (recombinants) © 2008 Paul Billiet ODWS

  9. Neo-Darwinism • The great synthesis of the 20th century • Mendel’s work was able to explain some of the patterns of inheritance through the mixing that occurs during meiosisand fertilisation • Darwin could not explain the origin of new variants • This had to wait until the 1920s and 1930s when work began on mutationsafter the discovery of radiation © 2008 Paul Billiet ODWS

  10. Deduction 2: Survival of the fittest • There will be a struggle for survival between the members of the population • Individuals with advantageous variations will breed and produce more offspring © 2008 Paul Billiet ODWS

  11. Natural selection in action • As generations pass by, the proportions of the alleles for the different variants will change in favour of those that provide the best adaptations • Natural selection has been observed at work in populations of species over the past century • Examples include: pesticide resistance in insects, antibiotic resistance in bacteria, industrial melanism in moths, tolerance to heavy metals in plants © 2008 Paul Billiet ODWS

  12. The Origin of Species by Natural Selection • Darwin and Wallace argued that if natural selection proceeded for a long enough period of time it could bring about the evolution of new species • Darwin himself favoured a long period of slow changes • Recently this has been refined to include the possibility of rapid changes over a short period of time (punctuated equilibrium) © 2008 Paul Billiet ODWS

  13. Natural selection is not the only way • Whether fast or slow, observing the evolution of a new species is unlikely in the lifetime of a scientist • That species evolve is a fact but that they evolve by natural selection remains a theory • Other mechanisms exist that can also lead to the evolution of species (e.g. genetic drift) © 2008 Paul Billiet ODWS

  14. Thylacine Thylacinus cynocephalus Wolf Canis lupus Convergent Evolution • Species exposed to the same selective pressure in different parts of the world tend to develop the same adaptations • Even though they may be completely unrelated • e.g. the placental wolf and the marsupial thylacine or Tasmanian wolf © 2008 Paul Billiet ODWS

  15. Divergent Evolution • Populations of a species that are separated and evolve under different selective pressures develop different adaptations as they diverge • They are usually geographically separated so that there is a barrier to the mixing of genes © 2008 Paul Billiet ODWS

  16. Adaptive radiation Species that populate a new area where there are vacant niches will diverge and specialise as they fill the vacant niches (e.g. Darwin’s Finches on the Galapagos) © 2008 Paul Billiet ODWS

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