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Darwin’s Theory

Darwin’s Theory. Evolution by Natural Selection. More organisms are born than can survive…. This hypothetical population of grasshoppers changes over time as a result of natural selection.

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Darwin’s Theory

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  1. Darwin’s Theory Evolution by Natural Selection

  2. More organisms are born than can survive…. • This hypothetical population of grasshoppers changes over time as a result of natural selection. • Grasshoppers can lay more than 200 eggs at a time, but only a small fraction of these offspring survive to reproduce.

  3. There are variations among organisms…. • In this population of grasshoppers, heritable variation includes yellow and green body color. • The variations already exist – they do not start existing because they provide an advantage

  4. Where do the variations come from? • Ultimately – all variations arise from random mutations. • An organism can not direct the mutations or will mutations into existence. • The mutations must already exist before environmental pressures (selective pressures) can cause one to become an adaptation.

  5. Organisms with favorable variations (characteristics) are selected for and will tend to leave more offspring • The green grasshoppers blend into their environment and so are less visible to predators. • Adaptation are refering to variations that can increase an individual’s chances of surviving and reproducing. • In this case, Green color is an adaptation.

  6. Organisms that survive longer will pass on their favorable variations to their offspring • Because their color serves as a camouflage adaptation, green grasshoppers survive and reproduce more often than yellow grasshoppers do. • Green grasshoppers have higher fitness.

  7. Survival of the fittest? • The ultimate goal is to survive long enough to reproduce and pass on your genes. This means that your genes will stay in the population. • Even if an organism is strong and can escape from predators – their fitness is zero if they do not survive OR do not reproduce

  8. Over time, the collective variations can cause a population of organisms to change (evolve) • Green grasshoppers become more common than yellow grasshoppers in this population over time.

  9. Natural Selection in Action: Pesticide Resistance

  10. Natural Selection in Action: Antibiotic Resistance

  11. Natural Selection in Action: Industrial Melanism

  12. Natural Selection in Action: Darwin’s Finches

  13. Myths about Natural Selection….

  14. Myth 1: Organisms can become adapted to a change in the environment. • If the variation does not already exist it is not going to become existent just because there is a need for it. Variations come from random mutations. Example: A rat can not become resistant to a pesticides just because they are used. The rat would have to be born with a variation that allows it to resist the pesticides.

  15. Myth 2: An individual can evolve • Groups, populations and species can evolve • An evolutionary change only happens when there is a shift in the genes of a population • An individuals genes can not change • Those with genes that allow them to survive more – reproduce more • As a result the next generation will shift to show more members of the populationwith those adaptive genes

  16. Myth 3: Evolution progresses to make more complex individuals • Just because an organism is simple does not mean it is not well adapted to its environment • Bacteria, insects and rodents are really good at surviving and reproducing

  17. Sometimes organisms will become simplified Organisms will branch out/evolve to fill empty ecological niches – not necessarily to make them more complex

  18. Directions of Natural Selection

  19. Myth 4: Evolution produces perfect organisms • Evolution can only work with the variations that already exist – this does not mean that the traits that come into existence through mutation are the perfect traits

  20. Myth 5: An organism can become perfectly adapted to its environment • If the environment changes – then there are new selective pressures. • Environmental changes can include: climate change, temperature change, drought, extra rain, pollutions, toxins, volcanoes, earthquakes, introduction of predators or competitors, extinction of species, introduction of diseases/parasites • What was once a positive trait may because detrimental under different environmental conditions

  21. Did Darwin get it right? • Darwin thought that natural selection would create a gradual change in living things

  22. New ideas…..punctuated equlibrium • It is now commonly believed that evolutionary change comes in spurts of rapid change followed by periods of relative inactivity. • This would make sense if the environment changed and then stabilized • This also explains the close juxtaposition of two fossil forms in sedimentary rock without fossilized intermediates.

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