The evidence for natural selection

• natural selection - explains how the features of the environment apply a selective force on reproduction of individuals in a population
• Charles Darwin

  • not his idea for the theory of evolution, he proposed a mechanism (natural selection) for the process that one species might evolve from another
  • made it easier to believe in the theory of evolution however caused commotion in the Victorian era as it counteracted the religious beliefs
• Charles Darwin 2

  • during a five-year trip around the world, in a ship called HMS Beagle, he discovered a large number of unusual species in Galapagos Islands

  →however they were similar to the species in South America mainland :

  • noticed the clear variation between members of the same species found on different islands
  • noted that what appeared to be a wide variety of same species were actually all closely related finches; concluded that one species had arrived on islands from mainland then had evolved to form many different species
• Charles Darwin 3

  Darwin realised that variation was key to understanding how species change

  • he saw that when too many young are produced, there is competition for food + resources

  →as all offspring are different, some may be more adapted

  • the better adapted individuals obtain enough + survive long enough to reproduce

  →these individuals can pass their characteristics to next generation + therefore population can change or evolve to become better suited to its environment
• Alfred Russel Wallace

  another naturalist who independently came to same conclusion as Darwin

  • Wallace had made collection in both Amazon + South East Asia

  →first publication was joint papers on subject of evolution by natural selection + soon followed by The Origin Of Species by Darwin
• Darwin’s conclusions :

  • there’s a struggle to survive
  • better-adapted individuals survive + pass on their characteristics
  • over time, a number of changes may give rise to a new species
• Evidence for evolution (fossil evidence)

  known fossils showed that :

  • in past, the world was inhabited by species different from present ones
  • old species have died out, new ones arise
  • the new species that have appeared are often similar to older ones found in same place

  he said fossil species gave more rise to modern species (better-adapted)

  however he saw many differences such as many fossil species were more larger than modern species
• Biological Molecules

  • the fact that certain molecules are throughout living world is evidence itself

  → we can suggest all species came from an original ancestor (if one species gives rise to another, both are likely to have same biological molecules)

  • two closely related species will have separated only relatively recent - their biological molecules are likely to be identical or very similar
  • In species that took separate evolutionary paths a long time ago, the biological are likely to differ more
• Biological Molecules 2

  • evidence from molecules such as cytochrome c + other protein shows this pattern of changes

  Structure of DNA can be used in a similar way to that of cytochrome c; genes can be compared by sequencing the bases in the DNA. The greater the number of similarities between gene sequences, the more closely related the species + more recent their evolution. It also shows that more distantly related species have more differences in their DNA.
• evolution

  process by which organisms change over time
• Natural Selection

  organisms that are better adapted to their environment have a higher chance of surviving + reproducing, passing on their alleles to their offspring
• mechanisms of natural selections

  different characterstics due to mutation

  • Selection pressure

  advantageous as organism is more likely to survive + reproduce + pass on allele to offspring

  -> frequent allele over time
• Example Evolution of drug resistance

  1. genetic variation for drug resistance/ drug non-resistant bacteria in population
  2. drug resistant bacteria have a greater chance to survive + reproduce
  3. offspring inherits allele that codes for drug resistance (opposite for non-resistant bacteria)
  4. increased frequency of drug-resistant bacteria over time