Evidence for evolution

• Evolution is the theory that describes the way in which organisms evolve, or change, over many years as a result of natural selection
• Charles Darwin realised that organisms best suited to their environment are more likely to survive and reproduce, passing on their characteristics to their offspring
• A species gradually changes over time to have a more advantageous phenotype for the environment in which it lives. We now know advantageous characteristics are passed on from on generation to the next by genes in DNA molecules
• Darwin was born in 1809. At this time, most people in Europe believed God directly created all life on Earth the way it is now. They believed this ocurred only a few thousand years before.
• In 1831 on the HMS Beagle, Darwin read 'Principles of Geology' written by his friend Charles Lyell. In the book he suggested fossils were evidence of animals that lived millions of years ago.
• Darwins observations on finches in the Galapagos Islands:

  • different islands had different finches
  • the birds were similar in many ways so must be closely related but their beaks and claws were different sizes
  • the design of the beaks was linked to the foods available on each island
  • he concluded a bird born with a beak more suited to the food available would survive longer so have more offspring passing on its characteristic beak
  • over time the finch population of the island will all share this characteristic
• At the same time as Darwin, Alfred Wallace was working on his own theory of evolution in Borneo. He sent his ideas to Darwin for a peer review. Their ideas were so similar that they proposed the theory of evolution through a joint presentation of two scientific papers to the Linnean Society of London
• Sources used to study the process of evolution:

  • Palaeontology = the study of fossils and the fossil record
  • Comparative anatomy = the study of similarities and differences between organisms' anatomy
  • Comparative biochemistry = similarities and differences between the chemical make up of organisms
• Fossils are formed when animal and plant remains are preserved on rocks
• Over long periods of time, sediment is deposited on the earth to form layers (strata) of rock. Different layers correspond to different geological eras
• Within the different rock strata, the fossils are different, forming a sequence from oldest to youngest which shows organisms have changed over time. This is known as the fossil reccord
• Fossils of the simplest organisms like bacteria and simple algae are found in the oldest rocks whilst fossils of more complex organisms are found in more recent rocks. This supports the evolutionary theory that simple life forms gradually evolved over a very long time to become complex ones
• The sequence in which organisms are found matches their ecological links to each other.

  E.g. plant fossils appear before animal fossils. This is consistent with the fact that animals require plants to survive
• By studying similarities in the anatomy of fossil organisms, scientists can show how closely related organisms have evolved from the same ancestor
• Fossils allow relationships between extinct and living organisms to be investigated
• The fossil record is not complete:

  • many organisms are soft bodied and decompose quickly before they have a chance to fossilise
  • The conditions for fossils to form are not often present
  • many other fossils have been destroyed by the Earth's movements, like volcanoes, or still lie undiscovered
• A homologous structure is a structure that appears superficially different (and may perform different functions) in different organisms, but has the same underlying structure
• Vertebrate limbs are used for a wide range of functions including running, jumping, and flying.
• You expect the bone structure of the limbs in flying vertebrates to be very different to walking and swimming vertebrates but they are actually very similar. The same bones are adapted to carry out the whole range of different functions. An explanation is that all vertebrates have evolved from a common ancestor so vertebrate limbs have all evolved from the same structure
• The presence of homologous structures provides evidence for divergent evolution
• Divergent evolution describes how, from a common ancestor, different species have evolved, each with a different set of adaptive features
• Divergent evolution will occur when closely related species diversify to adapt to new habitats as a result of migration or loss of habitat
• Comparative biochemistry = the study of similarities and differences in the proteins and other molecules that control life proceses
• Although proteins and other molecules can change over time, some important molecules are highly conserved among species. Slight changes that occur in these molecules can help identify evolutionary links. Two of the most common molecules studied are cytochrome c, a protein involved in respiration, and ribosomal RNA
• The hypothesis of natural evolution states that most of the variability in the structure of a molecule doesn't affect its function. Most variability occurs outside of the molecule's functional regions. Changes that don't affect molecule's function is called neutral. Their accumulation is not affected by natural selection so neutral substitutions occur at a fairly regular rate (different rate for different molecules)
• To discover how closely two species are related:

  • the molecular sequence is compared (looking at the order of DNA bases or at the order of amino acids in a protein)
  • the number of differences are plotted against the rate the molecule undergoes neutral base pair substitutions
  • from this info the point at which the two species last shared a common ancestor can be estimated
• Species that are closely related have more similar DNA and proteins. Those who are distantly related have far fewer similarities
• Ribosomal RNA has a very slow rate of substitution so it is commonly used together with fossil information to determine relationships between ancient species