GCSE AQA Biology 2 Fossils, Extinction & Speciation

Fossil formation
1. Fossils: the remains of organisms from many years ago, which are found in rocks
  1. Provide evidence that organisms lived a long time ago
2. Formed in 3 ways:
  1. From gradual replacement by minerals
    1. Things like teeth, shells & bones, which don't decay easily, can last a long time when buried
      1. Eventually they are replaced by minerals as they decay, forming a rock-like substance shaped like the original hard part
        Surrounding sediments also turn to rock but fossil stays distinct in rock until it's dug up
  2. From casts & impressions
    1. Organism is buried in soft material (like clay), clay hardens around it & organism decays, leaving cast behind
      1. Burrows & roots can be preserved as casts
    2. Footprints can be pressed into these materials when soft - leaving an impression when it hardens
  3. From preservation in places where no decay happens
    1. In amber & tar pits there's no oxygen/moisture
    2. In glaciers it's too cold
    3. In peat bogs it's too acidic
No one knows how life began
1. Fossils show how many species have evolved - but where did the first living thing come from....
  1. There are various hypotheses - but no one knows
  2. First life forms came into existence in primordial swamp? Simple organic molecules were brought to Earth on comets?
    1. Can't be supported/disproved - lack of valid & reliable evide
  3. Lack of evidence - early organisms were soft-bodied & soft tissue tends to decay away completely (fossil record is incomplete)
  4. Fossils may have been destroyed by geological activity
Extinction
1. Species can become extinct if:
  1. The environment changes too quickly
  2. A new predator kills them all
  3. A new disease kills them all
  4. They can't compete with another (new) species for food
  5. A catastrophic event happens that kills them all
  6. A new species develops
Speciation
1. Species: group of similar organisms that can reproduce to give fertile offspring
2. Speciation: development of a new species
  1. Occurs when populations of the same species become so different that they can no longer breed together to produce fertile offspring
3. 1) Species is isolated due to physical barrier
  1. 2) Conditions on either side will be slightly different - different characteristics will become more common in each population due to natural selection...
    1. 3) Each population shows genetic variation due to wide range of alleles
      1. 4) In each population, individuals with characteristics that make them better adapted to their environment have a better chance of survival & are more likely to breed successfully
        5) Alleles that control beneficial characteristics are more likely to be passed onto the next generation
        6) Eventually, individuals from different populations will become so different that they won't be able to breed with one another to produce fertile offspring - 2 groups are now 2 separate species

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GCSE AQA Biology 2 Fossils, Extinction & Speciation

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