GCSE AQA Biology 2 Fossils, Extinction & Speciation

GCSE AQA Biology 2 Fossils, Extinction & Speciation

1. Fossil formation

   • Fossils: the remains of organisms from many years ago, which are found in rocks
     • Provide evidence that organisms lived a long time ago
   • Formed in 3 ways:
     • From gradual replacement by minerals
       • Things like teeth, shells & bones, which don't decay easily, can last a long time when buried
         • 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
     • From casts & impressions
       • Organism is buried in soft material (like clay), clay hardens around it & organism decays, leaving cast behind
         • Burrows & roots can be preserved as casts
       • Footprints can be pressed into these materials when soft - leaving an impression when it hardens
     • From preservation in places where no decay happens
       • In amber & tar pits there's no oxygen/moisture
       • In glaciers it's too cold
       • In peat bogs it's too acidic

2. No one knows how life began

   • Fossils show how many species have evolved - but where did the first living thing come from....
     • There are various hypotheses - but no one knows
     • First life forms came into existence in primordial swamp? Simple organic molecules were brought to Earth on comets?
       • Can't be supported/disproved - lack of valid & reliable evide
     • Lack of evidence - early organisms were soft-bodied & soft tissue tends to decay away completely (fossil record is incomplete)
     • Fossils may have been destroyed by geological activity

3. Extinction

   • Species can become extinct if:
     • The environment changes too quickly
     • A new predator kills them all
     • A new disease kills them all
     • They can't compete with another (new) species for food
     • A catastrophic event happens that kills them all
     • A new species develops

4. Speciation

   • Species: group of similar organisms that can reproduce to give fertile offspring
   • Speciation: development of a new species
     • Occurs when populations of the same species become so different that they can no longer breed together to produce fertile offspring
   • 1) Species is isolated due to physical barrier
     • 2) Conditions on either side will be slightly different - different characteristics will become more common in each population due to natural selection...
       • 3) Each population shows genetic variation due to wide range of alleles
         • 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