C8

• STEPS OF FOSSILISATION

  1. The organism dies 
  2. Organism is rapidly buried with sediment 
  3. If conditions are suitable, the remains decay slowly (e.g. no scavengers, no oxygen) 
  4. Pressure compresses sedimentary layers to form sedimentary rock and the organism is preserved as a fossil 
  5. Erosion of sediments exposes fossil on the surface 
• TYPES OF FOSSIL

  Physical fossils (direct evidence) 

  • The remains of all  or part of the structure of organisms 
  • Impression fossils (moulds and casts) 
  • Mineral fossils? 
  • Mummified fossils? 

  Trace fossils (indirect evidence) 

  • Preserved biological activity of organism 
  • Faces, footprints, Eggs 

  Biosignatures (inferred evidence) 

  • Biomarkers that provide evidence of inferred past life existence 
•  Moulds and casts: These occur when sediments harden around a decaying organism .An impression or mould is made. This space fills with sediments that then harden. The result is a cast of the organism.
•  Trace fossils: Trace fossils are the evidence of an organism, rather than the organism itself. This includes items such as footprints, eggshells and dung.
•  Petrified fossils: The organic remains of an organism have been replaced with minerals and have a stone like appearance.
• True form fossils (body fossils): These are the original remains of the organism that have been preserved in amber, tar or ice.
•  Films: A film is the outline or indentation of an organism. The organic material has been compressed leaving only an outline.
• Transitional fossils: A transitional fossil is any fossilised remains of a life form that exhibits traits common to both an ancestral group and its derived descendant group
• Index fossils 

  • A fossil of known age is used to correlate strata -> determines relative age of fossils 
  •  Strata is a layer of rock 
  • Distinctive  
  • Abundant 
  • Wide geographic distribution 
  • Existed for a short geological timeframe 

  A) index fossil

  
• Transitional fossils 

  • An intermediate form of fossil between one group of organisms and another (ancestor + descendants/modern species) 
  • Archaeopteryx is a species  that has both reptilian and avian (bird) features  
  • Evident to have evolved from a reptile (ancestor) to a bird 
• Absolute dating 

  • How old a fossil is
  • Provide more precise age estimate of a fossil through radiometric techniques
  • Half life -> time it takes for half of atoms in an isotope to decay to its radioisotope 

  
• Carbon dating 

  • Used for recent fossils up to 50,000 - 70,000 years old 
  • Human and animal.

  1. An organism has both carbon-12 and carbon-14 
  2. Carbon-14 turns back into nitrogen and Carbon-12 does not 
  3. Compare the ratio of remaining carbon-14 to carbon-12 -> determines how much carbon-14 has decayed into nitrogen 
  4. Use half life of carbon-14 to determine how long it took for carbon-14 decay into nitrogen 
• Potassium-Argon dating 

  • Used when fossil (only igneous rocks of surrounding or adjacent strata to strata of fossil) is older than 50,000 - 70,000 years old  
  • Minerals and rocks

  1. Potassium decays into argon 
  2. Compare the amount of potassium 40 to the stable potassium isotope and use half-life to determine absolute age 

   
• SPECIES

  • Group of genetically similar individuals that produce fertile, viable offspring when interbreeding 
  • Species only exist in a population (look at definition) and in isolated gene pools from gene pools from other species 
• REPRODUCTIVE ISOLATING METHODS

  • Prevents individuals (species) from different populations from interbreeding 

  1. Prezygotic -> isolation prior fertilisation/prevents mating 
  2. Postzygotic -> isolation post fertilisation 
• Prezygotic isolation type 

  Description 

  Temporal 

  • Breeding cycles/active population times are not concurrent 
  • E.g. nocturnal and diurnal species + flowering periods 

  Geographic 

  • Physical locations creates barriers between species 
  • E.g. Mountains, rivers, valleys 

  Behavioural 

  • Sexual signals/rituals are not interpreted by each species 
  • Does not elicit sexual activity 

  Mechanical 

  • Incompatible physical traits -> sexual organs 
  • E.g. Bird and mouse 
• Postzygotic isolation type 

  Description 

  Incompatible gametes 

  • Sperm is unable to interact with eggs of other species 
  • Fertilisation does not occur 

  Zygote mortality 

  • Fertilisation occurs but zygote fails to develop 

  Non-viable zygotes 

  • Zygote is only able to develop into embryo  
  • Cannot develop beyond embryo 

  Sterile of hybrid zygotes 

  • Hybrid species survive and cannot produce offspring 
• ALLOPATRIC

  • Geographic barrier
  • Can form different species

  Steps:

  1. Moving into new environments
  2. Geographical isolation
  3. Formation of sub-species
  4. Reproductive isolation
•  

  SYMPATRIC

  • Same area
  • Different things isolating

  Prezygotic barrier

  • Different behaviours
  • Temporal
  • Habitat

  Postzygotic barriers

  • After fertilisation
  •  unable to reproduce
  • Survival rate low

   
• Carbon dating 

  • Used for recent fossils up to 50,000 - 70,000 years old 
• Potassium-Argon dating 

  • Used when fossil (only igneous rocks of surrounding or adjacent strata to strata of fossil) is older than 50,000 - 70,000 years old  
• Allopatric speciation of Galapagos finches

  1. Ocean barrier separates finch populations, preventing gene flow.
  2. Different environments create distinct selection pressures (e.g., food types).
  3. Certain beak shapes provide a selective advantage (e.g., large beaks for nuts, thin beaks for cactus).
  4. Advantageous traits are passed on to offspring.
  5. Over time, genetic divergence occurs due to changes in allele frequencies and mutations.
  6. If the barrier is removed, populations cannot interbreed, leading to reproductive isolation.
  7. Isolated populations become separate species.
• Steps to Sympatric Speciation

  1. Temporal or behavioral isolation separates populations with some gene flow.

  2. Different selection pressures act on isolated populations.

  3. Certain phenotypes gain a selective advantage.

  4. Advantageous traits are passed on to offspring.

  5. Genetic divergence occurs over time due to changes in allele frequencies and mutations.

  6. Populations become reproductively isolated and cannot interbreed, forming separate species.
• carbon half-life: 5730