Where we fit in the world and how we came to be here

Lamarck and Darwin

• Lamarck's Theory
  • Altered environment changes of phenotype
  • This happens DURING LIFE (after organism is born)
  • Change is passed on to its offspring
• Darwin's Theory (this is the accepted theory)
  • Phenotype randomly changes during sexual reproduction
    • Mutation and recombination of alleles changes genes/phenotype
    • This allows variation (in appearance, behaviour, ...) which is essential for natural selection
    • Variation may be beneficial or harmful
  • Resources (food, light) in the environment are limited
  • Beneficial variation allow adaptation to the environment
    • Organisms are more likely to make full use of resources (food, light)
    • Increased survival and reproduction
    • Successful alleles are passed on to their offspring
  • Harmful variation
    • Organisms have limited access to resources/too much competition
    • Shorter survival and reproduce less often
    • Phenotype is not passed on
    • Organisms with the harmful characteristic eventually die out
  • Organisms over-reproduce allowing more random changes/variation
    • Higher chance that a better phenotype will develop

Natural selection

• Responsible for development of new species from existing ones
• New environment + mutation → adaptation
  • Affects survival rate of phenotype before reproduction
    • Otherwise population would become extinct
  • Organisms better adapted survive, reproduce, and pass on their alleles/genes
  • Allele frequency of the advantageous gene increases
  • Changes frequencies of alleles in gene pool / phenotype in population
• Stabilising selection
  • Natural selection favours "average" organisms best adapted to that environment
  • Organisms with extreme forms of characteristics/mutations are selected against
  • Heaviest and lightest babies have highest mortality
  • Less likely to survive, reproduce, pass on their alleles
  • [Graph] Normal distribution curve with thinner bell-shaped curve
• Directional selection
  • Natural selection favours organisms with one extreme form of a characteristic
  • Pesticide resistance (warfarin - poison used to kill rats)
    • Resistant rats → require a lot of vitamin K → disadvantage / selected against → small population
    • New environmental effect: warfarin → kills normal rats
    • Resistant rats survive, reproduce, pass on resistance gene
    • New population forms by directional selection
  • Antibiotic resistance (penicillin resistance)
    • Resistant bacteria / unnecessary enzymes / selected against
    • New environmental factor: penicillin → kills normal bacteria
    • Resistant bacteria survive, reproduce, pass on resistance gene
  • [Graph] Bell-shaped curve shifts to the right

Speciation

• Splitting of one into more species / transformation of one into a new species over time
• Emigration/immigration moves alleles between populations
• Changes allele frequency by genetic variation in meiosis

Reproductive Isolation

• Premating
  • Habitat isolation: populations inhibit different local habitants within one environment
  • Temporal isolation: same environment but are reproductively active at different times
  • Behavioural isolation: two populations have different courtship patterns
  • Geographical separation: populations inhabit different islands, continents, ...
• Postmating
  • Gametes mortality: sperm cannot reach or fertilize egg
  • Zygote mortality: fertilisation occurs, but zygote fails to develop
  • Hybrid sterility: hybrid survives (viable) but is sterile and cannot reproduce (no meiosis)
  • Hybrid inviability: incomplete development

Allopatric Speciation (geographical isolation)

• Physical barrier (H2O, mountains, dessert) divides a population
• Two different environments (abiotic, biotic)
• Natural selection
• Genetic drift changes genotype and phenotype
• Two populations evolve separately
• Reproductively isolated / 2 distinct species

Sympatric Speciation (reproductive isolation)

• Genetic isolation by mutation / reproductively isolated / but inhibit same habitat
• Drift can cause further divergence between isolated gene pools
• Hybridisation in plants
  • Offspring produced from parents of two different species
  • Chromosomal number doubles / polyploidy
  • New species is reproductively isolated by a postmating mechanisms
• Can only reproduce with other polyploids, backcrosses with (2n) parents are sterile

Fossil dating

• Radioactive isotopes
  • Slowly decay over time
  • Measured by using its half-life
  • Not affected by temp, pressure, ...
• Stratigraphy
  • Fossil contained within a rock can be dated by assessing its number of layers
  • The older the rock, the more layers present, the older and deeper the fossil
  • Limitations
    • Earthquakes or volcanic eruptions may displace fossils within rocks
    • Fossil may be washed out and found at a different place
• Potassium-argon dating
  • Decays into 40Ar and 40Ca
  • Limitation: found in volcanic rocks only
• Carbon dating
  • Radioactive carbon (14C) is found in CO2
  • CO2 is used and stored by plants
  • Animals eat plants and store 14C in their body
  • Limitation: 14C is decayed after 50 000yrs - cannot be used in samples older than that