Organims 2

• • The earth formed approx. 4.6 billion years ago.
• • Early earth had no atmosphere, soil, or oceans
• • Prokaryotic life is thought to have formed approx. 4 billion years ago
• First fossils appear:• Prokaryote: 3.5 billion years ago• Single-celled eukaryotes: 1.8 billion years ago • Multicellular eukaryotes: 1.3 billion years ago • Animals: 0.6 billion years ago
• • Alexander Oparin and J.B.S. Haldane (1920s)
  • “Primordial soup theory”
  • Compounds characteristic of life formed slowly over time from simple molecules present in the prebiotic environment
• Oparin-Haldane hypothesis: Early atmosphere consisted of simple compounds (list them)

  • With an energy source, these compounds came together to form more complex organic molecules
• • Stanley Miller and Harold Urey
  • Circulated a mixture of; water, hydrogen, methane and ammonia.
  • Energy source: electrical spark
  • Results: In a week, 15% of the carbon in the mixture was converted to ‘organic’ compounds such as: amino acids, urea, fatty acids
• Simple inorganic molecules formed and began to accumulate in the early oceans. Over time:

  1. Abiotic synthesis of small organic molecules
  2. Formation of macromolecules (e.g. proteins, nucleic acids)
  3. Packaging of macromolecules into membranes to form protocells
  4. Origin of self-replicating molecules and true cells
• List the simple compounds the early atmosphere consisted of.

  Water vapour, hydrogen gas, ammonia, carbon dioxide, methane and limited free oxygen
• Could life on earth start again?

  • Probably not...

  • Early atmosphere was strongly reducing (lacked oxygen gas)

  • These conditions were conducive to prebiotic synthesis of life 

  • Present atmosphere is strongly oxidizing

  • Molecules necessary for life can’t be synthesized outside of cells (unstable in the presence of oxygen).
• Chemical Evolution

  • Formation of complex organic molecules from simpler inorganic molecules through chemical reactions (abiotic)

  • First step in the development of life on this planet.
• Organic Evolution

  • The process by which changes in the genetic composition of populations of organisms occur in response to environmental changes

  • Descent with modification
• • Darwin and Wallace independently conceived of the idea of natural selection
  • Foundation for this theory was already in place: • Geology: History of life on earth is long and changing • Economics: population pressures • Embryology: similarities among organisms
  • Evolution as a theory was accepted before Natural Selection (i.e. the mechanism)
• Five theories summarize Darwin’s views:

  1. Perpetual Change
  2. Common Descent
  3. Multiplication of Species
  4. Gradualism
  5. Natural Selection
• Which of the 5 theories summarizing Darwin's view is accepted as having universal application throughout the living world?

  Perpetual change, common descent and multiplication of species
• Which of the 5 theories summarizinf Darwin's view have some aspects that are unresolved?

  Gradualism and Natural Selection
• 1. Perpetual Change

  • Living world is always changing

  • Evidence: fossil record

  • However, fossil record is biased:

  -- Hard structures (skeletons, shells, teeth) are preserved best

  -- Soft-bodied animals such as jellyfish and worms are under-represented
• Common Descent

  • All organisms are descended from a common ancestor
  •  Last Universal Common Ancestor (LUCA)
  • Evidence: organismal form, cellular structure, and genetics
  • Life’s history forms a branching tree called a phylogeny
• Evidence for Common Descent

  Shared characteristics of living things

  1. Chemical uniqueness
  2. Complexity and hierarchical organization
  3. Reproduction
  4. Possession of genetic program (Conserved genetics --> e.g. codon usage)
  5. Metabolism
  6. Development
  7. Environmental interaction
  8. Mouvement
• Possession of a Genetic Program

  • Genetic code: correspondence between sequence of nucleotides in DNA and sequences of amino acids in protein

  • Codon: a sequence of three nucleotides that encode for an amino acid

  • Arose early in the history of life
• More Evidence for Common Descent

  • Homology

  • Homology = Similarity of parts or organs of different organisms caused by evolutionary derivation from a corresponding part or organ in a common ancestor
  • Homologous features are transmitted to all descendant lineages (unless they are subsequently lost)
• Multiplication of Species

  • Evolution produces new species by splitting and transforming older ones
  • New species form through the appearance of reproductive barriers
  • Biological factors that prevent interbreeding (i.e. morphological, physiological, ecological, and behavioral factors)
  • Often arise because of geographical barriers
• Patterns in Macroevolution (Gradualism)

  Gradualism (Darwin)

  • Small changes accumulate steadily over time

  • Sudden changes are more likely to have negative side-effects

  • However, gradualism is not always supported by the fossil record
• Patterns in Macroevolution (Gradualism)

  Punctuated Equilibrium - Niles Eldridge, Stephen Jay Gould (1977)

  • Long periods of stasis (equilibrium), punctuated by brief events of speciation
  • Speciation: lasts on average 10 000- 100 000 years
  • Species survive for 5-10 million years on average
  • Proposed in response to lack of evidence for Gradualism
  • Animal breeding also suggests that changes can be substantial and sudden (not gradual)
• Stasis

  • Some lineages don’t change much, even over millions of years (‘Living Fossils’)
  • E.g. Coelacanth: 80 million years of morphological stasis
• Extinction

  • Frequent or rare within a lineage
  • Can occur simultaneously across many lineages(‘mass extinction’)
  • Two possible evolutionary fates for every species: 
  • become extinct.
  • give rise to new species
  • Over 99% of the species that have ever lived on Earth are extinct!
• Natural Selection

  • Natural selection: the process whereby organisms better adapted to their environment tend to survive and produce more offspring.

  • Natural selection explains adaptation

  • Adaptation = a change or the process of change by which an organism or species becomes better suited to its environment.
• Natural Selection 101

  • There is variation in traits
  • There is differential reproduction
  • There is heredity
  • More advantageous traits become more common in a population
• Life: Bacteria, Archaea, Eukaryota
• Eukaryota: Plantae, Protista, Unikonta
• Unikonta: Ameobozoa, Opisthokonta
• Opisthokonta = Fungi + choanoflagellates + Metazoa
• Metazoa = Porifera + Cnidaria + Ctenophora + Platyhelminthes + Annelida + Mollusca + Nematoda + Arthropoda + Echinodermata + Hemichordata + Chordata + (24 others)
• • Porifera: sponges
  • Cnidaria: jellyfish, anemones, corals
  • Ctenophora: comb jellies
  • Platyhelminthes: flatworms, tapeworms, flukes
  • Annelida: marine worms, earthworms, leeches
  • Mollusca: snails, clams, octopuses, squid
  • Nematoda: round worms
  • Arthropoda: Lobsters, crabs, spiders, insects centipedes, millipedes
  • Echinodermata: sea stars, sea urchins, sea cucumbers
  • Hemichordata: acorn worms
  • Chordata: vertebrates
• Origins of Life

  • Next stage – formation of polymers • (e.g. proteins, nucleic acids)

  • Probably within semipermeable amphiphilic membranes

  • • Why? Semipermeable membranes could provide protection

  In aqueous solution, polymers would tend to be hydrolysed
• Common Misconceptions about Natural Selection

  • Does ‘survival of the fittest’ mean survival of the strongest?

  • Genes from the individuals that leave the most offspring will survive
  • Survival of the most reproductively fit (not strongest)
• Common Misconceptions about Natural Selection: Is Evolution random? Is it driven by random forces? No! Genetic variability in populations has a random aspect to it because it is created by mutations. Mutations are random. The force that drives evolution is natural selection and it is not random
• Common Misconceptions about Natural Selection

  Are mutations favourable or unfavourable?

  • Both. Most mutations are unfavourable or neutral. But some mutations confer favourable phenotypes. Genetic variability within a population is also favourable because it allows a population to respond to environmental change.
• Can every feature be explained as an adaptation?

  • No Features can be 1) the result of history (e.g. the amino acid code), 2) just a by-product (e.g. the colour of blood), or 3) an outdated adaptation (e.g. relationship with extinct organism)
• How do complex traits evolve?

  1. Advantageous Intermediates

  Intermediate step might be advantageous in itself