Evol Lecture 7- Macroevolution

Evol Lecture 7- Macroevolution

1. The fossil record

   • Fossils = living organisms + dead organisms = better understanding of the past
     • Preserved remnants (traces) of past life
       • Mineral components
       • Petrified organic material
       • Casts
       • Trace fossils (footprints etc)
     • More direct view of evolutionary history than from living organisms
       • Past ecosystems, climate, sea levels
   • Take gene sequences of animals and examine their differences
     • Comparing anatomical differences
   • Sedimentary rock
     • Particles suspended in water, heat + pressure form layers into rock
   • Dating the fossil record
     • Relative dating
       • Oder of appearance in strata
         • Without actually putting time on them
       • 'Index fossils' to compare locations
         • Produced really commonly
           • e.g. common shelf producing marine species have really wide distributions
     • Absolute dating
       • Putting actual # and time
       • Younger technique
       • Relies on isotopes (isotopic decay)
         • Radiometric methods
           • Potassium-40
           • Argon-40
       • Volcanic, not sedimentary rock

2. Mass extinctions and adaptive radiations

   • Many species extinct in very short time
   • Causes: large environmental changes
     • massive volcanic activity
     • impact by asteroid/comet
     • changes in ocean circulation patterns
   • Major importance in the history of life
   • 5 big ones, mostly marking ends of Eras/periods
     • 1st: 450 million yrs ago
     • 252 million yrs ago
       • biggest mass extinction
         • boundary between paleozoic and mesozoic era
           • end-permian
             • extinction of 90% species on earth
             • many large taxa went totally extinct
               • 50% of families
             • volcanic activity
     • most recent: end-cretaceous
       • due to asteroid dust
       • ~50% of species on earth
         • e.g. dinosaurs other than birds
         • several marine invertebrate groups
   • Adaptive radiations
     • Rapid speciation and evolutionary change in UNDEREXPLOITED habitats
     • Regional
       • colonization of new island chains
         • e.g. Hawaiian "Silversword alliance"
           • plant group, ENDEMIC to Hawaii
             • very isolated, 'young' island group
           • ~50 species
             • wide variety of morphologies
           • All descended from single WEEDY species in last ~5 million years
     • Worldwide
       • following mass extinction events
         • e.g. of mammals after end-cretaceous extinction
           • 65 MYA: extinction of dinosaurs
           • large land animals (dinosaurs) and mammals all small-medium sized --> large land animals, almost all mammals
       • replacement in fossil record

3. Evolution of complex adaptations

   • How do these complex adaptations evolve?
     • Darwinian evolution proposal
       • Evolution through many small steps
         • Every step should improve fitness
     • Camera eye
       • Convergent evolution
       • Adaptive evolution
     • Intermediates that are actually capable of functioning
       • Simpler forms of complex structures ARE functional
       • Evidence: organs of diff. complexity in related species
       • Step-wise evolution plausible
     • Modification of existing structures with different functions
       • Exaptation
         • Structures adapted for one function, are coincidentally useful for another function
         • e.g. powered flight in birds
           • Asymmetric contour feathers
           • Honeycomb bones
             • Hollow central shaft with "barbs"
           • Birds share ancestry with dinosaurs
             • Some dinosaurs had feathers, almost all couldn't fly
               • Feathers were present as insulation and for display
                 • A feather that is used for insulation/display is MUCH simpler than a feather used for flying, but it's a LOT easier for it to adapt into something useful for flight because it's already useful for something else
       • Evolution gets a "head-start" on making a complex adaptation... a fairly complexstructure is an adaptation for one function and somewhat beneficial for another function, and continues to adapt to be beneficial for both functions.
     • Larger steps than imagined by Darwin
       • Changes in developmental regulation
         • Development involves "cascades" of genes
           • There are some genes that tell other genes what to do in development, and those genes tell other genes what to do as well (gene messengers)
         • Small genetic change may result in large, coordinated changes in phenotype
           • Small changes in the protein sequences that they specify could result in large changes of how organisms develop
         • Mutations affecting genes that control development
         • e.g. Homeotic genes
           • e.g. Hox genes
             • control identity of segments along developing animal body
             • Ultrabithorax
               • homeotic mutant w/ 2 pairs of wings
           • determine fundamental structure of the body
       • Origin of novel genes
         • Gene duplication
           • Mistake (copying error) in DNA synthesis
             • One gene is accidentally copied in twice
               • i.e. chromosome in the organism has 2 copies of one gene
           • Eventually, the 2 genes take on different functions
             • Resemble each other less and less by accumulating mutations by selection
               • Common ancestor but different function
                 • Two related genes in one genome = paralogs
                   • e.g. hemoglobin and myoglobin
                     • hemoglobin
                       • four subunits, tetramer, 2 different kinds: alpha and beta, 4 polypeptides, blood
                     • myoglobin
                       • single subunit, one polypeptide, muscle
                     • both are protein complexes that carry oxygen
                     • used to be only one "globin" gene, an oxygen carrying protein
                       • gene duplicated
                         • duplicated evolved into myoglobin and hemoglobin
                           • hemoglobin duplicated again to form alpha and beta descendants
           • New genes do not arise by many little mutations, but ONE gene duplication event
         • Lateral/Horizontal Gene transfer
           • transfer of genes btwn species
           • common among unicellular organisms

4. The geological record

   • Earth = ~4.6 billion years old
   • Microbial life arose early (lecture 8)
   • Fossils of animals & plants common in the last ~550 million years
     • Phanerozoic Eon
       • Divided into 3 "eras"
         • Each "Era" subdivided into several "periods"
           • Periods are all different lengths because they were defined before absolute dating
             • Found discontinuities where things changed very suddenly

5. Eons

   • Phanerozoic
     • Eras
       • Cenozoic
       • Mesozoic
         • Period
           • Cretaceous
       • Paleozoic
         • Period
           • Permian
   • Proterozoic
   • Archaean