LECTURE 1:  LINEAGES, POPULATIONS AND GENETIC VARIATION

I.  Meet your ancestors

     A.  All organisms are descended from a single common ancestral species that lived 3.5 - 3.8 billion years ago.

     B.  Evidence for common descent from a single ancestor.

          1.  Shared cell structures (e.g. lipid bilayer membranes, ribosomes)

          2.  Shared biochemical machinery in cells of all organisms
               (e.g. glycolysis, pathways of amino acid synthesis, transcription and translation)

          3.  Shared genetic code.

               a.  No thermodynamic reasons to believe that the common genetic code is any better than a large number
                    of other possible genetic codes.

               b.  Therefore, the particular genetic code in use today is an accident of history.

               c.  Given the large number of possible genetic codes (roughly 64! 21⁶⁴/ 33! = 6 x 10⁹⁵ approximately),
                    even if only a small fraction of them are thermodynamically appropriate, it is inconceivable that two
                    independent origins of the genetic code would yield the same code.

               d.    Consequently, although there may have been multiple origins of life with different genetic codes, all
                      surviving organisms (and presumably all that have lived during the past 3.5 billion years) almost
                      certainly trace their ancestry back to just one of these origins.

II.  Implications of single common ancestor.

     A.  Organismal Diversity

          1.  Estimated 10 million species alive on earth today.

          2.  Species alive today estimated to constitute less than 5% of all species that have lived in the past.

          3.  Consequently, at least 200 million species have existed at one time or another during the history of
               life on our planet.

     B.  Common ancestor of this diversity implies operation of two primary evolutionary processes:

          1.  PHYLETIC EVOLUTION (Anagenesis): accounts for why most organisms do not resemble the common
               ancestor of all living things.  Focus of first part of course.

          2.  LINEAGE SPLITTING (Speciation or Cladogenesis):  accounts for why there is more than one type of
               organism at any time.

            1.  POPULATION:  A group of similar organisms living at the same locality that interbreed.  A population constitutes
                           a particular generation of the LINEAGE of which it is a part.

             2.  LINEAGE:  A series of ancestral-descendent populations through time.  May be depicted by an extended
                          GENEOLOGY, which diagrams the parent-offspring relationships between successife generations
                         (POPULATIONS) in a lineage.

      D.  Visual portrayals of  phyletic evolution and lineage splitting.  (in-class Computer Simulation)

III.  Phyletic Evolution (or Evolution, for short)

     A.  Initial definition: a average change in one or more characteristics of individuals of a lineage over time.

          1.  Two processes may produce such a change:

               a.  Environmental change with no genetic change
                    --character state is controlled by environmental cues (e.g. temperature effects on size)
                    --temporal changes in average environmental cues causes change in mean character state (e.g. increase
                       in average temperature over time cause change in average size)

               b.  Change in average genetic constitution of individuals
                    --character state is controlled by genetic makeup of an indiviudal (e.g. color pattern in many butterflies
                       determined by which alleles are inherited)
                    --temporal change in genetic makeup causes change in typical character state of individual (e.g.
                       substitution of one allele for another causes change in butterfly color pattern)

          2.  Only the latter constitutes evolution.

          3.  Hence, one definition of Evolution:  a change in one or more characteristics of individuals of a lineage over
               time due to change in the genetic constitution of a population.

IV.  Distinguishing between Environmental and Genetic causes of change in average character state
        within a lineage

          A.  Issue typically arises when differences are noted between individuals of two closely related populations.

          B.  Common garden experiment used to distinguish between Environmental and Genetic causes of change

               1.  Grow individuals from different populations in common gardens, each of which  represents the enviornments
                    of one of the populations

               2.  Within each garden, RANDOMIZE position of individuals from different populations

               3.  Characterize phenotypes of each individual

               4.  Differences among populations in the same garden reflect genetic (and hence evolved) differences among
                    the populations, since by growing in a common garden, all individuals experience the same environment

               5.  Differences among gardens for individuals of the same population reflect environmental (and hence NOT
                    evolved) differences among individuals at different sites, since the collections individuals from the same
                    population in different gardens are genetically  identical.

          C.  Example:  Experiment of Clausen, Keck and Hiesey on Potentilla glandulosa

                              [Back to Lectures | Bio 120 Home Page | Department of Biology | Duke University ]