GEOS 26400/36400/BIOS 23255/EVOL 32300: Principles of Paleontology

Systematics, I

I. Populations and species
      -variation as raw material for evolution (heritable change)
            **evolution as heritable change in trait frequency in pop'n.
            **ev'n as sorting of heritable variation
      -importance of scaling changes in mean trait value to variation 
	  -evolutionary change expected to be proportional to variance
	  -variation affects statistical confidence that change exceeds chance expectation
      -heritable (genetic) versus environmental or ecophenotypic variation
      **All traits have both heritable and environmental basis.
      **Heritability assessed statistically from parent-offspring resemblance.
            **Heritability of variation in trait, not of trait
      -natural selection and genetic drift
      -Biological Species Concept (BSC)
            -array of populations that are actually or potentially interbreeding under natural conditions
            -reproductive cohesion within species and isolation from other species
            -difficulties applying this concept in practice
                  -ability to interbreed difficult to verify
                  -hybridization, asexual reproduction, horizontal transfer
            -practical use of morphological species (morphospecies)
      -Alternatives to BSC include:
	-Evolutionary Species Concept (species as continuous lineages)
        -Phylogenetic Species Concept (species as smallest diagnosable monophyletic units)
      **Species definition vs. species recognition

II. Morphological and paleontological species
   -Try to establish fossil species with morphological differences
	comparable to the differences among living species.
   -Problems
     -Time-averaging inflates variance of sampled population
	-Hunt study suggests this may be of minimal importance in practice
	  -Fossil populations not systematically more variable than living ones
	  -Variance barely increases, on average, with scale of time-averaging
	  -Implication of Hunt study: predominance of stasis (little net evolutionary change) within spp.
     -Continuum of form may be oversplit because of emphasis on extremes (Kummel and Steele ammonoid study).
     -ecophenotypic variation
     -polymorphic species
     -cryptic species (behavioral or ecological differences without morphological differences)
     -Jackson and Cheetham bryozoan studies demonstrate congruence between morphological and genetic species.
	-morphologically defined species are discrete (greater variation between- than within-species)
	-variation is heritable (offspring more similar to their own mothers than to other individuals)
	  -implication: ecophenotypy not a dominant source of variation
	-all morphologically defined species have diagnostic genetic differences
	  -implication: differences probably not mere polymorphisms
	-no diagnostic genetic differences between different populations of the same species
	  -implication: populations are not cryptic species
	-magnitude of genetic difference correlated with magnitude of morphologic and cladistic distance
	-other studies yield similar results, but with an asymmetry:
	*If there are morphological differences, there are usually corresponding genetic differences.
	 -many "classic" cases thought to reflect ecophenotypic differences now seen as genetic
	*But if there are no discernible morphological differences, 
	there are sometimes genetic differences nonetheless.
	*I.e., polymorphic species not much of a problem, but cryptic species may be.
	*Implications for studying evolution in the fossil record  
	 -(more on this when we cover punctuated equilibrium later...)
		
III. Speciation
      -sympatric vs. allopatric populations
      -attainment of reproductive isolation within population
      -allopatric speciation as a three-stage process:
            -subpopulation formation
            -subpopulation persistence
		-failed speciation may reflect extinction of isolated subpopulation
            -subpopulation divergence
      -importance of allopatry for speciation
            -reduced gene flow
            -heterogeneous selection pressures (different environments)
      -examples of sympatric speciation
            -polyploidy (chromosome multiplication), especially in plants
            -host shifts