PBIO 250 Lecture Notes

James L. Reveal

Norton-Brown Herbarium, University of Maryland


Evolution, Variation and Biosystematics

Origin of species and speciation
G. Ledyard Stebbins (1906- )
Variation and evolution in plants (1950)
evolution and the evolution of species are not synonymous
species definition stresses the importance of genetic and morphological continuity and the discontinuity among closely related species
infraspecific entities are based on incomplete morphological separation but reinforced by ecological and geographical isolation
taxa are maintained by limited gene exchange and adaptations to local environmental conditions.
Verne E. Grant (1917- )
Plant speciation, ed. 2. (1981)
speciation mechanisms by Barry Sinervo - REQUIRED READING
reproductive isolation
isolating mechanisms:
     (1) spatial distance
     (2) environmental factors
     (3) reproductive biology
prezygotic mechanisms prevent fertilization

  • geographical - spatial isolation beyond normal dispersal
    Platanus occidentalis and P. orientalis
    Platanus x acerifolia
    allopatric vs. sympatric
  • ecological - differentiation in habitat requirements
    Eriogonum kingii and E. argophyllum
    seasonal - flowering at different times
    Camissonia brevipes and C. clavaeformis
  • ethnological - incompatible behavior patterns
    Pedicularis groenlandica (back) and P. attollens (face)
    1. Kubitzki, K. 1983. Dispersal and distribution: An international symposium. Hamburg.
  • mechanical - structural differences
    Asclepiadaceae and Orchidaceae
    developmental failures: pollen tube or sperm failure
    postzygotic mechanisms function after fertilization
  • hybrid inviability - combination failures
    hybrid zygote dies
    endosperm disintegrates
    fruit fails to set
    seed reduction
    seed fails to germinate
    seedlings fail
    anthers abort
    gametes are sterile
  • hybrid sterility - hybrids produced but sterile or nearly so
    partly developmental
    chromosomal disruption
    hybrid breakdown - failure of subsequent hybrid generations
    isolating mechanisms develop as a byproduct of evolutionary divergence

    Speciation: the evolutionary divergence and differentiation of a formerly homogeneous population system into two or more distinct entities; requires genetic variation and isolation - A summary

    Concepts of species - REQUIRED READING
    The species problem by Donald Ugent - REQUIRED READING what is a species? definitions of speciation (from Rieseberg & Brouillet, 1994)
    1. biological: species are groups of interbreeding natural populations that are reproductively isolated from other such groups.
    2. cohesion: the most inclusive group of organisms having the potential for genetic and/or demographic exchange-ability.
    3. ecological: a species is a lineage (or a closely related set of lineages) which occupies an adaptive zone minimally different from that of any other lineage in its range and which evolves separately from other such lineages.
    4. evolutionary: a single lineage of ancestral-descendent populations of organisms which maintains its identity from other such lineages and which has its own evolutionary tendencies and historical fate.
    5. phenetic: dense regions within a hyperdimensional environmental space.
    6. phylogenetic: an irreducible (basal) cluster of organisms, diagnosably distinct from other such clusters, and within which there is a parental patterns of ancestry and descent.
    geographical theory of speciation
    Ernst Mayr (1904- )
    spatial distance a prerequisite for speciation
    before spatial isolation there must be natural selection pressures to diverge
    the divergency must be sufficient enough to prevent gene exchange with adjacent populations
    phyletic evolution: a change from one state to another
    divergent evolution: a single population differentiated into two evolutionary lines; divergence leads to isolation and isolation promotes speciation
    sympatric speciation: divergence without geographical isolation the differentiation of closely related species that co-inhabit a single geographical area, e.g., sympatric
    punctuated equilibrium [-ia]
    Stephen J. Gould and Niles Eldridge
    gradualism (according to Darwin)
    1. Eldridge, N. 1985. Time frames: The rethinking of Darwinian evolution and the theory of punctuated equilibria. New York.
    2. -- & S.J. Gould. 1972. "Punctuated equilibria: An alternative to phyletic gradualism," pp. 82-115. In: J.M. Schopf (ed.), Models in paleobiology. San Francisco.
    3. --. 1988. Punctuated equilibrium prevails. Nature 332: 211-212.
    4. Gould, S.J. & N. Eldridge. 1986. Punctuated equilibrium at the third stage. Syst. Zool. 35: 143-148.
    existing species are the survivors of long lines of continuous evolution influenced by periods of environmental change
    organisms are involved in a constant struggle to survive
    only the fittest survive to produce the greatest number of offspring
    features critical to survival tend to be gradually accumulated until a new form, many generations later and distinct from its ancestors, is in place
    homeostatic equilibria that are disturbed infrequently by rapid and episodic events of speciation brought about by catastrophic events
    Mentzelia, Astragalus, Eriogonum, Centaurium, Ivesia, Castilleja
    role of hybridization in speciation
    Helianthus, Quercus, Aquilegia
    introgressive hybridization
    Edgar S. Anderson (1897-1969)
    Introgressive hybridization (1949)
    Cowania, Purshia, Gilia
    phylogeny and reticulate evolution
    polyploidy
    Grant estimates 43% in dicots, 58% in monocots and 47% in all angiosperms
    polyploid series
    Chenopodium: n= 9, 18
    Chrysanthemum: n= 9, 18, 27, 36, 45
    Crepis occidentalis (x= 11) consists of a series of 2x, 3x, 4x, 5, 7x and 8x
    Potentilla can be 16-ploid.
    polyploidy is rare in cycads and ginkgo, common in Ephedra. Probably common in ferns, Equisetum is 2n= ca 216, but rare in fungi and most algae.
    Ophioglossum reticulatum has 2n= 1260.
    types of formation: somatic doubling and non-reduction in meiosis
    polyploidy is promoted by a combination of:
         (1) long-lived plants with vegetative means of propagation
         (2) primary speciation accompanied by chromosome repatterning
         (3) occurrence of natural interspecific hybridization
    polyploids tend to occur more frequently at higher latitudes and elevations
    polyploids tend to be more widely distributed and more common than their diploid relatives
    Atriplex canescens sw Can. s. to n Mex (n= 18, 36) and A. canescens var. gigantea on sand dunes near Lynndyl, Juab Co., Utah (n= 9)
    multiple polyploidization - most polyploids are of a multiple origin, genetically variable, dynamic units of evolution
    aneuploidy
    Phacelia n= 7, 8, 9, 10, 11
    Lesquerella n= 5, 6, 7, 8
    Eriogonum subg. Oregonium n= 9, 11, 12, 17, 18, 20, 22
    Clayton virginica 2n= 12, 14, 16-20, 22, 24, 26, 28, 30-32, 34, 36, 41, 48, 72
    Poa pratensis 2n= 22-147
    true hybrid origin of new species is rare with only 8 confirmed cases
    Tragopogon
    Gilia
    Helianthus
    1. Rieseberg, L.H. 1997. Hybrid origins of plant species. Ann. Rev. Ecol. Syst. 28: 359-389.
    apomixis
    "microspecies"
    agamospermy is seed formation without fertilization
    Citrus, Potentilla, Taraxacum, Rubus, Crataegus
    vegetative reproducing clones
    Quercus, Iris, Populus

    Other Sites of Interest
    The Theory of Evolution: A review
    Principles of Genetics: A review
    The process of speciation by Timothy L. Killeen
    Evolution and Natural Selection by Timothy L. Killeen
    The Genetic Basis for Evolutionary Change by Timothy L. Killeen
    Rapid Evolutionary Change in Plants and Animals by Timothy L. Killeen
    Geography and Ecology of Species Distributions: A good discussion of zoogeography

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    Last revised: 21 Jan 1999