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)
- 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
(1) morphological-based species as distinct reproductive units (1682-1859)
(2) dynamic species concept (1859-present)
(a) reproductive-based biological species concept
formulated (1896-1916)
(b) genetic-based biological species concept formulated (1935-1942)
(c) gene-based biological ("phylogenetic") species concept formulated (1937-present)
what is a species?
- Mechanistic Species Concept
(1) Biological species concept
(2) Ecological species concept
These concepts define species in terms of the evolutionary process of speciation: One must know what a species is to ascertain how it evolved. If species are the basic unit of evolution then species are individual and discreet population then any aggregation of morphologically similar populations is an artificial construct.
- Historical Species Concept
This concept proclaims that species are the end products of evolution rather than participants in the process.
- Phylogenetic Species Concept
A species is "the smallest aggregation of populations (sexual) or lineages (asexual) diagnosable by a unique combination of character states in comparable individuals." The following are the assumptions:
(1) Species are the small diagnosable units
(2) The line between reticulating and hierarchical relationships does matter
(3) The terms "monophyletic" and "paraphyletic" can not apply to species
(4) Delimitation of species is not arbitrary
(from Luckow, 1995)
Question: Is the population or the individual the next level of organization below the rank of species? Doyle (1995) says "there may be no phylogeny, only reticulate relationships (tokogeny) among individuals." It may be that only when gene pools are identified and defined among individuals can one infer a phylogeny.
- Andersson, L. 1990. The driving force: Species concepts and ecology. Taxon 39: 375-382.
- Archibald, J.D. 1994. Metataxon concepts and assessing possible ancestry using phylogenetic systematics. Syst. Biol. 43: 27-40.
- Baum, D.A. 1992. Phylogenetic species concepts. Trends Ecol. Evol. 7: 1-2.
- -- & M.J. Donoghue. 1995. Choosing among alternative "phylogenetic" species concepts. Syst. Bot. 20: 560-573.
- --, Coyne, J.A., H.A. Orr & D.J. Futuyma. 1988. Do we need a new species concept. Syst. Zool. 37: 190-200.
- Cracraft, J. 1987. Species concepts and the ontology of evolution. Biol. & Philos. 2: 329-346.
- Cronquist, A. 1978. Once again, what is a species. Beltsville Symposia Agric. Res. 2: 3-20.
- Davis, J.I. & K.C. Nixon. 1992. Populations, genetic variation, and the delimitation of phylogenetic species. Syst. Bot. 41: 421-435.
- De Queiroz, K. 1992. Phylogenetic definitions and taxonomic philosophy. Biol. & Philos. 7: 295-313.
- -- & M.J. Donoghue. 1988. Phylogenetic systematics and the species problem. Cladistics 4: 317-338.
- --, -- & J. Kim. 1995. Separate versus combined analysis of phylogenetic evidence. Ann. Rev. Ecol. Syst. 26: 657-681.
- -- & J. Gauthier. 1992. Phylogenetic taxonomy. Ann. Rev. Ecol. Syst. 23: 449-480.
Dobzhansky, T. 1935. A critique of the species concept in biology. Philos. Sci. 2: 344-355.
- --. 1937. Genetics and the origin of species. New York.
- Donoghue, M.J. 1985. A critique of the biological species concept and recommendations for a phylogenetic alternative. Bryologist 88: 172-181.
- Doyle, J.J. 1995. The irrelevance of allele tree topologies for species delimitation, and a non-topological alternative. Syst. Bot. 20: 574-588.
- Du Rietz, G.E. 1930. The fundamental units of biological taxonomy. Svensk. Bot. Tidskr. 24: 333-428.
- Frost, D.R. & A.C. Kluge. 1994. A consideration of epistemology in systematic biology, with special reference to species. Cladistics 10:259-294.
- Ghiselin, M.T. 1974. A radical solution to the species problem. Syst. Zool. 23: 536-544.
- Gould, S.J. 1992. What is a species. Discover 1992(Dec.): 40-42.
- Grant, V. 1992. Comments on the ecological species concept. Taxon 41: 310-312.
- --. 1994. Evolution of the species concept. Biol. Zent. Bl. 113: 401-415.
- Hull, D.L. 1975. Are species really individuals? Syst. Zool. 25: 174-191.
- Huxley, J.S. 1938. Species formation and geographical isolation. Proc. Linn. Soc. London 150: 253-264.
- --. 1940. The new systematics. Oxford.
- Jordan, D.S. 1905. The origin of species through isolation. Science 22: 545-562.
- Kluge, A.G. 1990. Species as historical individuals. Biol. & Philos. 5: 417-431.
- Levin, D. 1979. The nature of plant species. Science 204: 381-384.
- --. 1993. Local speciation in plants: The rule, not the exception. Syst. Bot. 18: 197-208.
- Lewis, H. 1966. Speciation in flowering plants. Science 152: 167-172.
- Luckow, M. 1995. Species concepts: Assumptions, methods, and applications. Syst. Bot. 20: 589-605.
- Masters, J.C. & H.G. Spencer. 1989. Why we need a new genetic species concept. Syst. Zool. 38: 270-279.
- Maynard-Smith, J. 1966. Sympatric speciation. Amer. Naturalist 100: 637-650.
Mayr, E. 1942. Systematics and the origin of species. New York.
- -- (ed.). 1957. The species problem. Washington, D.C.
- --. 1969. The biological meaning of species. Bot. J. Linn. Soc. 1: 311-320.
- --. 1982. The growth of biological thought. Cambridge, MA.
- --. 1988. Toward a new philosophy of biology. Cambridge, MA.
- Mishler, B.D. & R.N. Brandon. 1987. Individuality, pluralism and the biological species concept. Biol. & Philos. 2: 397-414.
- -- & M.J. Donoghue. 1982. Species concepts: A case for pluralism. Syst. Zool. 31: 491-503.
- Nixon, K.C. & Q.D. Wheeler. 1990. An amplification of the phylogenetic species concept. Cladistics 3: 201-209.
- O'Hare, R.J. 1992. Systematic generalization, historical fate, and the species problem. Syst. Biol. 42: 231-246.
- Otte, D. & J.A. Endler (eds.). 1989. Speciation and its consequences. Sunderland, MA.
- Poulton, E.B. 1903. What is a species? Trans. Royal Entomol. Soc. London 51(Appendix): lxxxvi-cxvi.
- Ramsbottom, J. 1938. Linnaeus and the species concept. Proc. Linn. Soc. London 150: 192-219.
- Ray, J. 1682. Methodus plantarum nova. London.
- Rieppel, O. 1986. Species are individuals: A review and critique of the argument. Evol. Biol. 20: 283-317.
- --. 1991. Things, taxa and relationships. Cladistics 7: 93-100.
- Rieseberg, L.H. & L. Brouillet. 1994. Are many plant species paraphyletic? Taxon 43: 21-32.
- Sokal, R.R. & T. Crovello. 1970. The biological species concept: A critical evaluation. Amer. Naturalist 104: 127-153.
- Stebbins, G.L. 1950. Variation and evolution in plants. New York.
- Templeton, A.R. 1980. The theory of speciation via the founder principle. Genetics 94: 1011-1038.
- Van Valen, L. 1976. Ecological species, multispecies, and oaks. Taxon 25: 233-239.
- Vrana, P. & W. Wheeler. 1992. Individual organisms as terminal entities: Laying the species problem to rest. Cladistics 8: 67-72.
- Vrba, E.S. 1985. Species and speciation. Pretoria.
- Wiley, E.O. 1978. The evolutionary species concept reconsidered. Syst. Zool. 27: 17-26.
definitions of speciation (from Rieseberg & Brouillet, 1994)
- biological: species are groups of interbreeding natural populations that are reproductively isolated from other such groups.
- cohesion: the most inclusive group of organisms having the potential for genetic and/or demographic exchange-ability.
- 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.
- 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.
- phenetic: dense regions within a hyperdimensional environmental space.
- 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)
- Eldridge, N. 1985. Time frames: The rethinking of Darwinian evolution and the theory of punctuated equilibria. New York.
- -- & S.J. Gould. 1972. "Punctuated equilibria: An alternative to phyletic gradualism," pp. 82-115. In: J.M. Schopf (ed.), Models in paleobiology. San Francisco.
- --. 1988. Punctuated equilibrium prevails. Nature 332: 211-212.
- 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
- 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