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PBIO 250 Lecture Notes

James L. Reveal

Norton-Brown Herbarium, University of Maryland


Evolution, Variation and Biosystematics

Processes of evolution and sources of variation
Introduction to Evolutionary Biology by Chris Colby [REQUIRED READING]
origin of species
diversity, structural complexity and adaptative nature of plants
mechanisms and processes of how plants:
    (1) direct their evolution
    (2) influence their their patterns of variation
    (3) speciate
evolution: change over time
organic evolution: "a series of transformations in the genetic materials of populations through time."
primary cause of organic evolution is: "natural selection in response to changing environmental interactions."
speciation describes "the evolutionary divergence among similar populations."

anagenesis: (phyletic evolution) - gradual change over evolutionary time until the resulting descendents can be recognized and accorded status as a new species
cladogenesis: (branching evolution) - the concept that an ancestral species gives rise to two (or more) co-existing "daughter species" without a reticulation event
biosystematics
Wendell Holmes Camp (1904-1963)
Charles Louis Gilly (1911- )

  1. Camp, W.H. & C.L. Gilly. 1943. The structure and origin of species. Brittonia 4: 323-385.
biosystematics originally implied the production of a system of classification that expresses explicit relationships and use of experimental, genetic, cytologic and population approaches to systematic problems mainly at the species level.

Charles Darwin: History behind evolutionary theory - right; image from the University of Hamburg

Sources of variation
  • variation forms the basis of both evolution and classification
    three components of variation:
         (1) developmental variation
         (2) environmentally induced variation
         (3) genetic variation
    adults often differ significantly from seedlings
    developmental differences may be determined by garden and greenhouse studies
    developmental differences are significant in understanding morphology and evolution
    phenotype
    genotype
    phenoplasticity: "changes in appearance."
    causes include variations in light, water, nutrients, temperature and soils detected by growing cloned plants under different conditions
    Clausen, Keck and Hiesey
    phenotypic and genotypic plasticity - see also this site - with genotypic variation (e.g., heritable changes) caused by:
         (1) mutation
         (2) gene flow and recombination
    mutation: transmissible change in the hereditary material; includes both genic or point mutations and chromosomal mutations
    gene flow: exchange of genes among individual populations or species
    recombination: bringing together, via meiosis and fertilization, novel genes or alleles carried by different individuals mutation is the ultimate source of genetic variation
    recombination provides immediate sources of variation in sexually reproducing plants
    gene mutation: an alteration in the sequence of nucleotides
    significance of gene mutation depends on:
         (1) the amount of effect upon the organism
         (2) the adaptive advantage or disadvantage of mutants
         (3) the role of the mutant gene in population environment interactions
    multiple genes: the hereditary differences in quantitative characters are usually determined by two or more independent genes acting cumulatively
    chromosome mutations: polyploidy, aneuploidy, gross structural changes
    causes include: changes in segments (losses or duplication), translocations (the interchange of segments), and inversions rearrangements provide unique gene combinations, arrangements or linkage groups
    gene flow and recombination caused by cross-fertilization
    gene flow is often limited between populations
    cross-fertilization is a major source of variation in sexual plants - see these notes
    self-fertilization or other nonsexual means limit variation

  • methods of reproduction:
         (1) cross-fertilization or allogamy
         (2) self-fertilization or autogamy
         (3) vegetation propagation
         (4) seeds without sex or agamospermy
    allogamy: the mode of reproduction where the zygote results from the fusion of two haploid nuclei of the same species
    allogamy promotes heterozygosity; autogamy promotes homozygosity
    dichogamy: the temporal separation of male and female functions
    protandry: where the male matures prior to the female
    protogyny: where the female matures before the male
    agamospermy: the formation of a seed without fertilization
    agamospermy produces a series of identical individuals
    1. Cresti, M., S. Blackmore, & J.L. van Went. 1992. Atlas of sexual reproduction in flowering plants. Berlin.
    2. -- & A. Tiezzi (eds.). 1992. Sexual plant reproduction.. New York.
  • coevolution: any situation in which a pair of organisms act as selective agents for each other - variously defined
    sequential evolution: the subsequent evolution of one of the two organisms, as a result of the association, without an impact on the other
    reciprocal evolutionary: change when one member of the pair has a gene that affects the relationship for which the other member has a gene to counter this effect. escalation: enemies (competitors, predators and prey) are the most important agents of natural selection, and responses bring about long-term evolutionary trends to one of the pairs
    1. Gilbert, L.E. & P.H. Raven. 1975. Coevolution of animals and plants. Austin.
    2. Shields. O. & J.L. Reveal. 1988. Sequential evolution of Eupholotes (Lycaenidae: Scolitantidini) on their plant host Eriogonum (Polygonaceae: Eriogonoideae). J. Linn. Soc., Biol. 33: 51-93.
    3. Vermeij, G.J. 1994. The evolutionary interaction among species: Selection, escalation, and coevolution. Ann. Rev. Ecol. Syst. 25: 219-236.
  • adaptation is a consequence of natural selection
    natural selection operates by differential reproduction of certain genotypes, those favored being reproductively more successful
    levels of selection - REQUIRED READING - see also Adaption, Ecology and Selection both readings by Barry Sinervo.
    kinds of selections:
         (1) stabilizing: favors "normal" individuals
         (2) directional: operates in a unidirectional manner
         (3) disruptive: operates in a heterogeneous environment
    in the absence of natural selection, gene frequencies remain constant
    genetic drift is most significance in small populations
    founder principle: chance distribution of genotype to new area
    genetic revolution: chance reduction of population to few genotypes; see also catastrophic selection
    natural selection is the basic mechanism for adaptive evolution, it acts on the variation produced by genetic changes
    neutrality theory: any random change in a protein
    evolution at the molecular level consists of a gradual replacement of one amino acid sequence by another
    selectionist theory: all changes are evolutionarially significant
    catastrophic selection: loss of all but a few exceptional genotypes, results in quantum speciation
    Clarkia
    Encelia virginensis is of a hybrid origin between E. actonii and E. frutescens - all diploids
    1. Allan, G.J., C. Clark & L.H. Rieseberg. 1997. Distribution of parental DNA markers in Encelia virginensis (Asteraceae: Heliantheae), a diploid species of hybrid origin. Pl. Syst. Evol. 205: 205-221.
  • ecotype: a local population or series of populations with genetically based unique features and adapted to particular conditions
    clinal variation: change over continuous space
    mosaic variation: change over discontinuous space
    Gote Wilhelm Turesson (1892-1970)
    ecotype: genetically adapted ecological races
    genecology: study of the ecology of ecotypes
    ecocline: observable gradient of genetically based changes
    cline: ecologically adapted continuous variation over space
    ecocline: an ecological gradient
    topocline: a geographic gradient
    electrophoretic data
    useful in determining the identity of progenitors of taxa of hybrid origin
    enzyme data allow direct comparison of the products of individual genes; allelic frequency data can be obtained, these data being useful in measuring genetic similarities and differences among species, infraspecific taxa, and populations.

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