skunk cabbage symplocarpus foetidus araceae n.
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Skunk Cabbage – Symplocarpus foetidus - Araceae. Visit to Missouri Botanical Garden. Meet at 7 am at the heating plant Bring a notebook, camera (optional) Bring lunch, money for food on way home Plant Classification.

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Skunk Cabbage – Symplocarpus foetidus - Araceae

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Presentation Transcript
visit to missouri botanical garden
Visit to Missouri Botanical Garden
  • Meet at 7 am at the heating plant
  • Bring a notebook, camera (optional)
  • Bring lunch, money for food on way home
process of classification
Process of Classification

Classification has two desirable goals:

1. The arrangement of groups into a pattern that accurately reflects their evolutionary relationships

2. The placement of groups into a reference system so their major features are easily and efficiently described and identified (information storage and retrieval)

traditional classification
Traditional Classification
  • Most traditional classification systems derived from Linneaus and other early taxonomists depend not on evolutionary relationships, but rather on similarity in form or organization – taxonomic groups are based on organisms having a particular “grade”
  • For example the grass family Poaceae is made up of a grade of organisms having jointed stems, leaves with sheathing bases, and greatly reduced flower parts
numerical taxonomy
Numerical Taxonomy
  • With numerical taxonomy, each character a plant has is assigned a value of one, then we simply determine how many characteristics a pair of species share – the higher the percentage shared, the higher their relatedness – strict statistical criteria can then be applied to determine how related a pair of species should be to occur in the same genus, same family, etc.
problems with numerical taxonomy
Problems with Numerical Taxonomy

1. Should all characteristics be given the same weight or are some more important than others? Perhaps ovary position is more important than other characteristics like stamen number

2. Some features in common between species may be the result of parallelism or convergence and not homology – this system fails to recognize that

3. Numerical taxonomy assumes all characters have evolved independently but that is not likely to be true – woodiness may have evolved for support as well as protection from herbivory and probably both – and has happened repeatedly in many different groups of plants

cladistic classification
Cladistic Classification
  • Another system developed to introduce rigor into classification is cladistics – it is the most successful (and currently accepted) set of rules - the goal of cladistic classification is to determine the evolutionary histories of organisms and then to express those relationships in tree-like diagrams called cladograms (evolutionary trees)
rules of cladistics
Rules of Cladistics
  • A clade is the entire portion of a phylogeny which descended from a common ancestor
  • A common ancestor is the single species from which several other species have descended
  • A cladogram is an evolutionary tree which shows evolutionary relationships by showing points at which lineages diverged from common ancestral forms
  • We construct a cladogram by comparing traits among groups of organisms
cladistic traits
Cladistic Traits
  • Ancestral traits - traits shared with a common ancestor - almost all flowering plants have a flower with four main sets of structures – sepals, petals, stamens, and pistil
  • Derived trait - a trait that differs from the ancestral trait in a lineage – some groups have reduced or highly modified sepals and petals – such as grasses
homologous traits
Homologous Traits
  • Any two traits descended from a common ancestral structure are said to be HOMOLOGOUS
  • General homologous traits are shared by many organisms -
  • The flowering plants all share the flower as a reproductive structure and that is also thought to have arisen once
  • Special homologous traits are shared by a few closely related species – such as the composite flowerhead of Asteraceae
  • A trait may evolve more than once so that it is possessed by more than one species but it is not found in their most common recent ancestor - that is called homoplasy - here structures are modified to perform a common task - wings in birds, and bats are homoplasies because common ancestor lacked wings
homoplasy woodiness
Homoplasy - woodiness

Tree fern – up to 20m tall

Red fir – up to 60m tall

rules of thumb
Rules of Thumb
  • Hennig suggested that if two species possess the same trait, we should provisionally assume the trait is homologous (until proved otherwise)
  • Hennig also stated that general homologous traits could be distinguished from special homologous traits by comparing whether or not an outgroup has this trait
  • An outgroup is a group related to the groups in question, but which branched off from those groups earlier in the evolutionary sequence
molecular phylogeny
Molecular Phylogeny
  • Molecular phylogeny, also known as molecular systematics, is the use of the structure of molecules such as DNA or RNA or proteins to gain information on an organism's evolutionary relationships. The result of a molecular phylogenetic analysis is expressed in a phylogenetic tree – usually developed using rules of cladistics
defining plant species
Defining Plant Species
  • Biological Species Concept – A group of organisms capable of interbreeding and producing fertile offspring – Ernst Mayr
defining plant species1
Defining Plant Species

1. The individuals should bear a close resemblance to one another such that they are always readily recognizable as members of that group

2. There are gaps between patterns of variation exhibited by related species; if there are no such gaps then there is a case for consolidating the taxa as a single species

3. Each species occupies a definable geographical area (wide or narrow) and is demonstrably suited to the environmental conditions which it encounters

4. In sexual taxa, the individuals should be capable of interbreeding with little or no loss of fertility, and there should be some reduction in the level of success (measured in terms of hybrid fertility or competitiveness) of crossing with other species


Rosa carolina – pasture rose, from North America

Rosa rugosa – rugosa rose, from Asia

below species ranks
Below Species Ranks
  • subspecies - a population of several biotypes forming a more or less distinct regional group of a species - primarily a geographical race or ecotype
  • variety - a population of one or several biotypes, forming more or less distinct local groups of a species - primarily local race, or ecotype of very small habitat - many people feel this term shouldn't be used as it is too uncertain what it means
  • form - a population of one or several biotypes occurring sporadically in a species population in one or several distinct characters - a genetic variant mixed in with other distinct genetic variants - may be variation in flower color or secondary chemical compounds

Acer rubrum

var. rubrum

Acer rubrum

var. trilobum




Prunus persica

var. persica


Prunus persica

var. nectarina



Two forms of Viola palustris – blue-runner violet



Several cultivars of the tomato – Lycopersicon esculentum