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

• http://www.mobot.org/visit/maps/VS_MBG_Map2012.pdf

Plant 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

• 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

Poaceae

Downy Hawthorn – Crataegus mollis

White Hawthorn – Crataegus monogyna – flowers, and fruits of 4 species of Hawthorn

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.

Sokal Distances

Problems with Numerical Taxonomy1. 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

• 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)

Willi Hennig – founder of cladistics1913 - 1976

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

Hennig’s Figure – Groups in Circles All Represent Clades

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

Convergence – wind pollinationpine staminate cone vs. birch catkin

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

Leaf Modifications

Homoplasy

• 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

Tree fern – up to 20m tall Red fir – up to 60m tall

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

Progymnosperms

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

Michael Donoghue

Defining Plant Species

• Biological Species Concept – A group of organisms capable of interbreeding and producing fertile offspring – Ernst Mayr

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

SpeciesRosa carolina – pasture rose, from North America Rosa rugosa – rugosa rose, from Asia

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

Pitcher Plant - Sarracenia rubra subsp. gulfensis

Pitcher Plant - Sarracenia rubra subsp. wherryi

Acer rubrum – Red Maple - Aceraceae

Acer rubrum var. rubrum

Acer rubrumvar. trilobum

Rosaceae

PeachPrunus persicavar. persica

NectarinePrunus persicavar. nectarina

Violaceae

Two forms of Viola palustris – blue-runner violet

Plant Varieties – domesticated often known as cultivars

Solonaceae

Several cultivars of the tomato – Lycopersicon esculentum