Skunk Cabbage – Symplocarpus
foetidus - Araceae
Visit to Missouri Botanical Garden
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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 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
• 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 cladistics
1913 - 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 pollination
pine 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
Species
Rosa 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 rubrum
var. trilobum
Rosaceae
Peach
Prunus persica
var. persica
Nectarine
Prunus persica
var. 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