UNIT II – PLANT DIVERSITY
Lecture 8: Plant Systematics
and Darwinian Evolution
Today: Part I
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Plant systematics
Taxonomic hierarchy
Concept of species
Carolus Linnaeus
Evolution
Charles Darwin
Natural Selection
Rates of evolution:
gradualism vs. punctuated equilibrium
PLANT SYSTEMATICS
• Common names
• Have evolved over centuries in a multitude of
languages
• Sometimes used only in a limited geographical
area
• Problem with common names:
• One plant may be known by several names in
different regions, and the same name may be
used for several different plants…
Scientific names
Similar plant species form a group called a genus
(plural: genera)…
Genera are grouped into families…
Families into orders, classes, divisions and kingdoms
Kingdom
Division
Class
Order
Family
Genus
Species
“King David Came Over For Great Spaghetti”
“King David Conquered Our Fifty Great States”
Species name
Each species has a single correct scientific name in
Latin called a binomial (two names) – it is always
italicized or underlined.
First name is genus name.
Second name is species name
Human: Homo sapiens
Cat:
Felis catus
Dog: Canis familiaris
Wolf: Canis lupus
Examples
Genus of maple trees is Acer
It has many species including:
Common name
“Red maple”
“Sugar maple”
“Black maple”
Scientific name
Acer rubrum
Acer saccharum
Acer nigrum
Taxonomic hierarchy
Species that have many characteristics in common
are grouped into a genus.
Related genera that share combinations of traits are
grouped into families.
Families are grouped into orders.
Orders into classes
Classes into divisions (or phyla for animals)
Related divisions/phyla are grouped into kingdoms
(e.g. house, street, city, county, state, country, continent, planet)
5 KINGDOMS
• Living organisms are classified in five
kingdoms:
• Animalia: animals
• Plantae: plants
• Fungi: fungi
• Protista: algae
• Monera: bacteria
What is a species?
Species: a set of individuals that are closely
related by descent from a common ancestor
and ordinarily can reproduce with each other,
but not with members of any other species.
Biological species: a group of interbreeding
populations. Offspring are fertile.
Species
Some members of same species look very
different…
Same species, are capable of interbreeding, but
Morphologically look very different.
All these are same species!
Examples in plants: species of oaks and sycamores;
Broccoli, kale, cabbage, califlower: members same
species! Brassica olearea
Definition of species
• However, some plants look the same, but
due to polyploidy
(more than the diploid number of
chromosomes), they cannot interbreed.
• For example: Ferns; evening primrose
Carolus Linnaeus
• Swedish scientist – Carl von Linne
(doctor and botanist)
born in 1707.
• Called the “Father of Systematic Botany”
• Established modern system of nomenclature
Linnaeus
• Carolus Linnaeus used risque language for his
time…
• Classifying plants by their flowers, he compared
flower parts to human sexuality: stamens were
husbands (many) and the pistil was the wife – the
flower was the bed!
• Many were shocked. Dr. Johann Siegesbeck:
“such loathesome harlotry as several males to one
female would not be permitted by the creator…
Who would have thought that bluebells, lilies and
onions would be up to such immorality?”
Linnaeus legacy
His binomial system of nomenclature, in
which the genus and species names are
used.
He classified 12,000 plants and animals,
published Systema Naturae in 1753, and
many of the names he first proposed are still
in use today…
EVOLUTION
A genetic change in a population of organisms that
occurs over time, often adapting to an
environment or way of life.
Evolutionary changes must be genetically
inherited, not acquired.
Creationism was the main belief – all organisms
were “specially created”, unchanging
Evolution of Evolutionary thinking
(Pre-Darwinian)
Jean-Baptiste Lamarck (1744-1829) – French
naturalist, proposed a theory that organisms
were driven by some inner force toward greater
complexity. But thought that org. could pass on
traits to their offspring that they acquired during
their lives. (“Lamarckism”, proposed in 1809)
Lamarckism
• Lamarckism holds that traits acquired
(or diminished) during the lifetime of an
organism can be passed to its offspring.
• Lamarck based his theory on two observations
thought to be true in his day:
1.“Use it or lose it” - Individuals lose
characteristics they do not require and develop
those which are useful.
2.Inheritance of acquired traits - Individuals
inherit the acquired traits of their ancestors.
Lamarckism
• Examples include: the stretching
by giraffes to reach leaves leads to
offspring with longer necks;
• Strengthening of muscles in a blacksmith's arm
leads to sons with like muscular development.
This theory was later disproved!
Charles Darwin
• Charles Darwin (1809-1882)
• Born in England, studied medicine,
theology…
• Takes a 5-year trip around the world as a
naturalist on the HMS Beagle.
• Observes plant and animal species in Galapagos
Islands, Australia, NZ, etc.
• Observed: Island animals are similar to mainland
animal species (descended), but they show
differences due to the conditions on their island.
On the Origin of Species…
• Came home, marries cousin,
worked for 16 years
analyzing his data
• Darwin publishes the most influential text of
all times: On the Origin of Species by Means
of Natural Selection in November 24, 1859.
• (The entire printing (2500 copies) was sold
that same day!)
Controversy…
• The Origin of Species… caused great arguments
between scientists and philosophers – both noting
the theories failures and strengths.
• Huxley’s famous debate in June 1860, at Oxford.
• Archbishop Samuel Wilberforce ridiculed
evolutionary theory:
– “Do you descend from an ape on your grandmother's
side or your grandfather's side?”
Huxley replied: – "I would rather be the offspring of two
apes than be a man and afraid to face the truth."
Darwin’s revolutionary thoughts
• Darwin thought of organisms not as
constant, unchanging or “specially
created beings”.
• Could not believe that organisms
today appeared as they have always
appeared
• Darwin changed biological thought
forever with the concept of Natural
Selection!
Natural Selection
Has four premises:
1) Variation – Members of a population have
individual differences that are inheritable
2) Overproduction – Natural populations
reproduce geometrically
3) Competition – Individuals compete for limited
resources
4) Survival to reproduce – Only those
individuals that are better suited to the
environment survive and reproduce
Natural Selection
• 1. Variation: Member within a species exhibit
individual differences – these differences must
be inheritable
• Natural selection won’t work in a population of
clones! Remember that a key to variation is
sexual reproduction.
Natural Selection
• 2. Overproduction: Natural populations
increase geometrically, producing much
more offspring than will survive…
140
120
100
80
60
40
20
0
Natural Selection
• 3. Competition: Individuals compete for
the same, limited natural resources.
• Darwin called it: “Struggle for existence”
Natural Selection
• 4. Survival to reproduce: Only those
individuals that are better suited to the
environment will survive and reproduce
(“Survival of the fittest”).
• Fit individuals pass on to a portion of their
offspring the advantageous characteristics.
Natural Selection
• Works on the individual phenotype  which in
turn changes the population gene pool.
• Time – long periods of time
must be available in order to
change to a completely
different species;
changes are slow..
Natural Selection
• Offspring that inherit the advantageous traits
(“favorable genes”) are selected for
• Their chances of survival are greater
• May live to reproductive age
• May pass on those desirable attributes to future
generations
Natural Selection
• Those that do not inherit these traits
(“unfavorable genes”), are not likely to
survive/reproduce.
• Gradually, the species evolves (changes) as
more individuals carry these traits.
• Over time, enough changes  New species
Artificial Selection
• Selective breeding as practiced by humans on
domesticated plants and animals….
• For example: Dogs
Plant artificial selection
Teosinte
vs. modern corn
Tomato
Rates of evolution
• Two interpretations about the pace/speed of
evolution – based on the fossil record:
• 1. Gradualism (a traditional view) states that
• Evolution occurs as a slow and steady
accumulation of changes in organism
(Darwinian evolution)… Not much evidence.
Rates of evolution
• 2. Punctuated Equilibrium – evolution
proceeds with periods of inactivity, followed by
periods of very rapid evolution (Gould &
Eldridge model).
Punctuated Equilibrium
• Fossil record supports this view:
• Long periods of stasis (no change in species)
• Followed by rapid change
• However, fossil record is evidence only of Morphology
(structure), while evolution encompasses: morphology,
ecology, biochemistry, and behavioral changes…
• That is, there may be stasis in morphology, while there
is still active evolutionary changes going on…
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UNIT II – PLANT DIVERSITY