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Chapter 18: Classification and Systematics (Concepts (About 400 million…
Chapter 18: Classification and Systematics
Concepts
The task of taxonomists was to discover and identify new species and give them unique names.
With the discovery of evolution by natural selection, the basis of naming plants suddenly changed.
Natural selection showed that all organisms are related to each other genetically, all are part of a large phylogeny.
Most scientifically valid system of assigning names to species would be one that reflected evolutionary relationships.
1.) Developing a
natural system of classification
, a system in which closely related organisms are classified together.
2.) Assigning plant names on the basis of phylogenetic relationships.
About 400 million years ago, a long series of mutations and natural selection gave rise to the plants that are alive today.
phylogeny
: the evolutionary history of each member of the group
Some phylogenetic studies attempt to create models of the evolutionary relationships of very large groups.
Ex: The steps that were involved as plants, animals, and fungi evolved from some early ancestor.
Other studies focus on the relationships of smaller groups, such as the species of the legume family (e.g., peas, beans, and lentils).
systematics
: the branch of biology that deals with classification and nomenclature
Goal:
Understand each of these evolutionary lines.
Have a system of names that reflect their relationships accurately.
nomenclature
:a system of names and terms used in a particular field of study or community
Because our knowledge is incomplete and imperfect, the current classification systems are only approximations.
The Major Lines of Evolution
All organisms are grouped into 3 domains
Archaea
Eukarya
Bacteria (with cyanobacteria)
Most significant event in evolution, about 3.5 billion years ago.
The first organisms were simple, consisting of a cell membrane, protoplasm, and some means of inheritance, probably RNA.
Almost certainly had no distinctive nucleus or other membrane-bounded organelles.
Such organisms, either living or extinct, are prokaryotes.
Significant step in prokaryote evolution was the development of a type of photosynthesis that liberates oxygen and is based on chlorophyll
a
.
Next major step was the conversion of a prokaryote into a eukaryote, having a membrane-bounded nucleus.
A significant aspect was the origin of mitochondria.
This was a gradual process in which a bacterium capable of aerobic respiration began living inside the protoplasm of an early eukaryote whose own capacity for aerobic respiration was less sophisticated.
Organisms with an organization similar to that of early eukaryotes were formerly classified in a "kingdom Protista".
Classification is based on shared derived characters (synamorphies), not a lack of them.
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The old classification of protista was a
grade classification
(as opposed to a clade):
"Protistans" were placed together because they had a low level (low grade) of evolutionary advancement.
Similarly, ferns and other plants that reproduce without seeds used to be classified together as the grade "Cryptogamae."
The criterion for classifying organisms in kingdom Protista was simply that they were eukaryotes but lacked the derived characters of plants, animals, or fungi.
No longer use classifications based on lack of characters, but the old names are often convenient.
Therefore, use them informally, never capitalized, and occasionally in quotation marks to emphasize that "protists" do not form a clade.
But that classification was never satisfactory because it included members that were extremely diverse and obviously not closely related to each other.
Diversified rapidly into numerous clades, as evidenced by the presence of the thousands of living species of bacteria, cyanobacteria, and archaeans.
kingdom Plantae
Contain photosynthetic pigments which are present in plastids.
They are autotrophic mode of nutrition.
Eukaryotic cells.
Plants are non-motile, live anchored to a substrate.
Multicellular organisms with walled and frequently vacuolated.
Structural differentiation lead to development of various organelles for photosynthesis, anchorage and support.
Reproduction is primarily sexual.
Levels of Taxonomic Categories
Development of Concepts of Evolution and Classification
binomial system of nomenclature
: a system of providing scientific names to organisms, each name consisting of the genus name and the species epithet
inheritance of acquired characteristics
: the incorrect idea that all cells of the body produced fluids that diffused to the genitalia, where the fluids were concentrated and formed into sperm cells or egg cells
scientific names
: the binomial name of species, consisting of the genus name and the species epithet
Most fundamental level of classification:
species
Ideally and theoretically is a set of individuals closely related by descent from a common ancestor.
genera
: group of closely related species
Taxonomists referred as "lumpers", believe that even relatively distantly related species should be grouped together in large genera.
Taxonomists referred as "splitters", prefer to have many small genera, each containing only a few species that are extremely closely related.
polyphyletic group
: derived from more than one common evolutionary ancestor or ancestral group and therefore not suitable for placing in the same taxon
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monophyletic group
: a group of organisms that are classified in the same taxon and share a most common recent ancestor
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family
: each composed of one, several, or often many genera
Level above genus.
Levels above family:
class
division
order
kingdom
species epithet
: the word that distinguishes this species only from the other species of the genus
taxon
: refers to any of the above groups in a general way
Ex: Some systematists study smaller taxa such as species and genera.
Ex: Others are concerned with higher taxa such as orders and divisions.
Other Types of Classification Systems
artificial system of classification
: a classification not based on evolutionary, phylogenetic relationships
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Classification Systems for Fossils
form genera
: all fossils with the same basic form or structure are classified together
Ex: A piece of fossil wood similar to the wood of modern pines, spruces, and larches is classified in the form of genus
Pityoxylon
.
If the piece of fossil wood was part of a branch with leaves and cones attached, there would probably be enough characters present to allow us to assign the wood to a natural genus.
If the fossil contains only wood, not enough characters are present to determine whether it came out from an ancient pine, spruce, larch, or some other group that has since become extinct.
Cladistics
Understanding Cladograms
cladogram
: a diagram that shows evolutionary patterns by means of a series of branches
node
: each point at which a cladogram branches
Represents the divergence of one taxon into two, and all of the branches that extend from any particular point represent the descendants of the original group.
That ancestor is their
common ancestor
.
Any ancestor (any node) and all of the branches that lead from it constitute a
clade
(thus, this is a cladogram).
The divergence represented as a node with two branches coming from it.
apomorphy
: a feature present in one (autapomorphy) or several (synapomorphy) derived members of a group, but which is not present in the ancestral members.
ancestral condition
: the ancestors of another group; can refer to the ancestral group that gave rise to just one species or to a set of species
Most recent common ancestor
at node 1.
paraphyletic group
: one that does not contain all the descendants of the most recent common ancestor
parsimony
: the concept of minimum complexity; in cladistics, a cladogram with the least number of steps is the most parsimonious
equally parsimonious
: the scientific principle that when there are competing hypotheses for a set of data, it is the one that makes the fewest assumptions that is preferred, all else being equal
symplesiomorphy
: possession by two or more organisms or taxa of a shared primitive or ancestral character not exclusive to or characteristic of them in particular; the shared character itself
unresolved polychotomy
: sometimes there is simply not enough data, and then usually the various species are shown as arising from the same node
uninformative
: those that do not give information about evolutionary relationships among taxa
Cladograms and Taxonomic Categories
basal angiosperms
: the several clades of angiosperms that arose before the rest of the angiosperms diverged into the monocots and eudicots
eudicot
: this is the clade of angiosperms that contains most species formerly known as dicots; they have broad leaves and pollen with more than one germination pore
A species evolves into two species as different populations accumulate distinct alleles.
As they continue to evolve, each acquires its own mutations,and because they cannot interbreed, they cannot share the new alleles.
cladistics
: a method of analyzing these phylogenetic, evolutionary relationships
Complicated due to resemblance by two reasons:
1.) They have descended from a common ancestor.
2.) They have undergone convergent evolution.
Results when two distinct evolutionary lines of plants respond to similar environments and selection pressures.
Under these conditions, natural selection may favor mutations in each line that result in similar phenotypes.
Features like this are
homoplasies (analogous features)
and should never be used to conclude that plants are closely related.
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synapomorphies (homologous features)
: features similar to each other because they have descended from a common ancestor
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Critically important for making taxonomic comparisons and the only ones that can be used to conclude that species are related.
At node B (for liverworts) and clade c (vascular plants), "multicellular sporophyte" is a synapomorphy.
Taxonomic Studies
Declaring a new species is easy; proving and describing it properly are difficult.
To overcome this, taxonomists from all over the world have established an
International Code of Botanical Nomenclature
Describes precisely the steps necessary for naming a new species.
Name must be accompanied by a detailed description of the species in Latin and usually also in English, French, German, Spanish, or Russian.
The name and description must be published in a widely circulated journal, a step that prevents many problems.
A valid name, one never previously used, must be declared.
The journal's editors send the description to at least two independent specialists to verify that it is a previously unknown species and that the name has never been used before.
Description must include designation of a
type specimen
; this is a single preserved plant that truly carries the name.
To prevent a recurrence of this type disaster, other specimens, as similar as possible to the type specimen, are sent to many herbaria around the world; these are
isotypes
.
