chapter 26: taxonomy and systematics
TRANSCRIPT
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CHAPTER 26LECTURE
SLIDES
Prepared by
Brenda LeadyUniversity of Toledo
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Taxonomy and systematics Taxonomy
Science of describing, naming, and classifying living and extinct organisms and viruses
SystematicsStudy of biological diversity and the
evolutionary relationships among organisms, both extinct and modern
Taxonomic groups are now based on hypotheses regarding evolutionary relationships derived from systematics
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Taxonomy
Hierarchical system involving successive levels
Each group at any level is called a taxon Domain
Highest levelAll of life belongs to one of 3 domainsBacteria, Archaea, and Eukarya
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EukaryaBacteriaDomains: Archaea
Plantae Fungi Animalia
Excavata Alveolata Stramenopila Rhizaria Amoebozoa OpisthokontaLand plants and algal relativesEukaryoticsupergroups:
Large eukaryotickingdoms:
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Eukarya
lupus
Canis
Canidae
Carnivora
Mammalia
Chordata
Animalia
Domain
Species
Genus
Family
Order
Class
Phylum
Kingdom
~270
1
7
34
~5,000
~50,000
>1 million
OpisthokontaSupergroup >1 million
Taxonomicgroup
Gray wolffound in
Number ofspecies
~ 4– 10 million
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Binomial nomenclature
Genus name and species epithet Genus name always capitalized Species epithet never capitalized Both names either italicized or underlined Rules for naming established and
regulated by international associations
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Phylogenetic trees
Phylogeny – evolutionary history of a species or group of species
To propose a phylogeny, biologists use the tools of systematics
Trees are usually based on morphological or genetic data
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Phylogenetic tree
Diagram that describes phylogeny A hypothesis of evolutionary relationships
among various species Based on available information New species can be formed by
Anagenesis – single species evolves into a different species
Cladogenesis – a species diverges into 2 or more species
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Present
5
10
A
B
B
CD
E
F G HI J K
Mill
ion
s o
f ye
ars
ago
(m
ya)
Tim
e
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Monophyletic group or cladeGroup of species, taxon, consisting of the most
recent common ancestor and all of its ancestors Smaller and more recent clades are nested
within larger clades that have older common ancestors
Paraphyletic groupContains a common ancestor and some, but not all,
of its descendents
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D E F G
B C
A
D E F G
B C
A
D E F G
B C
A
(a) Monophyletic (b) Paraphyletic (c) Polyphyletic
H I J K L M N O H I J K L M N O H I J K L M N O
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Over time, taxonomic groups will be reorganized so only monophyletic groups are recognized
Reptiles were a paraphyletic groups because birds were excluded
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Reptiles Reptiles
Tu
rtle
s
Liz
ard
san
d s
nak
es
Cro
cod
iles
Bir
ds
Tu
rtle
s
Liz
ard
san
d s
nak
es
Cro
cod
iles
Bir
ds
Orders
Classes
(a) Reptiles as a paraphyletic taxon
(b) Reptiles as a monophyletic taxon
KEY
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Homology
Similarities among various species that occur because they are derived from a common ancestor
Bat wing, human arm and cat front leg Genes can also be homologous if they are
derived from the same ancestral gene
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Morphological analysis
First systematic studies focused on morphological features of extinct and modern species
Convergent evolution (traits arise independently due to adaptations to similar environments) can cause problems
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Molecular systematics
Analysis of genetic data, such as DNA and amino acid sequences, to identify and study genetic homologies and propose phylogenetic trees
DNA and amino acid sequences from closely related species are more similar to each other than to sequences from more distantly related species
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Cladistics
Study and classification of species based on evolutionary relationships
Cladistic approach discriminates among possible phylogenetic trees by considering the various possible pathways of evolutionary changes and then choosing the tree that requires the least complex explanation for all of the available data
Phylogenetic trees or cladograms
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Cladistic approach compares homologous traits, also called characters, which may exist in two or more character states
Shared primitive character or symplesiomorphy Shared by two or more different taxa and inherited
from ancestors older than their last common ancestor Shared derived character or synapomorphy
Shared by two or more species or taxa and has originated in their most recent common ancestor
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A
B
GD E F
C
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Branch point – 2 species differ in shared derived characters
Ingroup – group we are interested in Outgroup – species or group of species
that is assumed to have diverged before the species in the ingroup
An outgroup will lack one or more shared derived characters that are found in the ingroup
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(b) Cladogram based on morphological traits
Lamprey Salmon Lizard Rabbit
(a) Characteristics among species
Notochord Yes Yes Yes Yes YesNo Yes Yes Yes YesNo No Yes Yes YesNo No No Yes YesNo No No No
VertebraeHinged jawTetrapod
Lancelet Lamprey Salmon Lizard Rabbit
YesMammaryglands
Vertebrae
Hinged jaw
Tetrapod
Lancelet
Notochord
Mammaryglands
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Cladogram can also be constructed with gene sequences
7 species called A-G A mutation that
changes the DNA sequence is analogous to a modification of a characteristic
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Constructing a cladogram
1. Choose species
2. Choose characters
3. Determine polarity of character states Primitive or derived?
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4. Analyze cladogram based on All species (or higher taxa) are placed on tips in the
phylogenetic tree, not at branch points Each cladogram branch point should have a list of one
or more shared derived characters that are common to all species above the branch point unless the character is later modified
All shared derived characters appear together only once in a cladogram unless they arose independently during evolution more than once
5. Choose the most likely cladogram among possible options
6. Choose a noncontroversial outgroup as root
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Principle of parsimony
Preferred hypothesis is the one that is the simplest for all the characters and their states
Challenge in a cladistic approach is to determine the correct polarity of events It may not always be obvious which traits are primitive
and came earlier and which are derived and came later in evolution
Fossils may be analyzed
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Example
4 taxa (A-D) A is the outgroup
Has all the primitive states
3 potential treesTree 3 requires fewest
number of mutations so is the most parsimonous
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According to the principle of parsimony, tree
number 3 is themore likely choice
because it requires only five mutations.
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Molecular clocks Favorable mutations rare and detrimental mutations
eliminated Most mutations are neutral If neutral mutations occur at a constant rate they can be
used to measure evolutionary time Longer periods of time since divergence allows for a
greater accumulation of mutations Not perfectly linear over long periods of time
Not all organisms evolve at the same rate Differences in generations times
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Nucleotidedifferences ina homologousgene betweendifferent pairsof species
Evolutionary time since divergence of pairs of species(millions of years)
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Primate evolution example
Evolutionary relationships derived by comparing DNA sequences for cytochrome oxidase subunit II Tends to change fairly rapidly on an evolutionary
timescale 3 branch points to examine (A, D, E) Ancestor A
This ancestor diverged into two species that ultimately gave rise to siamangs and the other five species
23 million years for siamang genome to accumulate changes different from other 5 species
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Ancestor D This ancestor diverged into two species that eventually
gave rise to humans and chimpanzees Differences in gene sequences between humans and
chimpanzees are relatively moderate
Ancestor E This ancestor diverged into two species of chimpanzees Two modern species of chimpanzees have fewer
differences in their gene sequences
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Cooper and Colleagues Extracted DNA from Extinct Flightless Birds and Modern Species to Propose a New Phylogenetic Tree
Ancient DNA analysis or molecular paleontology Under certain conditions DNA samples may be
stable as long as 50,000 – 100,000 years Discovery based sciences- gather data to
propose a hypothesis Sequences are very similar New Zealand colonized twice by the ancestors of
flightless birdsFirst by moa ancestor, then by kiwi ancestor
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Horizontal gene transfer
Any process in which an organism incorporates genetic material from another organism without being the offspring of that organism
Vertical evolutionChanges in groups due to descent from a
common ancestor
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Due to Horizontal Gene Transfer, the Tree of Life Is Really a “Web of Life”
Vertical evolution involves changes in species due to descent from a common ancestor
Horizontal gene transfer is the transfer of genes between different species
Significant role in phylogeny of all living species Still prevalent among prokaryotes but less common
in eukaryotes Horizontal gene transfer may have been so
prevalent that the universal ancestor may have been a community of cell lineages
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Bacteria Archaea Eukarya
Common ancestral community of primitive cells
AnimalsFungi Plants
Horizontal gene transfer
KEYVertical evolution
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