Chapter 25: Phylogeny and Systematics
Valuable Information Can Be Derived From Classifying Organisms
•
A parasitic flatworm, Schistosoma, causes a blood infection in over 200
million people in S. America, Africa, China, Japan and Southeast Asia.
•
This flatworm inhabits two organisms during its lifecycle.
• a freshwater snail for one part
• people for another part. The larvae swim from the snail and penetrate
the skin when a person is contact with infected water. The flatworm
matures and lives in the abdominal blood vessels. It causes a slow
death.
1
Intro: Schistosoma
•
For most of the 20th century only one species was known to infect
humans, Schistosoma japonicum, transmitted by a single species of snail
with the genus Oncomelania.
•
In the 1970’s, a different snail was discovered to be transmitting the
parasite to human in the Mekong River in Laos.
•
Additional research into the types of snails in SE Asia lead to the
conclusion that S. japonicum was actually a cluster of at least 6 species.
•
They found that the evolutionary diversification from an ancestor
produced these modern snails. Three of these could host the
Schistosoma. Ten have a genetic trait that allows them to resist invasion
by the parasite.
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Intro: Schistosoma
•
So biologists use this information to determine whether or not a newly
discovered species is likely to be a host for the parasite.
•
Controlling the presence of just these types of snails and not all
freshwater snails would then be undertaken.
•
Systematics, the process of inferring evolutionary relationships among
organisms, allows us to:
• determine the evolutionary relationships among the snails that are
hosts.
• determine the number of times the genes resisting infection in the
snails arose.
3
How Are Phylogenetic Trees Reconstructed?
Evolutionary Agents Have Been At Work
•
Mutations
•
Migration of alleles or genetic drift
•
Natural Selection
•
Bottleneck Effect and Founder Effect
Phylogeny is the history of descent of a group of organisms from a common
ancestor.
•
Through the process of speciation, we organize lineages of organisms as
branching “trees.”
•
These phylogenetic trees show us the order in which lineages split.
4
Phylogeny
•
The phylogenetic trees are based on evolutionary changes in the traits of
organisms.
• Darwin noticed that closely related species, those that shared a
common ancestor, were very similar. That is, they would share many
characteristics that they inherited from the common ancestor.
•
Systematists expect traits inherited from a common ancestor in
the distant pass to be shared by a large number of species.
•
The sharing of traits by a group of species indicates that they
MAY be descendents of a common ancestor.
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Phylogeny
All present-day taxa descending from a
common ancestor are shown at the right
—
+
Most Ancient
Ancestor
Ancestor
Snail
The distance between branches means nothing
—
+
Species that can transmit
Schistosoma are shown as (+)
A split indicates
a division of a
population into
two, forming new
species.
The curves indicate
nothing about
evolution
But the positions of the
branches on the time axis tells
us the order in which
populations split.
The red inhibit icons indicate
evolution of genes blocking the
transmission of Schistosoma to
humans.
+
—
—
—
—
—
—
—
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—
Phylogeny
Homologous Traits
•
any two features descended from a common ancestral feature.
•
These features could be anatomical structures, behavior patterns,
nucleotides in a DNA sequence
•
Traits that are shared by most or all organisms in any lineage being
studied are likely to have been inherited relatively unchanged from an
ancestor that lived very long ago.
• All vertebrates have a vertebral column and fossil ancestral vertebrates
also had one. Therefore it is homologous.
7
Phylogeny
Derived Traits
•
Derived traits differ from their ancestral form.
•
Evolutionists infer the state of the trait in some ancestor and then
determine how it has been modified. This is not easy because:
• Similar features may evolve similarly due to similar environmental
pressures (Convergent Evolution). Wings of bats, birds evolved
independently.
• Similar developmental processes occurred in distantly related
organisms. (Parallel Evolution). Wing development in butterflies and
moths produce similar patterns on their wings.
• Sometimes a character will revert from a derived state back to the
ancestral one. (Evolutionary Reversals). Most frogs lack teeth on their
lower jaw, but ancestors of frogs had teeth. One frog genus has reevolved teeth in its lower jaw.
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Phylogeny
Identifying Ancestral Traits
•
This is difficult because traits can become so different from the ancestral
type.
•
One way to distinguish the ancestral trait from a derived trait is that the
ancestral trait is not only in the group you are studying (focal group) but
also in an outgroup, a lineage that branched off from the focal group
below its base on an evolutionary tree.
•
Traits found only in the focal group then must be derived traits.
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Phylogeny
Reconstructing a Simple Phylogeny
Derived Trait
Taxon
Jaws
Lungs
Claws/
nails
Feathers
Fur
Mammary
Glands
4-Chambered
Heart
Hagfish
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-
-
-
-
-
-
Perch
+
-
-
-
-
-
-
Salamander
+
+
-
-
-
-
-
Lizard
+
+
+
-
-
-
-
Crocodile
+
+
+
-
-
-
+
Pigeon
+
+
+
+
-
-
+
Mouse
+
+
+
-
+
+
+
Chimpanzee
+
+
+
-
+
+
+
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Phylogeny
• Hagfish are considered from distantly related to the other vertebrates than the
other vertebrates are to each other. Hagfish will therefore be our outgroup.
• Derived traits are those that have been acquired by other members since their
separation from the hagfish.
Forming the Phylogenetic Tree
•
The chimpanzee and the mouse share two characteristics. These are
absent in the outgroup and other species. Therefore we infer they are
derived characteristics from a common ancestor.
Fur; mammary glands
Mouse
Chimpanzee
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Phylogeny
•
The pigeon has one unique trait- feathers. Crocodiles, pigeons, mice and
chimpanzees all have 4-chambered hearts. So this evolved before the
feathers.
Crocodile
4-chambered
heart
Pigeon
Feathers
Fur; mammary glands
Mouse
Chimpanzee
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Phylogeny
•
Claws and Nails are shared by lizards, crocodiles, pigeons, mice and
chimpanzees so this structure was present in an ancestor of all of these
organisms. Since lizards do not have 4-chambered hearts they diverged
along a different lineage.
lizards
Claws/nails
Crocodile
4-chambered
heart
Pigeon
Feathers
Fur; mammary glands
Mouse
Chimpanzee
13
Phylogeny
• Lungs are common between salamander, lizards, crocs, pigeons, mice
and chimps so lungs developed prior to these organisms. But the salamander
does not have claws or nails.
salamanders
Lungs
lizards
Claws/nails
Crocodile
4-chambered
heart
Pigeon
Feathers
Fur; mammary glands
Mouse
Chimpanzee
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Phylogeny
• Jaws are common between perch, salamander, lizards, crocs, pigeons,
mice and chimps so lungs developed prior to these organisms. But the perch
does not have lungs so it must have branched off.
perch
Jaws
salamanders
Lungs
lizards
Claws/nails
Crocodile
4-chambered
heart
Pigeon
Feathers
Fur; mammary glands
Mouse
Chimpanzee
15
Phylogeny
•
Common ancestor had a vertebral column and the hagfish has no derived
traits.
Common
Ancestor
hagfish
perch
Jaws
salamanders
Lungs
lizards
Claws/nails
Crocodile
4-chambered
heart
Pigeon
Feathers
Fur; mammary glands
A particular trait appears after each
branch point.
Mouse
Feathers arose after
the lineage leading to
birds and crocodiles
separated.
Chimpanzee
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Phylogeny
Various Methods of Reconstructing Phylogenetic Trees
1.
Parsimony Principle
a)
The simplest hypothesis is preferred to explain the known facts.
b)
This minimizes the number of evolutionary changes that need to be
assumed for all the traits in a tree.
c)
“The best hypothesis is the one that requires the fewest explanations
using of :”
i.
Convergent Evolution
ii.
Parallel Evolution
iii.
Evolutionary Reversals
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Phylogeny
Various Methods of Reconstructing Phylogenetic Trees
2.
Maximum Likelihood Method
a)
This uses molecular data
b)
It uses computer programs to analyze mutation frequencies.
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Traits Used in Reconstructing Phylogenies
1.
Morphology and Development
a)
Use the size and shape of body parts
b)
Fossil record helps to differentiate between ancestral and derived traits.
c)
Early developmental stages of many organisms are similar.
i.
Sea squirts have a rod of tissue, the notochord, during their larval
stage but it disappears when they are adults. Vertebrates also have
this same rod of tissue so this supports the belief that sea squirts are
more closely related to vertebrates than would be expected by
examination of the adults alone.
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Traits Used in Reconstructing Phylogenies (cont’d)
2.
Molecular Traits
a)
Protein Structure
i.
b)
Determine the number of amino acids for a particular protein that
have changed since the lineages have diverged.
DNA Base Sequences
i.
Chloroplast genes are used extensively in the phylogenetic
relationships among plants
ii.
Medina is used with animals.
iii. 10,000 base pairs of nuclear DNA were sequenced of a nonfunctional
sequence formed in early primate evolution by duplication of a
hemoglobin gene. This revealed a new genus
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A Phylogeny of Anthropoid Primates
622
Common
Ancestor
Spider Monkey (Ateles)
457
128
Rhesus monkey
199
Orangutan
150
94
70
14
Gorilla
92
Chimpanzee
76
Humans
Numbers indicate the number of base-pair changes in the globin
region of DNA. So humans and chimps share a more recent
ancestor with each other than they do with gorillas.
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Figure 25.7 Hierarchical classification
•
Each taxonomic level is more
general than the one below it.
• Carolus Linnaeus devised
this system to assign species to
particular groups but he did
not have an evolutionary theme
in mind.
•
Taxon: simply a grouping of
organisms at a given level. So a
species is a taxon; so is a genus
or a phylum.
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Biological Classification and Evolutionary Relationships
Current Biological Classifications Reflect Evolutionary Relationships
•
Taxonomic groups should be monophyletic.
• A monophyletic group is also called a clade which contains all the
descendants of a particular ancestor and no other organisms.
• A clade is a group of organisms that can be removed from a
phylogenetic tree by one “cut” in the tree.
Common
Ancestor
Common ancestor to D, E, F
D
E
Common ancestor to E, F
F
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Biological Classification and Evolutionary Relationships
• Polyphletic: A taxon consisting of members with more than one recent
common ancestor. (B, C and D)
A
Common
B
Ancestor
C
D
Common ancestor to A,
B and C
Common ancestor to B
and C
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Biological Classification and Evolutionary Relationships
• Paraphyletic taxon includes some but not all descendants of a single
ancestor
A
Common
B
Ancestor
Common ancestor to A,
B and C
Common ancestor to B
and C
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Figure 25.9 Monophyletic versus paraphyletic and polyphyletic groups
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