25.3
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Concept 25.3: Phylogenetic systematics informs the construction of phylogenetic
trees based on shared characteristics
Maggie, Will, Namroo, Austin, Rex
Cladistics
-Cladogram: shows patterns of shared characteristics
-Clade: a group of species including ancestral and descendants in the tree
Groupings in Phylogenic Trees
I. Monophyletic: regular clade II. Paraphyletic: ancestor is present, but
not all descendants III. Polyphyletic: descendants are
present, but no ancestor
D
C
E G
F
B
A
J
I
KH
D
C
E
B
G H
F
J
I
K
A
D
C
B
E G
F
H
A
J
I
K
Shared primitave and shared derived characters
“Character” refers to any feature that a particular taxon process
Shared primative character – A character that is shared beyond the taxon we are trying to define
Shared derived character – An evolutionary novelty unique to a particular clade
Outgroups
Outgroup comparison is used to differentiate between shared derived characters and shared primitive characters
Ingroup – The various species we are studying
Outgroup – A species or group of species that is closely related to the ingroup.
Phylogenetic Trees and Timing
Phylograms – present sequences of events relative to each other.
Ultrametric Trees – Present sequences based on the actual times they occurred.
Phylograms
Length of branch corresponds to the amount of changes that occurred.
A long branch means more changes in DNA.
Drosophila
Lance
let
Amph
ibia
n
Fish
Bird
Human
Rat
Mou
se
Ultrametric Trees
The lengths of the branches are the same lengths for each lineages.
The tree draws information from the fossil record.
Droso
phila
Lanc
elet
Amph
ibia
n
Fish
Bird
Hum
an
Rat
Mou
se
Cenozo
ic
Meso
zoic
Pa
leozo
ic
Pro
tero
zoic
54
2
25
1
65
.5
Mill
ions
of
years
ago
Maximum Parsimony and Maximum Likelihood
Systematists:› Can never be sure of finding the single best tree in a
large data set.› Narrow the possibilities by applying the principles of
maximum parsimony and maximum likelihood.
According to the principle of maximum parsimony, we should first investigate the simplest explanation that is consistent with the facts.
The principle of maximum likelihood states that, given certain rules about how DNA changes over time, a tree can be found that reflects the most likely sequence of evolutionary events.
Among phylogenetic hypotheses› The most parsimonious tree is the one that requires the
fewest evolutionary events to have occurred in the form of shared derived characters.
Human Mushroom Tulip
40%
40%
0
30% 0 Human
Mushroom
Tulip
(a) Percentage differences between sequences
0
Figure 25.14 (a)
Tree 1: More likely
(b) Comparison of possible trees
Tree 2: Less likely
15%
5%
15% 20%
5%
10%
15%
25%
Figure 25.14 (b)
Phylogenetic Trees as Hypotheses
Phylogenetic trees represent a possible way of how the species in it are related.
Phylogenetic hypotheses can change with new evidence.
Usually the most parsimonious tree is most likely
Analogy-Homology issue The more matching base pairs the less
probability they evolved independently
Parsimony more reliable if with longer segments.
Accidentally mistaking an analogy for a homology is less likely to affect the tree if the clades are defined by several defined characters.
Strongest hypotheses are supported by lots of morphological and molecular evidence and fossil evidence.
Pg.502-503
APPLICATION In considering possible phylogenies for a group of species, systematists compare molecular data for the species. The most efficient way to study the various phylogenetic hypotheses is to begin by first considering the most parsimonious—that is, which hypothesis requires the fewest total evolutionary events (molecular changes) to have occurred.
TECHNIQUE Follow the numbered steps as we apply the principle of parsimony to a hypothetical phylogenetic problem involving four closely related bird species.
SpeciesI
SpeciesII
SpeciesIII
SpeciesIV
I II III IV I III II IV I IV II III
Sites in DNA sequence
Three possible phylogenetic hypothese
1 2 3 4 5 6 7
A G G G G G T
G G G A G G G
G A G G A A T
G G A G A A G
I
II
III
IV
I II III IV
A G G G
GG
G
Bases at site 1 for each species
Base-changeevent
1 First, draw the possible phylogenies for the species (only 3 of the 15 possible trees relating these four species are shown here).
2 Tabulate the molecular data for the species (in this simplified example, the data represent a DNA sequence consisting of just seven nucleotide bases).
3 Now focus on site 1 in the DNA sequence. A single base-change event, marked by the crossbar in the branch leading to species I, is sufficient to account for the site 1 data.
Species
THE END.
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