Download CLADISTICS: UNRAVELING EVOLUTION

CLADISTICS: UNRAVELING
EVOLUTION
Major groups of organisms:
Prokaryotes (Bacteria, Archaea)—no nucleus,
unicellular and simple multicellular
Protists—eukaryotes, ±multicellularity, ±motility,
±photosynthesis, various life cycles
Plants—eukaryotes, photosynthetic, sporic life cycles
Fungi—eukaryotes, simple multicellular, non-motile,
zygotic life cycles
Animals—eukaryotes, consumers, gametic life cycles
How are these organisms related?
What was the path of evolution to each?
What did the ancestors look like?
How do we figure out the answers?
Contrast terms:
Adaptation/natural selection: change in the
distribution of characters of a population
Speciation/divergence: emergence of two (or
more?) species from one ancestor species
Phylogeny: "origin of groups", groups=species,
genera, families, etc., the pattern of evolution;
formation of evolutionary "trees" (based on shared
characteristics, assuming lines of inheritance of the
genes for the characteristics)
Taxonomy: classification of organisms into groups
Phylogenetic systematics: taxonomy to
follow/illuminate phylogeny
Cladistics: mathematical system for phylogenetic
systematics
Assumptions of phylogeny
Common ancestors
Inheritance of traits
Speciation events lead to divergence of genes, traits
Thus finding traits that are similar implies a common
ancestor; traits that are different implies a speciation
event
For some "character," E,F,G share a "character state"
different from that of A,B,C,D (e.g., character =
vascular system, character states = secondary growth
(EFG) and no secondary growth (ABCD))
(e.g. character = organization of genes; character
states = nucleus, no nucleus)
Problems in recognizing phylogeny
Variable character states (environmental, not
hereditary)
Convergence of character states (convergent
adaptation)
Hybridization (anastomosis of genetic lines)
Examples: desert plants
Ocotillo: has leaves, but only in the rainy season
(environmentally determined character state)
Cactus vs euphorbes: adaptation to different deserts
(convergent evolution)
The most confusing
anastomosis: early evolution
of photosynthetic eukaryotes
by endosymbiosis
endosymbiosis
green algae
brown algae
red algae
chloroxybacteria
cyanobacteria
.
green algae
brown algae
red algae
chloroxybacteria
cyanobacteria
How can we "unravel" the path of evolution?
Traditional solutions: use lots of traits (“characters”),
judgment (ignore some traits: human, rattlesnake
Hb); geographical location (cactus vs euphorbes)
New solution: use molecular (gene sequence)
evidence, cladistics
Molecular evidence = lots of characters (each base
position in a gene), so convergence can be
discounted on a statistical basis
Cladistics: find most parsimonious trees (least
changes)
(See "Do a cladistic analysis" in Rost et al.)
Step 1: Identify conserved regions:
Step 2: Align conserved regions:
Step 3: Identify new conserved regions:
Step 5: Use non-conserved regions to build
"character matrix":
Step 6: Construct all possible cladograms (there are
15 possibilities for this matrix):
Step 7: Identify the base sequence changes needed
for each cladogram; choose the one with the least:
Step 8: Use an "outgroup" to "root" the cladogram
and show phylogeny:
Cladistic methods can also be applied to protein sequences:
(These are the amino acid sequences of cytochrome c)
Summary
•Speciation events can be used to suggest the course of
evolution
•Cladistics is a method for determining relationship trees
and infer speciation events
•Cladistic methods can be applied to DNA sequences, protein
sequences (and traditional character matrixes)