Download Cladistics - Cougar science rocks!

CLADISTICS
Topic 5.4
U: A clade is a group of organisms that
have evolved from a common ancestor
Clade = group of organisms that
includes a single ancestor & all
of its descendants
= unbroken lines of evolutionary
descent
Includes:
■ Common ancestor
■ Evolved species now extinct
■ Current species - can be
10,000 (birds) or just 1
(Ginkgo biloba)
Clade or not?
* Clades can be nested (smaller ones inside bigger ones)
* Human species forms a clade - a single branch within larger
clade of hominid lineage, which is a single branch within larger
clade of primate lineage, which is a single branch within larger
clade of mammalian lineage, and so on, back to most
encompassing clade of all: the entire tree of life!
U: Evidence for which species are part of a clade
can be obtained from base sequences of a gene or
corresponding amino acid sequence of a protein
■ If common ancestor is recent, then few differences in base or amino
acid sequence
■ If common ancestor is distant (millions of ya), then many differences
in base or amino acid sequence
U: Sequence differences accumulate gradually so there is a positive
correlation between number of differences between 2 species & time
since they diverged from common ancestor
■ Differences in base sequence are result of mutations
■ Differences accumulate over time
■ Evidence that mutations occur at constant rate, so can be used as a “clock”
■ # of differences correlates to how long ago species split from common ancestor
■ “Molecular Clock” = rate at which mutations occur, giving us a timeline
Based on mitochondrial DNA sequencing
U: Traits can be analogous or homologous
Analogous traits
Homologous traits
■ Similar because of convergent evolution
■ Similar because of similar ancestry
(similar yet evolved independently)
■ E.g. pentadactyl forelimbs
■ E.g. human eye & octopus eye
* Difficult to know if a trait is homo- or analogous, leading to mistakes in classifying
* Must use base or amino acid sequence to determine, instead of
morphology/structure (unless you want to include extinct species!)
U: Cladograms are tree diagrams that show the
most probable sequence divergence in clades
■ Cladogram = tree diagram based on similarities & differences
between species in a clade
■ Based on base or amino acid sequences
■ Principle of parsimony = the simplest explanation is usually the
correct one  http://science.howstuffworks.com/science-vs-myth/31651conspiracy-test-occams-razor-video.htm
■ “An ancestor of mine maintained that when you eliminate the impossible,
whatever remains, however improbable, must be the truth.” -Spock (Star Trek)
■ Clades show the most probable sequence of divergence
cladograms
■ Nodes = branching points (represent hypothetical ancestor)
■ Usually 2 clades branch off at a node, but sometimes more
■ Where do these Pterosaurs fit in? 
App: Cladograms including humans &
other primates
■ Closest relatives of humans are
chimpanzees & bonobos
■ Entire genomes have been
sequenced and compared
■ Molecular clock with mutation rate
of 10-9 yr-1
■ Primates = mammals that have
adaptations for climbing trees
(humans, monkeys, baboons,
gibbons, lemurs)
Primate cladogram
Cladogram: primates & most closely related mammals
Skill: analysis of cladograms to deduce
evolutionary relationships (DBQ p273)
Answers:
U: Evidence from cladistics has shown that classifications of
some groups based on structure did not correspond with the
evolutionary origins of a group of species
■ Cladistics = construction of cladograms and identification of
clades
■ Base and amino acid sequencing is relatively new (end of 20th
century)
■ As a result, some groups have been reclassified, revealing
unnoticed similarities and differences between species
NOS: falsification of theories:
Plant families have been reclassified as a result of
evidence from cladistics.
■ Important process in science: testing of theories and replacement of
theories found to be false with new theories
■ Example: reclassification of plants based on discoveries in cladistics (like
Figworts)
■ Classification of angiospermophytes into families based on morphology by
French botanist Antoine Laurent de Jussieu in 1789
■ Revised repeatedly
App: Reclassification of the figwort family
using evidence from cladistics
■ Until recently, Scrophulariaceae (figwort)
was 8th largest family of angiosperm (out
of 400+ families)
■ Due to cladistics discoveries, 5 clades
had incorrectly been combined into one
family
■ Some families were merged, some
genera were moved to another family
■ Less than half of species stayed in family
■ Figwort family now 36th largest family