Download DNA base sequences

5.4 Cladistics
Essential idea: The ancestry of groups of species
can be deduced by comparing their base or
amino acid sequences.
The images above are both cladograms. They show the
statistical similarities between species based on their
DNA/RNA. The cladogram on the left show bacteria
classified using the 16 S ribosomal RNA gene. The
cladogram on the right uses retroviral sequences which
have been incoporated into ape DNA and then inherited
to build a picture of the relationships between apes. It’s
an Interesting fact that humans have around 100,000
endogenous retroviruses incorporated into their DNA.
By Chris Paine
https://bioknowledgy.weebly.com/
http://www.faraday.st-edmunds.cam.ac.uk/CIS/Finlay/images/image3.jpg
http://openi.nlm.nih.gov/imgs/512/196/3364987/3364987_pone.0038062.g002.pn
5.4.U5 Cladograms are tree diagrams that show the most probable sequence of divergence in clades.
A cladogram is a tree diagram showing the similarities and
differences between different species
The branching points on
cladograms are called nodes.
Nodes denote a speciation
event when a common
ancestor splits into two, or
more, species.
Cladograms are mostly based on DNA base
sequences or the amino acid sequences in a
protein.
Software applications combine multiple
sequences* to find the most probable pattern of
divergence.
*Cladograms can show different relationships between species depending on the DNA/amino acid
sequences used in the analysis. It is important therefore to use multiple sequences when trying to
identify the true evolutionary relationships between species.
http://commons.wikimedia.org/wiki/File:The_Ancestors_Tale_Mammals_cladogram.png
5.4.U1 A clade is a group of organisms that have evolved from a common ancestor.
Species may evolve over time to form a new species. Consequently there are
groups of species derived from a common ancestor. Such groups are called Clades.
Clades can be small
Clades can be large (large clades
can contain smaller clades)
Not all members of a
clade may exist today, e.g.
dinosaurs
Common ancestors often do not exist, as they have
evolved into modern species or have become extinct.
http://evolution.berkeley.edu/evosite/evo101/images/reptileclade1.gif
5.4.A1 Cladograms including humans and other primates.
The DNA evidence used to build this
cladogram of primates,
including humans. can also be used to
estimate when species diverged and
hence when the common ancestor
existed.
https://sites.google.com/a/canacad.ac.jp/sl-hl-1-biology-4-ferguson/_/rsrc/1403598223535/unit-11-ecology/5-4-cladistics/primate%20cladogram.jpg
5.4.U2 Evidence for which species are part of a clade can be obtained from the base sequences of a gene or the
corresponding amino acid sequence of a protein. 5.4.U3 Sequence differences accumulate gradually so there is a
positive correlation between the number of differences between two species and the time since they diverged from
a common ancestor.
Mutations in DNA that persist and are inherited* occur at
a predictable rate#.
For example mitochondrial DNA from humans
and primates has been completely sequenced and
used to construct cladogram between them.
The rate at which mutations occur at can be
used as a molecular clock to calculate how long
ago species diverged.
If the DNA base sequences or two species are
similar …
… then few mutations have occurred …
… therefore the species only diverged relatively
recently
The length of the lines separating species on
cladograms is often used to represent the
estimated time since they diverged.
* In species that reproduce sexually for mutations to be
inherited they must occur duration the creation of sex cells.
#When considering multiple changes in the base sequence
of DNA. The sample size is large enough to estimate a
mean rate of mutation.
https://sites.google.com/a/canacad.ac.jp/sl-hl-1-biology-4-ferguson/_/rsrc/1403598223535/unit-11-ecology/5-4-cladistics/primate%20cladogram.jpg
5.4.S1 Analysis of cladograms to deduce evolutionary relationships.
1. What animal is most closely
related to a squirrel?
1. What animals are more
closely related to humans
that old world monkeys?
3. Tarsiers share a common ancestor with
which animals?
3. Elephant shrews, Elephants, Manatee and
dugong all share a common ancestor.
Which animal diverged from the ancestor
first?
http://commons.wikimedia.org/wiki/File:The_Ancestors_Tale_Mammals_cladogram.png
5.4.U4 Traits can be analogous or homologous.
Homologous structures are similar
Analogous structures are similar
because of common ancestry
because of convergent evolution
e.g. pentadactyl limbs
e.g. the human eye and the octopus eye
Both have similar structures to provide similar
function, but have evolved independently.
In the past relying on homologous structures to classification has led to mistakes in classification.
Distinguishing between homologous and analogous structures is not straightforward
http://evolution.berkeley.edu/evolibrary/images/limbs_homology_not.gif
5.4.A2 Reclassification of the figwort family using evidence from cladistics.
5.4.U6 Evidence from cladistics has shown that classifications
of some groups based on structure did not correspond with
the evolutionary origins of a group or species.
DNA evidence identified the different
common ancestors
The similarities among the members of the
old Scrophulariaceae family were
superficial.
The general flower shape and form of the
seed capsule evolved many times from
different ancestors (convergent evolution).
Different plant species adapted to similar
pollinators and adopted similar seed
dispersal strategies.
DNA evidence has cast doubt on some
traditionally classified groups, but equally it has
given support to others.
Though DNA evidence is very important, it must
be remembered that is not the only evidence
used to decide evolutionary relationships.
http://www.hopspress.com/Books/Images/Scrophulariaceae.jpg