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Keys\keys & trees lo2.ppt
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What are Dichotomous Keys?
 a method for determining the identity of something (like the name
of a butterfly, a plant, a lichen, or a rock) by going through a series
of choices that leads the user to the correct name of the item.
 A dichotomous key is a listing of characteristics, such as structure
and behavior, organized in such a way that an organism can be
identified or classified.
 Think of a dichotomous key as a type of scavenger hunt.
 Dichotomous means "divided in two parts".
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Slide 1
What are Dichotomous Keys?
What are Dichotomous Keys?
Duration: 00:00:36
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 a method for determining the identity of something (like the name
of a butterfly, a plant, a lichen, or a rock) by going through a series
of choices that leads the user to the correct name of the item.
 A dichotomous key is a listing of characteristics, such as structure
and behavior, organized in such a way that an organism can be
identified or classified.
 Think of a dichotomous key as a type of scavenger hunt.
 Dichotomous means "divided in two parts".
Notes:
What are dichotomous Keys?
These keys are a method for determining the
identity of something like the name of a butterfly,
a plant, or a rock. This is accomplished going
through a series of choices that leads the user to
the correct name of the item. A dichotomous key
is a listing of characteristics, such as structure
and behavior, organized in such a way that an
organism can be identified or classified.
Think of a dichotomous key as a type of
scavenger hunt. Dichotomous means "divided in
two parts".
Slide 2
Using a dichotomous key
Duration: 00:00:39
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Using a dichotomous key
At each step of the process of using the key, the user is given two
choices; each alternative leads to another question until the item
is identified.
1a. If the leaves are flat….go to question 4.
1b. If the leaves are needle-like….go to question 2.
2a. Are the needles in a bunch? Go to question 5
2b. Are they spread along the branch?“…pine tree
Eventually, when enough questions have been answered, the identity
of the tree is revealed.
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Notes:
Using a dichotomous key
At each step of the process of using the key, the
user is given two choices; each alternative leads
to another question until the item is identified.
For example
1a. If the leaves are flat….go to question 4.
1b. If the leaves are needle-like….go to question
2.
2a. Are the needles in a bunch? Go to question
5
2b. Are they spread along the branch?“…pine
tree
Eventually, when enough questions have been
answered, the identity of the tree is revealed.
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Slide 3
Dichotomous Key Example
Duration: 00:01:06
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DICHOTOMOUS KEY EXAMPLE
1. a. tail fins are horizontal—whale………………….go to 2
b. tail fins are vertical—fish……………………….go to 3
2. a. has teeth or tusk—toothed whale………………..go to 4
b. has no teeth………………………...BALEEN WHALE
3. a. has gill slits behind mouth—shark…….………...go to 5
b. has no gill slits……………………..NONSHARK FISH
4. a. black with white underside………….KILLER WHALE
b. tusk, gray with dark spots………………....NARWHAL
5. a. head is hammershaped……..HAMMERHEAD SHARK
b. tail is half the body length……….THRESHER SHARK
Notes:
Look at this key to figure out the name of the
whale below. You probably already know this is a
killer whale. But lets use the key to confirm that
this is a killer whale. We need to start at the top.
Number 1. Every time you use a key you
ALWAYS start at the top. Ok so when we look at
the key we see we have 2 choices. a. tail fins are
horizontal—whale………………….go to 2
or b. tail fins are vertical—
fish……………………….go to 3. Being that the
fins are on the horizontal we know this is a whale
and we need to go to number 2. Now lets look at
number 2. We have to chose from 2 statements.
a. has teeth or tusk—toothed
whale………………..go to 4 or b. has no
teeth………………………...BALEEN
WHALE
Being this whale has teeth we are going to go to
4.
Now lets look at 4. Again we have 2 choices. a.
black with white underside………….KILLER
WHALE OR b. tusk, gray with dark
spots………………....NARWHAL
Being that this whale is black with a white
underside we know it is a killer whale.
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Slide 4
Reading phylogenetic trees
Reading phylogenetic trees
Notes:
Another tool scientist use are called Phylogenic
trees. A phylogeny, or evolutionary tree,
represents the evolutionary relationships among
a set of organisms or groups of organisms,
called taxa that are believed to have a common
ancestor.
Duration: 00:00:19
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 A phylogeny, or evolutionary tree, represents the evolutionary
relationships among a set of organisms or groups of organisms, called
taxa (singular: taxon) that are believed to have a common ancestor.
Slide 5
Tips, Internal Nodes, Edges
Tips, Internal Nodes, Edges
Duration: 00:00:31
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 The tips of the phylogenetic tree represent groups of descendent taxa
(often species)
 The internal nodes of the tree represent the common ancestors of those
descendents.
 The tips are the present and the internal nodes are the past.
 The edge lengths in some trees correspond to time estimates –
Notes:
Lets look at how to read one of these keys.
The tips of the phylogenetic tree represent
groups of descendent taxa (often species)
The internal nodes of the tree represent the
common ancestors of those descendents.
The tips are the present and the internal nodes
are the past.
The edge lengths in some trees correspond to
time estimates – evolutionary time.
evolutionary time.
Slide 6
Sister Groups and a common
ancestor
Duration: 00:00:35
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Notes:
From these trees we can see sister groups and
which organisms have a common ancestor. Two
descendents that split from the same node are
called sister groups.
In the trees above, species A & B are sister
groups — they are each other's closest relatives;
which means that they have a lot of evolutionary
history in common and very little evolutionary
history that is unique to either one of the two
sister species and that they have a common
ancestor that is unique to them.
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Slide 7
Equivalent trees
Duration: 00:00:22
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Notes:
There are places on the tree where organisms
are equivalent. For any speciation event on a
tree , the choice of which lineage goes to the
right and which one goes to the left is arbitrary.
These three phylogenies are therefore
equivalent.
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