Download D.5 HL Phylogeny and Systematics D.5.5 Define clade and

D.5 HL Phylogeny and Systematics
D.5.5 Define clade and cladistics
A clade is a branch of a phylogenetic tree containing all the organisms descended from a
particular common ancestor.
In phenetics, the answer to this question is that the measurable similarities and differences of
anatomy should be used to arrange species into dichotomously branching trees. The product is a
dendrogram.
In cladistics, classification is based on when the branches arise in the taxonomic tree.
Cladistics is a system of analysis of relatedness. The product is a cladogram.
The differences between phenetics and cladistics are illustrated by lizards, crocodiles and birds,
for example. Lizards and crocodiles resemble each other more than either resemble the birds, but
crocodiles and birds share a common ancestor
D.5.6 Distinguish with examples between analogous characteristics, and homologous characteristics
Example of Analogous characteristics: resemblances between the wings of a bird and the wings of an insect are
superficial. While both are aerofoils (structures that generate lift when moved though the air), they are built from
different tissues and have different origins in development of the body. We say that the wings of birds and insects are
analogous structures.
Examples of Homologous characteristics: the limbs of all vertebrates suggest they are modifications of a common plan,
the pentadactyl limb, and indicate that vertebrates can be classified together. Structures built to a common plan, but
adapted for different purposes, are homologous structures. Another example is the structure of eyes. They are present
in both vertebrates and invertebrates.
D.5.7 Outline the methods used to construct cladograms and the conclusions that can be drawn from them
1. Make a list of all the organisms to be included in the cladogram
2. List as many as possible the characteristics the organisms possess. From this list, many of the traits will be clearly
derived characteristics. Examples of characteristics to consider are:
 Eukaryotic
 Backbone
 Amniote egg
 Limbs (presence or quality)
 Hair
 Opposable thumbs
 Multicellular
 Segmented body
 Jaws
 Placenta
 Mammary glands
 DNA sequences
 Biochemical data e.g. haemoglobin modifications
3. Pick the one characteristic common to all organisms. This ancestral trait is considered the primitive characteristic.
4. Make a table showing the derived characteristics along the top row and the names of the organisms in the first
column.
Example:
Multicellular




Sponge
Sailfish
Wombat
Elephant
Vertebral Column
X



Hair
X
X


Placenta
X
X
X

Totals
1
2
3
4
5. The cladogram is constructed with the first branch from the bottom belonging to the organism with the fewest
derived traits (the sponge). The organism with the most derived characteristics goes to the top of the last branch
wombat
elephant
sailfish
sponge
Oops. This is a bit sloppy. Everything should be on the same level. Oh well. I’m sure you guys can do better.
Conclusions:
1. Organisms whose branches start at the bottom of the cladogram are the earliest ones to have evolved and the ones at
the top are the ones which have evolved most recently among the organisms considered in the cladogram.
2. Each time there is a point where a branch forks into two, a split occurred between species to develop into 2 lineages.
This splitting point on a cladogram is called a node and it shows where a new species (and therefore a new clade) was
founded.