For this exercise you will use 7 mammals to look at evolutionary relationshis.
When people study evolutionary relationships of different species, they often use a large number of features. These features are called characters once they have been defined in a way that they can be given numerical scores. To generate an hypothesis of the evolutionary tree, we calculate the simplest evolutionary explanation for the distribution of characters in the species being studied. On a large scale (like all Mammals), this has been shown to work well by comparisons between DNA and morphology analyses. There is less consensus for groups where interbreeding might be possible.
For this exercise you will use a small number of specimens and features to try the method. Warning: this is not really a valid analysis because you will not be using a large enough set of characters or specimens, but it will give you a solid example of how the process works and some of its limitations.
Characters are organism features used to study evolutionary relationships that have been defined in a special way. ÔCharactersÕ are defined objectively so that the taxa presenting them can be clearly distinguished as having, or not having, the character. When a character has 2 states, present or not present, it is called a binary character-present or absent. In these case the scores usuallyhave the value 0=not present, 1=present.
In order to 'score' a character so that a computer can compute the simplest (most parsimonious) tree for the characters, you must define them in a manner that allows you to give them numerical scores. For example, if you used beak size as a ‘character’ in an analysis of hawks and eagles, you might objectively define a character as: Length from tip of beak to corner of mouth: score=0 for <1cm, score=1 for >1cm. This way you can run a computer program that will detect the most parsimonious evolutionary branching pattern (often called a tree) based on the taxa and characters.
In some cases a character might have more than 2 possible values, These are called multistate characters and they have more values than just present or absent. In multistate characters, each value must be clearly defined.For example,if you were analyzing cranial capacity you might want to score it so that there were 4 possible states (0=very small, 1=small 2=medium, 3=large).
For this exercise, 10 features are roughly outlined for two different groups, one set for hominids and one set for mammals. It is up to you to define the characters in a way that you can score them and then fill in a table that will allow you to input the numbers into a computer.
This table is called a data matrix. I provide you with the cranial capacities, but you will need to look at the casts to define the others. You will have to decide on how to score cranial capacity. You might, or might not, want to make cranial capacity a Ômultistate characterÕ.
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Cranial capacities |
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Rat |
~7 cc |
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Skunk |
~15 cc |
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Deer |
~115 cc |
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Gazelle |
~80 |
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Pig |
~100cc |
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Hyaena |
~110 cc |
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Chimp |
~375 cc |
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Homo sapiens |
~1,350 cc |
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Rat |
~1:40 |
Skunk |
~1:110 |
Deer |
~1:600 |
Gazelle |
~1:600 |
Pig |
~1:500 |
Hyaena |
~1:600 |
Chimp |
~1:70 |
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Homo sapiens |
~1:40 |
Cranial capacity (large 1, medium 2, small 3)
Brain size/body mass
Number of digits on forelimb(1, 2, 3, 4, 5)
Horns or ossicones are present in males (yes 1, no 2)
Number of neck vertebrae (less than 7 = 1, 7 = 2, more than 7 =3)
Postorbital bar (yes 1, no 2)
Sharp, pointed canine (yes 1, no 2)
Honing (self-sharpening) canine (yes 1, no 2)
Sagittal crest (yes 1, no 2)
Tail (yes 1, no 2)