11.1.2 Building a Cladogram

Question 1

building a cladogram

Answer 1

• Review: The branching nature of a cladogram reflects the evolutionary relationships between taxa (phylogenetic groupings).
• To build a cladogram:
1. Gather the organisms in question.
2. Determine homologies.
· General homologies are shared by all species of a clade.
· Special homologies are shared by only certain taxa of a clade.
3. Determine derived traits.
4. Determine the outgroup (first branch of the cladogram).

Question 2

contrsucting a cladogram

Answer 2

1. Gather the organisms in question. There are countless extinct and extant (still existing) species, and all are related to some degree. The process begins with narrowing the focus and choosing the organisms of interest.
2. Determine homologies. Homologies are equivalent
   structures resulting from a shared ancestry. Each branch point in the cladogram is defined by novel homologies unique to the group of species on that branch. Cladistics distinguishes between two types of homology:
   · General homology is a similarity common to all the organisms in question, including the common ancestor.
   · Special homology is a similarity common to a few of the organisms in question and the ancestor common to their particular branch of the tree.
   - General homology among humans, whales, and cats includes the radius and the ulna of forelimbs. Among cats, special homology includes specialized dentition for a carnivorous lifestyle.
   - Homoplastic (analogous) structures perform similar
   functions, but do not share a recent common ancestor.
   Homoplasy can result in a wrongly constructed cladogram. An example of analogous structures is the bat wing versus the insect wing. Looking at the developmental basis of the insect versus the bat wing reveals no homology between the two structures. As members of the animal kingdom, the two groups are related, although very distantly. Homoplasy can arise because of similar environmental pressures.
3. Determine derived traits. Special homologies, also called derived traits, distinguish a particular branch from the rest of the cladogram.
   - The species pictured on the top of the cladogram comprise the two closely related taxonomic families Ursidae and Procyonidae. All species of both families and the ancestral carnivore that gave rise to the groups share general homologies. Derived traits unique to the family Ursidae include a rudimentary versus a fully developed tail.

Question 3

note

Answer 3

• The outgroup is a species or group of species that are more closely related to each other than to other species in their clade. The outgroup is closely related to the other species being compared, but it branched off earlier on the evolutionary tree.
• Comparing members of the subphylum Vertebrata on the left, the agnathans, or jawless fish, share only ancestral characteristics with the other taxa. The agnathans are the outgroup of the vertebrates. All of the other taxa share derived traits that the agnathans lack.

Question 4

You are trying to construct a phylogenetic tree that will reflect the evolutionary branches among several related bird species found on an island. Which of the following would be most useful?

Answer 4

• Several characteristics that are thought to have evolved after the arrival of the ancestral birds.

Question 5

Which statement below gives the best description of a cladogram?

Answer 5

• a graphic representation of the evolutionary relationship between organisms

Question 6

Homologous traits are

Answer 6

• derived from a common ancestor.

Question 7

Which of the following correctly describes the role of the outgroup in cladistic analysis?

Answer 7

• The outgroup is used to distinguish between general and special homologies

Question 8

Which of the following is not a major way in which phylogenetic trees are used?

Answer 8

• Phylogenetic trees are used to help identify unknown organisms using identification keys.

Question 9

What is the first type of homology to be considered when building a cladogram?

Answer 9

• general homology