Answers
Activity 34.1 What Can We Learn about the Evolution of the Chordates by Examining Modern Chordates?
Fill in the chart on the next two pages to organize the major characteristics of key groups of chordates.
Using the information in the table, answer the questions.
1. a. According to Biology, 7th edition, what three characteristics are common to all chordates?
All chordates have a notochord, pharyngeal gill slits, and a dorsal hollow nerve chord at some stage in their life cycle.
b. Why don’t we find all three of these chordate characteristics in our own bodies?
The notochord and pharyngeal gill slits are present in the developing embryo. However, as development continues, they are replaced by other structures. The notochord is replaced by the vertebral column. The cells associated with the pharyngeal gill slits continue in development to form parts of the jaw and ear structures.
2. The chart organizes the chordates based on grade or shared body plan features. If you compare the characteristics of one group with the next, what key differences separate the groups from each other?
To answer this question, you need to compare the characteristics in a given column/phylum with those in the next. Those characteristics that change from one column to the next represent the key differences that separate the groups. For example, Chondrichthyes are separated from Agnaths by the presence of paired lateral fins and toothlike scales.
3. What unique combination of characteristics defines each group as separate from the others?
If you read down each column, you can determine the characteristics of each phylum.
No single characteristic defines each phylum. Rather, it is the specific combination of characteristics that is unique to and defines each phylum.
4. a. Looking across each of the rows in the chart, what major trends do you see in the evolution of the different organs and organ systems?
This chart is set up to allow you to visualize how the various systems changed in the evolution of the subphyla and classes of chordates. This arrangement also enables you to visualize major trends in this evolution. For example, evolution of limbs began in Chondrichthyes as paired fins. Amphibians evolved pentadactyl limb structure and this became highly modified in the evolution of reptiles, birds, and mammals.
b. Does this analysis provide evidence for or against the statement: “Evolution adds onto or modifies what already exists”? Explain.
As is apparent in this chart, modifications are built upon what already exists. For example, we see an evolution from a two-chambered heart to three- and four-chambered hearts as the classes of chordates evolved.
5 a. What major changes in structure and function are seen in terrestrial groups as compared with aquatic groups?
One of the major changes is in reproduction. Evolution of the amniotic egg freed terrestrial organisms from the water environment for reproduction. Another obvious change is from the use of gills to the use of lungs for respiration on land. Lungs provide more protection from desiccation.
b. Can these changes be related to differences in natural selection on land versus in an aquatic environment? Explain.
Early forms of gills probably evolved in an aquatic system and allowed fish to supplement respiration by gulping air. However, the continued evolution of lung structure to the forms we see in terrestrial organisms today is unlikely to have occurred in an aquatic system. Similarly, where desiccation is not a problem, it is unlikely that a structure like the amniotic egg would have evolved.