AP Biology : 7.7 Common Ancestry- Exam Style questions with Answer- FRQ

Question

Three new species (A, B, and C) of fossilized crocodile are discovered. The characteristics of these species are compared to those of the saltwater crocodile, Crocodylus porosus. The saltwater crocodile is found in Southeast Asia and Australia, and adults range in length from 5.5 to 5.8 meters and have 66 teeth. Data comparing the characteristics of the three fossilized crocodile species are shown in the table.

(a) Based on the data given, identify the fossil species that has the most in common with Crocodylus porosus.
(b) The number of teeth in a crocodile jaw correlates with increased predator efficiency. Identify the crocodile(s) that would be less efficient predators than C. porosus.
(c) Evaluate the claim that species B shares a more recent common ancestor with C. porosus than do species A or species C.
(d) Explain your reasoning for your response from part (c).

▶️Answer/Explanation

Ans:

(a) Species C has the most in common with Crocodylus porosus because they are both found in Asia, the number of teeth in species C is closest to the number of teeth in C. porosus, and the DNA sequence of species C has the highest percentage of homology with C. porosus.
(b) Species A and species B would be less efficient predators than C. porosus because they both have fewer teeth than C. porosus.
(c) The data do not support the claim that species B shares a more recent common ancestor with C. porosus than do species A or species C. In fact, species C has the most in common with C. porosus.
(d) The DNA from species B only has 90% homology with the DNA from C. porosus, while species C’s DNA has 98% homology with the DNA from C. porosus. Thus, species C likely has a more recent common ancestor with C. porosus than species B does.

Question

Terrestrial vertebrates are tetrapods (have four limbs). Tetrapods include amphibians, reptiles, birds, and mammals. Snakes are reptiles, but they lack limbs.
Describe the common ancestor of the tetrapods and explain why all terrestrial vertebrates are tetrapods.
Explain how snakes fit into this picture.
• Describe a piece of evidence that would support the purported relationship between snakes and the rest of the terrestrial vertebrates.

▶️Answer/Explanation

Ans:

The common ancestor of all the terrestrial vertebrates, including snakes, is a lobe-finned fish. This type of fish has fleshy (muscular) paired fins joined to the body by a single bone that can rotate in the joint socket. The ability to rotate their fleshy fins, instead of just wave them back and forth, allowed them to make the transition onto land. This joint eventually gave rise to your shoulder socket by divergent evolution. The lobe-finned fishes have paired appendages: one pair is anterior (toward the head) and the other, posterior (toward the tail). Our arms and legs derive from these
paired appendages. Tiktaalik, a genus of extinct sarcopterygian (a group that include the coelacanths, lungfish, and tetrapods), is the most recently discovered ancestor that “walked” out of water about 360 million years ago (the late Devonian). Tiktaalik is considered the link between fish and amphibians. There are a small but significant number of terrestrial vertebrates that lack legs. These include snakes, several species of legless lizards, and caecilians, but there are several more. Limblessness was selected for in these organisms. Let’s not forget the cetaceans, the whales and dolphins, which descended from terrestrial vertebrates. These species are still descendants of the four-limbed ancestor, but the reduction of limb size (even to no apparent limbs at all) would be accompanied by vestigial structures. For example, snakes have tiny hind leg bones buried in muscles toward their tail end.

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