Analogy & Homology Example
How Humpback Whales are More Closely Related to Cats Than Sharks
Two different species that possess a homologous trait are humpback whales and domestic cats. Humpback whales are large marine mammals adapted to life in the ocean’s depths. They can grow up to 60 feet long and weigh as much as 40 tons. Despite their large size, whales are very acrobatic, able to leap out of the water before slapping the ocean surface on their way down. Unlike whales, domestic cats are small terrestrial mammals adapted to life on land. They can stand about 8 to 10 inches high and weigh approximately 8 to 12 pounds. Lightweight, cats are fast and flexible, able to stalk prey across a variety of terrain before pouncing on their target.
One common trait that helps explain both species’ maneuverability are their forelimbs. Both species have a humerus (upper arm bone), radius and ulna (forearm bones), carpals (wrist bones), metacarpals (palm bones), and phalanges (finger bones). However, the structure and function of these bones have diverged significantly. In humpback whales, the forelimb bones are relatively short and compact, allowing their flippers to steer, balance, and maneuver through the water. In domestic cats, the forelimb bones are proportionally longer, slender, and more flexible, allowing their arms to walk, climb, and catch prey on land.
The common ancestor for the humpback whale and the domestic cat was a four-legged, land-dwelling mammal that lived over 50 million years ago. This mammal possessed a humerus, radius, ulna, carpals, metacarpals, and phalanges. We know this common ancestor possessed these bones because all air-breathing vertebrates and aquatic mammals (including whales and cats) inherited this same basic skeletal pattern. Therefore, the forearms in both humpback whale and domestic cats are a homologous trait, inherited by one common ancestor.
Two different species that possess an analogous trait are humpback whales (Megaptera novaeangliae) and great white sharks. Great white sharks are large marine fish adapted to life in the ocean’s depths. They can grow up to 20 feet long and weigh as much as 2 tons. Just like whales, sharks are very acrobatic, able to launch themself out of the water after latching onto their prey. Given the many similarities between great white sharks and whales, one might assume that they have homologous traits. After all, if cats and whales share common traits, surely sharks and whales do too. However, whales and sharks share analogous traits – resulting from their shared environment.
One common trait that both species have in common are humpback whale’s flippers and great white shark’s pectoral fins. Both flippers and fins are paddle-like appendages located on the sides of their bodies. However, rather than resulting from a common ancestor, these traits are the result of both animals’ shared environment and behaviors. Both animals need to steer, stabilize, and maneuver through open water, especially as they hunt for smaller prey. As a result, both animals developed similar limbs, allowing them to accomplish the same tasks in the same environment.
Although all pairs of organisms share common ancestors as you go back far enough in time, we know that these traits are analogous rather than homologous. We know this because whales are descended from four-legged vertebrates, while sharks are descended from cartilaginous aquatic animals. Furthermore, their anatomical structures are completely different, with whale flippers containing bones arranged like other mammals and shark fins containing cartilage supported by fin rays.
Thank you for using two animals that look completely different as examples of the Homologous Trait. The cat and the whale. One is small and cute, while the other is huge and highly aggressive. But their forelimbs are Homologous traits. I think the reason for the difference in their forelimbs is that they are under completely different environments and choice pressures. Whales need to swim quickly and steadily, and fin-like limbs can help them achieve this. Cats need to be extremely sensitive to catch small animals, insects and other foods, so they need to be long and thin.
ReplyDeleteHomology:
ReplyDelete1. (5/5) - Good opening description here.
2. (4/5) - I guess I'm not quite seeing the connection here between the skeletal structure and the function. How does a shorter structure help the whale in an aquatic environment? And how do longer, thinner bones help terrestrial cats? Explain. You can include soft-tissue descriptions as well if that helps clarify the connection between structure and function in their given environments. On the right track here, but keep going. Make it fully clear for your readers.
3. (5/5) - Good, although understand we also know this because of fossil evidence and phylogenetics, not just from anatomical evidence.
4. (5/5) - Good images, though consider offering some skeletal images for clarity.
Analogy:
1. (5/5) - Good description.
2. (5/5) - Good discussion of the environmental pressures producing the convergent evolution pathway here.
3. (5/5) - Good! I'm just going to offer my own "story" here for clarity. The common ancestor of the whale and shark is an archaic fish, who did possess these fin structures and also passed that trait onto extant shark species. So the question is, did the whale also inherit it's fin from that common ancestor? Whale "fins" evolved after whales split off from terrestrial mammals, i.e., long after the split with ancient sharks. This provides us with the evidence we need to confirm that this trait did evolve independently in at least one of these organisms, making these traits analogous.
4. (5/5) - Good images.