a. Two different species that posses a homologus trait would be a turtle and an alligator. The two species do not look alike, however, their anatomical structures have adapted in order to survive living in different environments. Turtles are reptiles of the order Testudines, while alligators are also large reptiles, they are part of the Alligatoridae of Crocodylia family.
b. The homologus trait the two species share would be found in the limbic structure. Their forelimbs share the structure of the humerus, radius, ulna, carpals, metacarpals, and phalanges despite how different each might appear on the outside. Both use their forelimbs to get around, however the shapes are much different. Turtles forelimbs are much smaller in size, and also the skin covering the forelimbs is much tougher on the alligator. Alligators use their limbs to slowly swim, however they mainly rely on their tail when trying to swim fast. Turtles on the other hand, rely on the limbs to walk and swim. The two exhibit differences because of the different environments. They had to learn to adapt in order to survive, which is why we see differences in speed, differences in length of the nails, and toughness of the skin.
c. The ancient lobe-finned fish might have been the common ancestor between the two. Although the fish has fins, the limbic structure (bones) contains the same components as that of the turtle and the alligator. The ancient lobe-finned fish also shares the same embryonic tissue as the others.
d. 
2. Analogy:
a. Two different species that posses an analogous trait would be a platypus and a duck. The platypus is a mammal that spends most of its time in the water. They are carnivorous and swim in order to find their food underwater most of the time. Ducks on the other hand, are aquatic birds that are part of the waterfowl family. They also spend most of the time in around the water. The platypus and the duck are similar, however, they have analogous traits because they do not have a common ancestor who possesses their similarities.
b. Some of the analogous traits would be seen in the similarities of the beak. They both use the beak for the same functions of finding food, both have their nostrils on the beak. Another similarity is seen in their webbed feet. Lastly, the main similarity would be that they both lay eggs instead of birthing live species.
c. I don't think the common ancestor (if any) possessed the analogous trait of laying eggs because platypus are mammals while ducks are birds. There is no relation between the platypus and the avian family. Their functions of the beak, webbed feet, ability to lay eggs shares no evolutionary similarity.
d. 
Good opening description of your homologous species pairing.
ReplyDeleteNote: "Limbic" has a specific meaning and is referring to the hypothalamus, the hippocampus, the amygdala structures in the brain, responsible for emotional responses and memory. It has nothing to do with "limbs". Words have very specific meanings in science. Make sure you say exactly what you mean.
I'm having trouble seeing the differences in structure and function in these species. There is a difference in size, but that is because of incidental differences in body size. If you look at a small, young crocodile, their limb structure is nearly identical to a turtle (the image is actually a tortoise). Both species use their limbs the same way on land. I agree that the crocodile uses his tail for propulsion and the turtle doesn't, but this is a difference in tail structure, not limbs.
If I was going to focus on a key difference, I would go with the structure of the "hand" or the phalanges. The tortoise/turtle is compact and not separated. That's in part due to the function of the front limbs for digging (in tortoises) and movement across rocky streams. Crocodiles needs the spread of the digits and webbing to move efficiently across muddy river banks and across the bottom of those rivers, where they hide and wait for prey. That is a much clearer example of homology.
Since both turtles/tortoises and crocodiles are reptiles, do we really need to go back to archaic fish to find a common ancestor? Wouldn't the ancestral reptile suffice? And we know from fossil evidence that these early reptiles possessed the ancestral limb structure you are comparing. That is what we need to confirm homology.
Great opening discussion on your analogous traits and good choice of analogy (the bill). This one is a classic. Note that you needed to focus on just one trait, not two or three.
You lose some focus in your description of the analogous trait because you try to cover three instead of just one. Yes, the "bills" are similar in structure and function, primarily in finding and obtaining food in marshy environments.
Correct, the platypus is a mammal and the duck is a bird, but is that sufficient to serve as evidence for analogy? Can we get more detailed? We know that the platypus is a strange, unique creature that arose from archaic terrestrial mammals and that the bill is a derived feature that evolved during this process... in other words, it arose independently from any common ancestor with birds. Likewise, the "bill" structure in ducks arose independently as ducks evolved from their avian ancestor as they moved into the watery environment. Again, independent evolution. So we have a case where both bill structures arose independently from the common ancestor (which would have been a reptile). That is the information we need to confirm analogy, though honestly, only one confirmation of independent evolution would have been necessary. The second one is bonus. For example, the duck could have inherited it's bill from the common reptilian ancestor (it didn't, but just as an example), but as long as one of the species (in this example, the platypus) evolved the trait independently, then these traits are analogous to each other with no common genetic origin.
Good images.