Homology:
Seals and humans are both
mammals that have the shared homologous trait of the bone structure of the arm
and hand(flipper). In humans, the skeletal structure is covered with flesh,
muscle and skin, and has fine mobility and flexibility to perform different
functions, including gripping and squeezing. The ability to move each digit
independently is a point of differentiation, and an advantage over most other
animals. In seals, the framework is covered with fat/blubber, thick skin with
fur for warmth, and the area between the bones is webbed so that the seal is an
efficient swimmer. The common ancestor would be an early reptile that developed
a similar structure as it was pressured to be able to traverse diverse terrain
in order to survive and reproduce.
Analogy:
Seals and ducks are two different species that spend a large
portion of their time in the water, though one is a mammal, and one is a bird.
They have the analogous trait of webbing on their foot/flipper. The seal has
large hand like appendages with furry webbing between the joints which help the
seal move about in easily in the water. Ducks, on the other hand, have a thick,
adapted, skin-like webbing over the skeletal structure and they move very
rapidly underwater to propel the duck, which sits atop the water. We know that
this trait is analogous because it is not a character that the common ancestor exhibited,
but rather, this trait would have evolved in each species as a response to
similar ecological pressures to adapt to a life spent largely in the water
Hi Robert,
ReplyDeleteFirst of all I want to say that is the cutest pictures! Its amazing how similar a seal and human limbs are. Whats more interesting is the fact their common ancestor is a reptile who would ever guess. As for the seal and duck it makes sense that they both have webb feet after all they both are animals of the water. Just like insects and birds they both have wings their environment is the same they fly in the air.
Interesting choice to compare, but like the professor said, it is fascinating the similarity the structure of the fore and hind limbs are between the two. I compared the dolphin to the human and even then it was interesting to see how they are much the same too. I think that when both have a very similar ancestor such as the reptile, and I think then it is likely to see many of the same similarities in animals with a common ancestor. The differences between the two both show how they are only advantages to each animal. Such as the webbed portion of the seal to help it swim and the ability to grip in a human. All advantageous traits specific to one another. The second part of your post is very well said, They are similar because they are water animals but they both developed or evolved to have this trait but they both have a different ancestor as opposed to the seal and baby.
ReplyDeleteJust for clarity, your choice of homologous trait is the skeletal structure of the "hand" in humans and the seal, correct? You discuss the superficial differences (what is covering the bones) but you don't discuss the differences in the skeletal structure themselves. You have great images of the bones of these parts of the bodies. What differences do you notice? And how can these differences be traced to their different functions and environments? That is what you needed to present as evidence that these are homologous traits, along with the information on common descent.
ReplyDeleteDo you need to go back to reptiles to find a common ancestor between humans and seals? Both are mammals who arose from a common mammalian ancestor who possessed that archaic limb structure. That is the genetic ancestry and the differences arose due to different functions in different environments.
Great analogous comparison. The first time anyone has raised the issue of webbing. Well done. Good explanation of the functional commonalities.
Correct on the ancestry. To be more specific, we know that this is a unique derived trait in ducks (non- aquatic birds don't possess this trait, so it didn't come from the bird common ancestor) and non-sea mammals don't posses it (so the common ancestor didn't possess it either... sea mammals started off on the land.)
Images helped a lot with this. Good.
As far as the skeletal structural differences, the most obvious is the lack of an opposable thumb in seals, compared to humans. Seals developed phalanges that are more evenly spread out, and larger proportionally to humans. These "hands" are used to prop the seal up and assist with balance when on land, and, equipped with webbing, used to effectively move water as they swim. On the other hand(no pun intended), humans, and other primates, evolved to posses opposable thumbs, which help with activities unique to humans like precision grip and the ability to hold a variety of objects and tools.
DeleteI also wanted to ask: is there a resource I am not taking advantage of which would help me to find specific common ancestors?
DeleteThank you for the response and clarification on the skeletal differences in that pairing.
DeleteThere is a link on the "Good Resources" sight that can let you calculate back in time to the common ancestor of any two organisms, but this issue just requires basic understanding of the phylogenetic relationships of the major groups vertebrate groupings (fish, amphibians, reptiles, mammals, birds) and that non-vertebrates separated early from the vertebrate groupings. The concept of derived vs. ancestral traits is helpful as well. There as stated in the guidelines, we aren't looking for a specific ancestor, just an understanding of how these organisms are broadly related on the evolutionary tree.