Surprisingly, humans and whales share a homologous trait. In the human hand and the whale fin, both species share "phalanges." For humans, the phalanges are the long bones in our hands and feet that are needed to grip things, or walk. Whales also have phalanges but in their fins, and they are also a lot longer than the phalanges in the human body. The whales use their phalanges to help them swim and move through the water. Although both humans and whales use their phalanges for movement, they are very much different as the human phalanges are much smaller than the large whale phalanges.
Analogy
Ducks and platypus both possess an analogous trait of laying eggs. Although both a platypus and a duck can lay eggs, they did not gain this trait from the same common ancestor as a platypus is a mammal and a duck is a bird. The platypus shares no other traits with birds besides laying eggs so it is a perfect example of an analogous trait.
From this image above, it appears that the closest ancestor between birds and platypus goes back to 325 Mya to a species called Amniotes. I believe that this species had to have carried the analogous trait of laying eggs because that seems like the only way that the platypus would gain the trait of laying eggs.
Sources:
http://www.bio.miami.edu/dana/160/160S13_5.html
http://www.buzzle.com/articles/analogous-structures.html
I wrote about the human arm and the whale flipper as well and found it very interesting. Seeing the side by side image of the bone structures is fascinating.
ReplyDeleteVery good description of the homologous structures, Good connections between differences in structure with the differences in function.
ReplyDeleteMissing the section on ancestry? Did the common ancestor possess the archaic structure and pass that onto these descendents? How do we know that?
Good discussion on your analogous trait, including your reasoning as to why these are analogs.
But then you contradict this with your discussion on ancestry. "Amniotes" are not a species. It is a group of organisms that share the common trait of laying eggs on land. To suggest that the ancestor was an amniote argues that the ancestor possessed this trait of laying eggs... and yet that would make this a homologous trait, wouldn't it, if it passed the trait onto both species?
In order for this trait to be an analog (which it is), it needed to evolve independently from a common ancestor in at least one of these species. So what do we know about these organisms that can help us with that? We know that mammals, in general, don't lay eggs. This is a trait developed independently in the platypus after it split from other mammals, i.e., it developed this trait independently from how birds inherited the trait. That is all you need to know to confirm it's analogous status.
Don't both birds and the platypus have bills? ;-) That is actually another example of an analogous trait.
I liked that you focused on the egg laying trait, but you also say that they share no other traits with each other even though they both have bills and webbing on their feet? I would have liked to have seen more details and information about the two species too.
ReplyDeleteGreat two examples. I think, for myself at least, it was diffuclt to do the ancestry on the analohous trait becuase as you go far enough back you can usually find a common ancestor.
ReplyDeleteI wasn't aware that Whales and Humans share the similar structure of phalanges, that is very interesting. Did you know that cats and dolphins also possess phalanges? I discovered that yesterday and was very surprised that such visually different animals could share such similar traits, as I'm sure you were to discover that Whales and Humans share similar traits.
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