Tuesday, January 17, 2017

Homology & Analogy

Homologous Trait

a) Humans are the only surviving species of the Homo genus, known for their erect posture and their bipedalism. Humans have more complex brains than most animals, gifting them with improved communication and survival techniques. Dolphins are a grouping of the order Cetacea, which includes around 40 different species of dolphins. They are highly social and intelligent animals that exhibit complex behaviors such as learning, teaching and even grieving. Both humans and dolphins are known to be some of the smartest species in the world.

b) The forelimbs of humans (arms) and dolphins (fins) are an example of a homologous trait. Dolphins use their fins to help them swim and traverse through the ocean, while humans use their arms for a variety of things including grabbing and reaching for items, using tools, and defending themselves. While they serve very different functions, if you go far back enough to find their common ancestor, you'll see that the forelimb started off as one design that further evolved to fit each descendant's respective environment. Evidence that dolphin fins and human arms are homologous is in their bone anatomy. Both limbs include a humerus, an ulna, radius and carpals as well as five-digit phalanges. Though they may be structured different to meet their respective needs, the anatomy is still the same, showing us that these limbs developed from a common ancestor.

c) You would have to go way back in time to find the common ancestor between humans and dolphins as dolphins split from primates long before humans existed. But the common ancestor must have been able to utilize their forelimbs for something than just walking. Perhaps the common ancestor exhibited traits of bipedalism that would have caused it to develop differently from their hind limbs.



Analogous Trait

a) Another example that features humans is the development of the eye, as compared to a cephalopod eye found in animals such as the octopus. The octopus is a mollusk, a highly diverse phylum that features invertebrate species.

b) Cephalopods such as the octopus have a camera eye that's similar in structure to the human camera eye. While they provide the same function and even developed quite similarly, the evolution of the eye in humans was vastly different to that of the octopus. It's an analogous trait because while humans and octopodes do not share a common ancestor, the development of the camera eye was crucial to both their respective ancestors to survive. Both human and octopus eyes share a similar anatomy; they each include an iris, a retina, nerve fibers and an optic nerve. They aren't completely the same however. As an example of convergent evolution, the environment of each respective species required similar, but different needs, so the eye isn't developed exactly the same. For example, in human eyes, the nerve fibers route before the retina, which helps block light, but also creates a blind spot in our view. In octopus eyes however, the fibers route behind, causing no blind spot, giving them better vision and defense from predators.

c) You would have to go quite a long time back to find the common ancestor between cephalopods and humans, however, I would imagine that that common ancestor had a very rudimentary photoreceptor cell to be able to tell when it was in the sun, in the dark or in shade. After the split between these two species, that trait developed to further meet their respective needs.


7 comments:

  1. Great opening description of your homologous species.

    Very good discussion on the comparison of your homologous traits, clearly explaining the differences in function and structure.

    The issue of ancestry is more straightforward than what you have presented. What you have written is accurate, but aren't both humans and dolphins mammals? The common ancestor would then have been an archaic mammal, and we know from fossil evidence that all ancestral mammals possessed the archaic mammalian limb, which was then passed onto these two species via common descent. That is all we need to know to confirm that these are homologous traits.

    Good description of your analogous species.

    You jump into ancestry in your description section, so I'll address it then. Actually, all organisms share a common ancestor if you go back far enough, so claiming a lack of common ancestor is not a valid piece of evidence for analogy. The question is not whether or not there is a common ancestor, but whether the common ancestor possessed the archaic eye structure and passed it onto these to modern species via modern descent. That is the information we need here.

    The eye is a great example of analogous traits, having arisen independently several times in Earth's evolutionary history. In the case of the octopus and human eye, we know from phylogenetics that the octopus and human eye are genetically different. We also know from their structure (the octopus eye is "backward" from the human eye and consequently is missing the blind spot), that the octopus eye is structured much differently from the human eye. This is the information we need to confirm that the evolutionary pathways for these two structures are different, via parallel (or convergent) evolution, confirming that these are analogous traits.

    Good images.

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    Replies
    1. More fully read your section on ancestry for your analogy section. If the common ancestor possessed the rudimentary structure for the eye and passed it onto these two modern species via common descent, wouldn't the mean these traits are homologous?

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    2. Thank you for your input. I understand the question, I didn't see that point as having contradicted the fact that eyes are analogous. My reasoning was that since they developed so differently, from such different species and in a different environment, their closest common ancestor (as far back as that had to have been) would have shown an inkling of that trait. However I do understand now that that would imply that the necessity for an eye (or an eye-like structure) would have to mean that it's a homologous trait. The common ancestor would not have possessed even a basic rudimentary eye, that trait must have developed well after the evolution of humans and cephalopods had split from that ancestor. Thank you.

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    3. Also, I did make an error with my wording in my analogous answer, I didn't mean to say that a common ancestor never existed between humans and octopodes, I just meant it as a way to emphasize eye structure as an analogous trait as opposed to homologous. Upon re-reading my answer, I realize I worded it incorrectly there.

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    4. Thanks for the response and the clarification, Milestones. I always appreciate it when I know students have read my comments and take the time to respond with a productive comment.

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  2. Hello,

    I found your Homologous trait response rather informing. It is almost hard to believe that dolphins and humans both are descendants of the archaic mammalian, but after learning more about the homologous similarities between the limbs of a human and a dolphin it makes more sense why research has developed such conclusion. You did mention that humans have more complex brains than animals. I liked that you implied that humans are animals. Although, we may not see each other in that form, we are living things that share similar ancestries to those we perceive as animals. Great post!

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  3. Hey!
    I found the description of your homologous traits and analogous traits quite interesting. I think dolphins are the cutest animals ever and to know we have a common ancestor is exciting. It is crazy to know that we share the same arm structure as a dolphin, because we share common ancestors with animals realizing that we are mammals as well.

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