Thursday, February 23, 2017

Week 3 Analogy/Homology


1.

    A. Two different species that possess a homologous trait is humans and horses. Both are mammals. Humans are bipeds (walk with two feet) and horses are quadrupeds (walk on all four legs).

    B. The homologous trait of humans and horses are the forearms. Forearm for horses and forearm for humans. In the horse the forelimb is very delicate since its responsible for 60% of the horse’s weight. Its located in the two from legs of the horse. As for humans, the forearm is between the elbow and our wrists. Humans use the forearm to grasp things and horses use their forearms for walking and running.

   C.  Aside from both species being mammals, I couldn’t find the ancestor of humans and horses; it’s safe to say that both are primates. Both share flexible and similar body parts of a primate.
  D.

2A. Horses and humans also possess analogous traits. An analogous trait in both humans and horses are the limbs in both species.

B. The structure of the limbs was independently evolved. Both limbs in horses and humans have different structures and evolutionary histories. Although, limbs are used for the same function (locomotion) in both humans and horses.

C. Common ancestry between humans and horses come from primates. The traits are analogous because they look different and are placed different in the humans and horses anatomy.
D.

2 comments:

  1. The opening section was an opportunity to describe the species you chose to give your reader an understanding of how they move, behave, eat, and live in general so we can understand the environmental pressures that have produced these traits. A little short on the actual description for both homologies and analogies.

    For your description of your homologies:

    " In the horse the forelimb is very delicate since its responsible for 60% of the horse’s weight."

    This was a puzzling comment, since it doesn't make sense for a "delicate" structure to have adapted for this function. What you need to do here is compare the horse structure directly to the human structure, in which case, the horse's skeletal structure (specifically what is called the cannon bone, an adapted metacarpal, which is blue in your image) is much thicker and sturdier than the same bone (the central metacarpal) in the humans. A focus like this would have helped your reader understand the relationship between the difference in function and structure in these two traits.

    No, horses are not primates. Primates are only prosimians, monkeys and apes. However, both are mammals, and we know from fossil evidence that archaic mammals possessed the primitive form of the forelimb, passing it onto these two descendants, with changes occurring due to environmental differences. That is what we needed to know to confirm homology.

    There are situations where you can find both homologies and analogies in the same two species, but this isn't one of them, particularly if you are comparing limbs. The image you are using here is incorrect (what was the source), as it is displaying *homologies* not analogies. This is essentially the same comparison you offered in your first section.

    In order for traits to be analogous, they must be the result of independent evolution, or homoplasy. That means the common ancestor did NOT possess the rudimentary structure but that each species developed the trait independently, such as the evolution of bird wings and butterfly wings.

    Same problem with issue of ancestry. See comments above.

    Let me know if you have questions on this.

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  2. I was a little confused about the horses and humans comparison. You said that they have the same function, however horses don't use their limbs like humans use their arms. So I was wondering if you could elaborate on that more?
    Thank you!

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