Thursday, June 28, 2012

Homologous and Analogous

Homologous Structures

It’s obvious enough to say that the homologous structure that these animals have is a hand, wing, flipper or paw. All in which help them live and do everything our human hand would personally need as well. As a human, we know that our hands are very important and everything we do incorporates the use of them.

Organs as different as a bat's wing, a seal's flipper, a cat's paw and a human's hand have a common underlying structure, with identical or very similar arrangements of bones and muscles. In 1843 Richard Owen reasoned that there must be a common structural plan for all vertebrates, as well as for each class of vertebrates. He called this plan the archetype. Richard Owen also distinguished homology from analogy, which he defined as a 'part or organ in one animal which has the same function as another part or organ in a different animal'.

Homologous structures are structures that are derived from a common ancestor; they have a 

common evolutionary ancestry. This is not to say that homologous structures have the same 

function, a whale's flipper is homologous to a human arm. These limbs are superficially different, but their internal skeletal structure is essentially the same. 

Bat's Wing

Helping them fly, allowing bats to maneuver more quickly and more accurately than birds. 

Seal Fin

On land, the use the flippers to drag hind limbs and in the water, using primarily 

their hind flippers for propulsion and their front flippers as rudders for steering.

Cat Paw

Cat's walk on their toes, providing sure footing for their hind paws when they 

navigate rough terrain. Also any other necessity a human hand can do, a paw is able.

Human Hand

As human's we use our hands to do every action in our lives. As human's life would be extremely 

hard and nearly impossible to do much without such a limb. 

Common Ancestor:

A common ancestor amongst all these animals would have to be a mammal. Mammal's all need 

some type of  limb to have multi-functions as in this does.

Analogous Structure

Octopus Eye vs Human Eye

Octopus Eye

Cephalopods as active marine predators, possess sensory organs specialized for use in aquatic conditions. They have a camera-type eye, which consists of a lens projecting an image onto a retina. Unlike the vertebrae camera eye, the cephalopod's form as invaginations of the body surface, and consequently they lack a cornea. A cephalopod eye is focused through movement, much like the lens of a camera or telescope, rather than changing shape as the lens in the human eye does. The eye is approximately spherical, as is the lens, which is fully internal. 

Human Eye

The human eye is an organ which reacts to light for several purposes. As a conscious sense organ, the mammalian eye allows vision. Rod and cons cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth. The human eye can distinguish about 10 million colors. In common with the eyes of other mammals, the human's eye non-image-forming photosenitive ganglion cells in the retina receive the light signals which affect adjustment of the size of the pupil, regulation, and suppression of the hormone melatonin and entrainment of the body clock.

Analogous Trait

Something known as the "camera eye" is what the octopus eye and the human eyes both evolved from. The name "camera eye" came from consisting of a lens projecting a representation onto a retina. The common ancestor of the octopus and of man possessed this analogous trait and modified it so it could see.

Common Ancestors:

Their common ancestor lived more than one-half billion years ago. Since it did not have a camera-like eye (like they now do), the fact that humans exchange a simple gaze with octopuses can only mean that such an eye evolved independently. This is a classic example of parallel evolution, the emergence of a similar biological feature, not be a descent from a common ancestor. But from organisms that are effectively unrelated. Yet biologists also know that this eye-type has evolved independently at least four other times. Both octopus and humans end up seeing in much the same way, even though their respective ancestors could not.


  1. I've tried time and time again to figure out what is going on on my blog... I am really technology challenged so I cannot figure it out. To read things, highlight an area! Sorry for the inconvenience!

    1. Did you solve the problem? Because I don't see a problem. Looks great! More comments below.

  2. I can see the writing just fine! Your tech skills are good!

    But about your content, the information about the homologous traits I found were very similar to those that I choose. You choose human hand, bat wing, seal flipper and the cat paw. I choose the human hand and porpoise flipper which is pretty much the same trait. The functions are different some for swimming, some for flying, and humans a mixture of many things.

    Where I really found interest in your piece was the analogous trait. The eyes of a human and that of an octopus was extremely interesting and something that I was unaware of. I learned a new term today, "camera eye,"which is what our eyes and the octopus evolved from. I really enjoyed reading your post, nice work!

    Jaqi Gibson

  3. Hi Kristina,

    I too have really enjoyed your post. I wasn't able to read it before with the black background, but I can read it fine now :) It is very informative, and detailed; great job!
    The "camera eye" is a new term for me as well. Great choice for your analogous trait. Helps me understand it better (as you know I sort of struggled with it).
    Thank you so much!

  4. This was honestly such a great blog! It had detail after detail and made me understand the topic more. It is obvious that you're knowledgeable about what you're writing. The analogous trait paragraph was spectacular. Those two would've never came to my mind and it was nice to read something new.

  5. You went over and above the call of duty on your homologus structures! Nice images and good background. My only questions is regarding the ancestor. You say, "they have a common evolutionary ancestry", but you don't try to identify the ancestor. This didn't have to be a specific organism but the general organism. Mammal? Reptile? Amphibian?

    Your analogous structure is excellent. The eye structure has evolved several independent times in the history of our natural world. These are two excellent examples.

    Well done.

  6. Well thank you everyone for being so supportive and positive towards my post! I really took pride and time to understand the concepts, finding valid examples amongst it. I suppose from the feedback I received from all of you, that hard work paid off! Thanks!

    I believe I was looking for a more specific answer than just the type of animal. I believe the octopus eye and the human eye ancestrally derived from mammals!