Biology: Homologous Structures

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We've been really moving through all of these modern evidences of evolution bringing together some of the past and some of the present. Well, once again, let's look where we've been. We have really looked at fossil evidences, we've looked at the fossil record and we've worked our way down through transitional fossils. We've looked at that in terms of animals, we've looked at it in terms of plants, and we've even looked at it through human beings. Then we said, okay, what about some of these things that seem to be in our bodies and they have no apparent function, so we talked about vestigial organs.
Well, now we want to take a look at some of the things, again, going back to the days of Buffon and Leclerc that they had noted and that they had put in their theory or Leclerc had put into his theory of use and disuse. And, how a Darwinian and the modern evolutionary theory will seek to explain these things. And, the thing I want to talk to you about are homologous structures.
Now, homologous structures, these are fun. I want you to think of a squirrel. Now, I would like you to think about a squirrel because squirrels they are not really closely related to us, but did you ever watch a squirrel eat a nut? You know when they get their hand up there and they pull that thing right up to their mouth. And, they seem to have some kind of prehensile ability to grip that nut and hold it just as I can grip a pen.
And, you know, that makes me think of limbs and it makes me think of hands and it makes me think of the fact that our arm is an unbelievably well adapted structure. And, we've really stressed the fact that our limbs and our traits and all of our physical attributes come our DNA, come from our genes. So, this arm is clearly, clearly a function of our DNA, clearly a trait that we have inherited from our parents. So, if I say to you, all right, look at you and look at the squirrel do you share limbs? You'd say, yes. And, therefore, do you share DNA? And, you'd probably say, yes. And, therefore, are you related? And, you'd say, well, I guess I'm closer related to a squirrel than I am to a spider. I'd say, well, you're right.
But, as we study more and more anatomy we find out some very, very interesting things. There are some creatures, and boy, I'm going to keep coming back to those marine dwelling mammals again, that have what we are going to call homologies or homologous structures to you and I. Structures that, as we said a second ago, homologous means that they literally seem to have the same structure, but may have a different function. That's the key. A homologous structure will have a different function, but will definitely have the same structure. What do I mean by that?
Well, there's a human arm, let's talk about whales still again, there's that whole whale's arm, remember that? There it is it is a little flipper and sure enough one, two, three, four, five fingers in their flipper, two bones right here equivalent to your radius and ulna, one bone right here equivalent to your humerus. I have to tell you, I love chicken wings and if you take a look at the wing of a bird think of that. The wing of a bird is homologous to your arm. When you get chicken wings you can get them two ways, and I don't mean hot or mild. When you get chicken wings, if you eat chicken wings, you'll know that one of them comes with one bone, it's kind of gory to point to a human arm and think of this, but I can't even imagine eating a whale flipper, but it comes with one bone. And, another type of wing, the other type, comes with two bones. And, you know, that homology doesn't stop there.
That DNA sequence that causes those traits doesn't stop there. Let's look at a cat's leg. Two, one, wrist, foot, now, what about like a flying mammal like a bat. Could their wing be anything like this? Well, sure enough, if you take a look at the wing of a bat, one, two, wrist, fingers all covered with membrane so they can fly. Well, what's the lesson with homologous structures? The lesson is this. Structure is determined by your genes, this backs up not just the fossil record it backs up the biochemical record. You would expect that your DNA would be similar to all of these things.
Well, let me ask you a question then. Bat, bird, moth they all fly they all have wings, danger, danger, red flag what I just made you start thinking about is something called analogous structures or what we like to refer to as convergent evolution. Convergent evolution is the fact that some adaptations converge even though they may not have common ancestry. Just because a moth flies doesn't mean that the moth is actually going to have the same ancestor as a bat or a bird. Just because a beetle walks doesn't mean that his legs are built the same way as you. You have to go to structure. You have to go the expression of that DNA. You can't just say, oh, look what it does.
And last but not least, I think the final and coolest evidence of this is something that we can do with embryos. Most of your DNA during the course of your life is turned off. So, when I look at what I have, most of my DNA, over 95 percent of it, is turned off at this point. But, there is a time in your life when a lot of your DNA is turned on and that is when you are an embryo. This is so neat to do, if you compare the embryos of certain creatures you find out that as an embryo you have a lot more homology than you do as an adult. You know you had a tail once? Sorry, you all had tails, some of you were even born with little tails sticking out and your parents had them chopped off and didn't tell you about it I'm sure.
But, sometimes, you see, because look at a reptile's embryo, a human embryo, a pig embryo, a bird embryo, and, indeed, even a fish embryo all have that tail. Oh, it gets even more interesting than that. See those right there? Gill slits, yes, you had gill slits. Were they functional gills, absolutely not, don't start thinking that you can breathe inside your, no, you don't, the gill slits were nonfunctional.
But, nevertheless, you had the genetic propensity to develop gill slits early in your embryology, they later went on and disappeared. So, the ultimate comparison of anatomical structures happens when so many of your genes, and so, we don't have to just look at arms and legs and noses and hair, etc. We can look at the development, because there is the ultimate structural comparison, the comparison of embryos.
So, we can take it from practically day one and compare your structures all the way through. Remember, it all ties together. The fossil record is backed up by homology; homology is backed up by DNA record. It's one big picture and the picture is crystal clear.
Evolution
Evidence for Evolution
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