Like what you saw?
Create FREE Account and:
- Watch all FREE content in 21 subjects(388 videos for 23 hours)
- FREE advice on how to get better grades at school from an expert
- FREE study tips and eBooks on various topics
M.Ed., Stanford University
Winner of multiple teaching awards
Patrick has been teaching AP Biology for 14 years and is the winner of multiple teaching awards.
So evolution through natural selection, is the bedrock idea that all modern Biology is based on kind of like the theory that the earth is round, is what modern Geography is based on. So it's really important that you understand it. Luckily it's one of the easiest concepts in Biology to get. And the evidence for it is all around us.
In some things like say I'm studying DNA, or I'm trying to teach my students about photosynthesis. That's at the molecular level and I can't just so easily point at a plant and say, "Hey kids let's watch that Rubisco enzyme do it's stuff." But with evolution, I can ask kids, who's heard about tuberculosis. And a bunch of kids will raise their hands and they'll rattle off how tuberculosis is becoming antibiotic resistant, because people have been using and misusing antibiotics to fight them.
Or say for example I could describe a bunch of rabbits. Some of the rabbits have coats that allow them to blend into background, while others their hair makes them stand out. Which one is the wolf going to find out, well the one that stand out and they'll be eaten. What happens in the next generation? Well the camouflage rabbits, and they start having babies like rabbits. And you wind up with a bunch of camouflage ones. Well the ones that look like circus freaks they are dead. And once you're dead you tend to cut down the number of babies you tend to have. So that's easy and that's why my student's love the Evolution unit, because it's the easiest one of year.
That being said, there are some new answers in this whole idea of evolution through natural selection. There is also a bunch of misunderstandings that some people have. That make it easy for you to make some careless mistakes on the AP test, if you don't really spend the time to get the basics of the idea. So what I'm going to do is I'm going to take you through this whole idea. I'm going to start off with what is Darwin's theory of evolution through natural selection, and what does that mean. Then I'll go over some of the evidence that was used to build up this idea, and also the evidence that's used by scientists today, to determine the evolutionary relationships between different species.
Now if I'm going to use the term species, I've got to define it. So I'll go through what a species is and then how are species kept separate, so that they don't wind up becoming just generic organisms. Last, I'll go through a number of the different evolutionary patterns that scientists have discovered, using that evidence that I discussed earlier about how evolution works.
Now to start this whole thing off, I wanted to go into first, some of the issues that people have with evolution. Some of the misunderstandings about what evolution is and what evolution is in. And this will make sure that we're all on the same page to start with.
A lot of people have misconceptions about what evolution is and what evolution isn't. So, to make sure we're all on the same page, let's start there. So evolution in actuality isn't a theory, it's a fact. Evolution is defined as genetic change in a population over time. A population for those of you who don't know, is a group of members of a particular species in a particular area.
And we see this all the time. Every year you know you need to get a new flu shot and why is that? Well that's because there is genetic change in the various strains of flu virus, and that's evolution. If there wasn't this generic change, you could get one flue shot and your done for the rest of your life. What Darwin's key thing was that he was the guy that came up with the idea, "Why the heck are there these genetic changes?" What he figured out is that, it happens through this process that he called natural selection.
So what is natural selection? Natural selection is this idea that if you have within a group of organisms, some creatures have genes that give them a particular trait, while other creatures don't have that gene. So don't have that particular trait. If that gene, that trait, gives you an advantage that allows a better chance of surviving long enough to have babies and they survive, then you'll have babies and pass in those genes.
All those organisms that don't have that trait, they tend to die off and not have so many babies because they're dead. And so, over a long enough period of time, you wind up having fewer of the ones who don't have the trait, more of the ones who do. That's called evolution.
So how does this work? Well let's take a look at an example. A 100 years ago, if you got a staph infection which is caused by a particular bacteria called staphylococcus, chances are, if you got that in a wound you're going to die. But as soon as they started using the antibiotics like penicillin, all of a sudden penicillin was like this nuclear bomb. "Wipe them out." It was the wonder drug. Nowadays however, it's pointed stick against most bacteria, and how that happened?
Well the first time people were taking penicillin, wiped out the bacteria. Now there maybe one or two survivors but their immune system could take them out. Now let's imagine within somebody, he takes a dose of penicillin but maybe a little bit lower than he should have really had. Now within the staphylococcus bacteria, there might have been a few mutants that may have a slightly greater chance of surviving, than the ones who didn't have that mutation. Well, they survived. The antibiotic however did wipe out all the competition for food, which is you. And so, these guys they start reproducing and pretty soon you have a new staph infection. Well no problem you take some new penicillin and wipe out 99% of them. Leaving the one percent that are a little bit resistant.
You keep doing this for hundreds and hundreds of generations, which for bacteria could be a week or two. And pretty soon, you have got a bacteria that there has been more mutations, and some have made them a little bit more resistant. Your starting to breed more and more resistant bacteria. That makes a lot of sense, doesn't it?
Now scientists recently have also realised that there is this thing called genetic drift, which are random changes.
That is another surprisingly major cause of genetic change over time or evolution. But the big key thing is natural selection. So let's take a look at what are the requirements for natural selection.
The first of them is, you have to have more offspring being born than will survive to adulthood. If everybody who's born gets to survive, so there is not going to be any changes. So, sorry, but that's the way things are. Next, you need to have competition for resources.
What do I mean by resources? That could be food, that could be shelter, that could be warmth, that could be mates. And well, if you think there is no competition for mates among humans, you don't watch VH1. There needs to be a genetic based variation within the population.
If Dr. Evil got his sharks with lasers and he decided to take out anybody with a Mohawk, not a problem. We just all shave our hair or comb it down. On the other hand if you decided to go after anybody whose is above five foot ten, that would cause major changes pretty quickly within the human population to wipe out most of the NBA.
Next up, some variations need to have an increased chance of surviving long enough to reproduce. Now this is key. This is called increased reproductive fitness. A lot of people confuse this with being survival for the strongest. Being strong isn't an advantage if it does not help you have surviving babies. To give you an example, think back to the crusades. Back in the crusades where did the strongest, toughest best at fighting people go? They went to the middle east for years and decades. Now as the little weak, not strong guys who would stay at home in Europe and they'll say, "Don't mind me my Lord, I'll take care of the Castle, and your wife."
Right now in Africa, scientists have seen that the size of tusks of elephants is going down. Now it's the tusks that allow the males to win their competition for mates. So you'd expect bigger and bigger tusks. Well guess what? Hundreds love elephants with big tusks and they've been shooting them. So now the advantage to have little tiny tusks and that way you're going to have a chance of surviving.
What about genetic drift? Now genetic drift is the random changes, the weird stuff that happens just because random stuff happens. Now this is usually far more common in small populations, as opposed to larger populations. Because if you had 10 people die in a car crash, sorry that's horrible. But if that was from a starting population of a village of a 100 people, that's a sizeable percentage of your population that just got wiped off randomly. Now if it was 10 people in a population six billion, that's barely even a little drip. So that's why genetic drift tends to focus in on small populations.
Now there is two general kinds of things. There's something called the Bottleneck effect and the Founder effect. The Bottleneck effect is when a portion of the population dies because of some random event, say a floods, or an earthquake or something. And those who survive, didn't survive because they had some genetic advantage, they just survived because they were lucky.
That creates what's called bottleneck. And you can see that in some of these Eastern European Jews population, where they tend to go through programs or other things that wiped out large portions of the population. And it wasn't because they had some genetic advantage versus those who died, it just was the way things worked out.
In a similar way but a little bit less deadly, there is the idea of the Founder effect. Now the Founder effect is, let's imagine you have a new environment and you bring a small population. And if the population that comes isn't an exact matched to their parental population, that may cause some changes in this small population. For example if you look at people who live in South Africa, the Afrikaans. They are all founded from a group of roughly 1000 settlers who were all members of the 30 families from the Netherlands.
Because they didn't go through the Netherlands, and take exactly average Dutch people, it wound up turning that in the Afrikaans gene poles there are some genetic diseases like porphyria. That they suffer a lot more than the average Dutch population does. And then there are some genetic diseases that they just didn't happen to bring to them to South Africa and they just don't suffer them. That's the two main kinds of mechanisms of genetic change, genetic drift and of course natural selection.
While natural selection just makes sense, unfortunately in life and in science, just because something makes sense, that doesn't mean it's true. In science you need evidence to back up your ideas. So what's some of the evidence for evolution? First off, there is that observed evolution. Scientists have actually seen it happen. One of the problems with observing evolution is that, these kind of genetic change I've been talking about, typically takes dozens, hundreds, even a thousands of generations for there to be a significant amount of change. And that's why it's been in the short lived creatures that we've had our best chances of seeing it. Whether it is the antibiotic resistant bacteria that I was talking about before, or in viruses and other short lived creatures. Now, besides seeing it in living creatures, scientists have also been able to look back in the time of the last three to four billion years that we've heard life on this planet. And that's using fossils.
Now looking at the fossilised imprints, or the fossilised bones of creatures that used to be on our planet, when you are looking at them, you have to look at some of their structures. And you have to bear in mind, a concept called homologous structures versus analogous structures.
Homologous structures are body parts that come from similar origins in the embryo. For example, when I look at my hand and I look at the wing of a bat, they look very different. But naturally, a bat's wing is just made out of the same kind of bones and tissue that made out of, just arranged differently.
That's an idea that's called diverging evolution. When you see homologous structures in two organisms that look very different, that indicates that they diverged.
Now, a lot of times scientists have run into troubles because there is things called analogous structures. Analogous structures are body parts or other portions that look or act the same and may outwardly appear, "these two things are related." But it turns out they are not related, they've just come up with the same solution to the same problem.
For example, take a look at a shark and take a look at a dolphin. They both generally have the same shape. Why? Because if you're going to go 30 miles per hour through the water, you need to kind of look like a dolphin or a shark. But if you've ever been playing and you think that's a bunch of dolphins swimming around you, and you turn out later to discover it's not, you can tell there is some significant differences. And that's a process called convergent evolution or sometimes parallel evolution. The idea that natural selection is wheedled away at two non-related species to make them eventually appear essentially the same.
Now in talking about embryology, looking at the embryos to discover these things, that's something that scientists have been able to use on living creatures. And that's looking at how their embryos develop. For example in us humans, down your back, when you were an embryo, you had this rode of cartilage called a notochord. Now ultimately in us it got replaced by our spinal cord. But all vertebrates actually develop that notochord at some point during their foetal development. While creatures that are not vertebrates, that are not closely related to us, like octopus or insects, part of the reason that we know they are not related, is they don't have that notochord.
Now, with the advent of modern molecular biology, scientists are now able to look at the DNA and other size of components of living creatures, or no longer living creatures. And we've actually have been able to look at their genes, and use the DNA differences to determine who's related and who's not. And this is causing some major revamping of our evolutionary past. To give you and idea of some of the revamp said it's had in our not so distant past.
Many years ago, the descendants of Thomas Jefferson, they were having one of their family reunions when this group of African-Americans walked in and said, "Hi we're members of we're your family. We're descendants of Thomas Jefferson's liaison on with his slave?" They said, "Oh! No you didn't." And the DNA said, oh yes he did. And now they're hopefully one big happy family.
Comparative molecular biology is this idea of using the comparison between DNA, comparisons between proteins, and other molecular components of organisms, to figure out this relatedness. It's even led to something called the molecular clock. And this is the idea that some mutations happen at a relatively predictable rate. Say in this one gene, if you tend to get one mutation say every 10,000 years, and then between two different species you see there is nine differences. That indicates that nine times 10, 90,000 years ago is when those two species branched off from each other.
So if Jerry Springer's audience can accept DNA testing as evidence of relatedness, then it seems that it's pretty good evidence that species can be related. But then that begs the question what the heck is a species?
Now, defining a species is a naughty problem in two senses of the word. It's a naughty problem that you're trying to come up with a human definition of something that happens in nature. And work as hard as we can, a lot of time nature just ignores us and breaks our rules. Then it's also naughty in that it's all about sex.
So let's take a look at what scientists best efforts they've come up with. And that idea is that, a species is a group of interbreeding, or potentially interbreeding natural population that is reproductively isolated. That's a whole bunch of whistle words. Let me put it a little bit more simply.
A species is a group of organisms that all tend to have sex with each other, and not with other kinds of organisms.
Now let's take a look at why they have these whistle words in here. And it's about interbreeding or potentially interbreeding, what's that about? Well, people in Italy are humans just like people in Kansas. But they are typically not breeding with each other unless they hop on a plane go to that bother. Now what's the whole thing about natural? Well, that means that these creatures typically never come into contact with each other, and so they never breed with other.
For example you and I would say, lions and tigers are members of different species, and lion and tigers would agree. But it turns out that sometimes in zoos, if you put lions and tigers in same cage, darken the lights and play some romantic music, in those rare occasions when they don't wind up attacking each other, sometimes you wind up with a liger or a tion.
So, what are some of the ways that we keep species separate? These are called reproductive isolating mechanisms. They are broken down into two categories. The first of which is what's called prezygotic. And this is any mechanism, any technique or anything that prevents the sperm from meeting the egg and fertilizing it. That's what creates a zygote, hence the name prezygotic.
Well what happens if you do somehow get the sperm and egg together? That's what are called postzygotic reproductive isolating mechanisms. And here something wind up creating weak or dead offspring. So let's take a closer look at those prezygotic reproductive isolating mechanisms.
First of which is habitat isolation. If you never meet each other, it's really hard for your sperm or your egg to come together with his or her sperm or egg. So by having things in different habitats, in different locations, like African and India, very different, those things tend to stay separate. You could also have two kinds of creatures in the same general vicinity. Let's say in a rainforest, there are some creatures that stay way up in the canopy, while others stay down in the rainforest floor and they never meet each other.
Mating seasons is another good way to keep species separate. Now we humans we're pretty freaky and we can have sex and successful sex. And by successful sex I mean you wind up having children and grandchildren and so on. Well if you have a mating season say in March, and something saddles up next to you in June. Try as hard as they may, nothing is going to come of it hopefully.
On to the next one, that's the different mating rituals. And this is one of my favourite areas because it's so rich in all sorts of examples. We see it all the time. Some spider walks up to you and starts doing this with its front legs, hopefully you're not going to go 'hubba hubba'. But what is doing is trying to send you the signals that, "Hi I'm here to mate, please don't kill me, yet." On the other hand if you walk up to a female spider and hand her some flowers and chocolates, and bust out your best dance moves, she's not going to have sex with you. That's because you have different mating rituals. And this helps makes sure we don't wind up creating spider-man.
Now in humans, what are some of our mating rituals? Watch any school dance and you'll see a bunch of them. In humans it's also suspected that intelligence is in fact part of our mating rituals. Becoming intelligent is an extremely difficult thing to do. It takes just the right combination of genes, just the right kind of foetal development, growing up development. And it's thought that, one of the reasons that we go through so much trouble to show off our intelligence, is to try to attract mates.
Now of course you can't think of anybody who has managed to achieve success with the opposite sex, even though they're kind of ugly looking, but they have been successful in the creative industry known as the music industry. Let's move on.
Another one is to put it delicately, the "Lock" and "Key" hypothesis. The idea that if the male's 'key' can't fit into the female's 'lock'. Then the sperm is not going to make its way to the egg. For example, let's imagine for some reason a male blue whale fell madly in love with a female guinea pig. Try as hard as they may, their only hope for children as adoption. Moving on.
Incompatible gametes. Even if somehow you manage to get the sperm together with the egg, there is all these enzymes or proteins on the surface of the sperm and egg that help ensure that fertilisation just can't happen. Imagine for example, you really fell in love with a sea urchin, and you went searching through the type hole you found your true love there. You'd done the research, you've figured out their mating season. You somehow figured out whatever the heck their mating rituals were. Well, even if you engaged in external fertilisation, so you got around the problem with the lock and key, your sperms aren't able to survive in the ocean. And so maybe just die. And even if they managed to somehow bump into the egg before they die, they still couldn't eat their way in to fertilise it.
Now let's take a look at the postzygotic mating rituals. Well a lot of time what happens is that you wind up getting very weak offspring. Because due to the chromosomal mismatches between one species and the next, the child may be missing some vital genes that are important. Or they may be regulated incorrectly.
Ultimately, sometimes you can get very strong viral offspring, but unfortunately they are infertile. An example of this would be the mule. Donkeys and horses are obviously closely related creatures and they can successfully create a mule offspring. Mules are great, they're strong, high endurance. They're wonderful pack animals, but they are all infertile. This is the way that they maintain that species barrier.
Now you know what a species is and how the differences between two species are maintained. We've also discussed some of the evidence that scientists use to determine evolutionary relatedness. So, now let's go into the patterns that I discussed before, the kinds of evolutionary changes that are seen. First, let me start off by talking about what's called gradualism and punctuated equilibrium.
Gradualism is the initial idea that Darwin came up with. The idea that species would gradually accumulate from one generation to the next, tiny little genetic changes. Until after thousands of generations, enough of these small little changes will have accumulated, to make the descendants different enough from their ancestors, that we'll all agree in they're different species. Okay, makes sense.
However when you look at the fossil record, you'd expect to see a lot of intermediate fossils, what are called transition fossils. And often you would find them but quite often you'd also not find them. A big part of that is because, becoming a fossil is a weird event. We don't often do that. I'm not going to become a fossil, and it's very unlikely you are.
So if you have a small population of organisms to begin with, the chances of any of them becoming a fossil are pretty rare. However, scientists were still kind of puzzled about this. And something they realised though was that, actually it doesn't have to happen that way. Instead, you could have a group of organisms, and if their form does match the environment, in fact natural selection would argue against mutation, because a mutant is messing with success.
And so natural selection removes the mutants. And that's what provides the equilibrium part for this whole thing. Now, so long as the environment stays the same, the organism stays the same. But, if there is some kind of relatively sudden environmental change, a storm or global warming, then all bets are off. And now the mutants aren't selected against and maybe, there may be some weird mutant that does have an advantage. And so you can have this relatively rapid evolutionary change. And that's where the punctuated part comes from this.
Now you have got to be careful because, when a biologist says rapid, they may mean, "It's only 700 or a 1000 generations." So that can be a rapid change taking 30,000 years. So now let's take a look at what's called allopatric versus sympatric speciation.
Speciation is the creation of new species. It happens in two basic ways based on; is the new species in the same geographic population as the parental population? Or is it in a different one?
Allopatric is when the new population is physically, geographically, separated from the parental population. Now, in this diagram here, let's imagine we start off with a group of trees. And through just standard erosion, a river eventually eats the ground away between parts of the population. Now if there is a big enough river, it's hard for the pollen from one group of trees, to get to the flowers of the trees on the other side. So over long periods of time, you can accumulate genetic differences between the two sides. If somehow, ultimately, some pollen from one side did make its way to the other side. If you have had enough time for the genetic differences to accumulate, then their reproductive will isolate, and the pollen won't be able to pollinate the flower. That's allopatric speciation.
So now let's take a look at sympatric.
With sympatric, the new sub population is in the same general location, geographic location, as the parental population. This is pretty rare amongst animals, because any time you have a small group of animals with a genetic trait, they just tend to start having sex with the larger population that's near them. And it just becomes a generic trait shared by the greater population.
Now, you could imagine perhaps if that sub group that has this new gene, maybe they develop different mating rituals at the same time or feeding patterns. Maybe behaviourally they'll be prevented from mating the bit of greater population, but that doesn't happen so often. Plants on the other hand, they do this all the time.
Now one of the things that makes this possible for plants, is that they can self-fertilise. Now, this means that you can have a new mutant by himself. He has some kind of weird mutation occur, and he just self-pollinates. And then there is a small crop of new trees with this mutation on location.
Now you can also have it when you have a high breed between two different species. But remember, we talked about that. That's one of those postzygotic reproductive isolating mechanisms, that high breed sterility. Why are high breeds sterile? Well a lot of times it's because, they're missing homologous chromosomes. Because their parents didn't provide two homologous chromosomes of every kind to the new high breed.
With animals, you're pretty much out of luck. With plants, they can do this trick called polyploidysm, where they accidentally double the number of every kind of chromosome in one of their cells through a mistake in mitosis.
This can go on to allow that plant to now have its homologous chromosomes. It can make gametes successfully and that allows them to create a new species. With animals, they couldn't survive this process. It would screw up their genes too badly, and especially their nervous system which is very sensitive to genetic differences.
They'll be totally screwed up. Why can plans do this? Well they're already mental vegetables.
The last thing I'm going to discuss is, different kinds of selection. And the first of which is directional selection. Now in this graph, orange represents the old population. So let's suppose this orange represents the bell curve or distribution of say, rabbits speed.
So this is the number of rabbits; these are slow rabbits, these are fast rabbits. Well if you're slow rabbit, and there is wolves in the population, what's going to happen? You get eaten. If you're fast rabbit, what happens? Your slow friends slow down the wolf and you escape. And so over time, we'd see a movement of the average speed of rabbit in that direction, that's why it's called directional selection. Seems logical enough.
The opposite of this, in certain degrees is what's called stabilizing selection. Let's take a look at that.
With stabilizing selection, again I'm using orange to represent the old average. Here, rather one extreme or the other be selected for, in this case both extremes are being selected against, and the average is being favoured. Now what would be an example of this? Well, with humans for a long time, we've had a fairly stable average birth weight,of somewhere around six to eight pounds. Much smaller than six or five pounds, and the baby winds up being born without a lot of internal resources to be able to survive their first critical months. Whereas if you were a large baby, 20 pound baby, lots of fat reserves. But that baby is probably not going to be able escape mummy and baby and probably mummy will also both die. And that kept birth weight pretty stable.
Let's move to the next one, disruptive selection.
Again, our starting population, here the average individual is being selected against and the extremes are being favoured. Now what would be an example of this? The one that sticks in my mind, and sticks in the mind of my students, is one that was shared with me by one of my college professors. There was this group of fish where the male fish would engage in a reproductive isolating mechanism. They would engage in battles with other fish of their same species and the males would drive off the other males. And generally the larger the male, the better his chance of winning the battles. What would you expect to see in the sizes of males over a period of time? You would expect to see larger and larger males. And you did see larger and larger males.
However, you also saw a fair number of small adult males. And the scientists will sit there and scratch their heads and go, "What's up with that." What they did is they watched and they would see, large male drive off these small males and they rid some away. But the large males wouldn't waste any of their energy chasing off the juvenile fish, the little young ones, because they weren't ready yet to mate. So no waste of energy. Well what would happen is that, the large guy will drive off all of his competitors and reclaim to a region of the string. The females will come over and do their dance and then it turns out that there is this little ninjas, adult males that are the same size as juveniles. They'd hide below a rock. Waiting till right the end of the dance and zip in between and spray his sperm out and zip away. And the big guy's concentration has been disrupted and often he would fail to release his sperm. So that little guy managed to fertilize all the eggs that the female sprayed out. This is called the sneaky 'efforts' hypothesis and F stands for fish.
That's a lot of ground to cover. But hopefully like I said, this stuff makes sense, except for those evil plans and those freaky deaky polyploidism. But if you understand the underlying principles. It's pretty straight forward and easy to understand.
For the test, you really need to know the requirements for natural selection. And remember, it's survival of the reproductive fit, and not just the strongest. It's all about getting your offspring to have their own offspring.
Don't forget about the random changes to genetic differences caused by that random genetic drift stuff I talked about. It's becoming more and more important than scientists originally thought. Be prepared for questions about evolution, and tossing out evidence of evolution; antibiotic resistant bacteria, comparative anatomy, the homologous structures versus the analogous structures, the comparative molecular biology and comparing the embryos of different creatures.
There could also be questions where they ask about, this development of new spaces. So be ready with the allopatric versus sympatric speciation. And also talk about the kinds of selection events that can occur. And just to keep yourselves entertained, next time valentines day rolls around, think about those prezygotic mating rituals, when you see somebody giving somebody else a gift of candy to demonstrate their ability to acquire resources. Roses are red violets are blue, I'm flaunting my reproductive fitness at you.
Please enter your name.
Are you sure you want to delete this comment?
- Organic Molecules 5,311 views
- Parts of a Cell 3,459 views
- Enzymes 2,993 views
- Cellular Transport 2,875 views
- Photosynthesis vs. Respiration 4,222 views
- Photosynthesis 3,347 views
- Respiration 2,844 views
- Cell Division 2,507 views
- Mendel's First Law 2,762 views
- Mendel's Second Law 2,547 views
- Non-Mendel Genetics 2,971 views
- DNA Structure 2,450 views
- DNA Replication 2,814 views
- Transcription 2,386 views
- Translation 2,121 views
- Mutation 2,291 views
- Biotech: Genetic Engineering 3,289 views
- Biotech: DNA Fingerprinting 2,700 views
- Hardy Weinberg Equilibrium 2,858 views
- Diversity of Organisms 2,919 views
- Animal Kingdom, Part A 2,740 views
- Animal Kingdom, Part B 2,697 views
- Water Transport in Plants 3,429 views
- Muscle Contraction 3,510 views
- Circulatory System 2,691 views
- Immune System 2,793 views
- Hormone System 2,670 views