Hi. In this lesson, we'll be talking about animal reproduction, which often is going to be sexual reproduction, which is when two organisms combine their genetic information, usually through the fusion of gametes. Now gametes are formed from meiosis, and these are going to be cells that have just one copy of each chromosome from the parent, making them haploid. Now if this sounds unfamiliar, I recommend you go check out our videos on meiosis and sexual life cycles. Now male gametes are known as sperm, and female gametes are known as eggs. Male gametes usually are smaller and motile, meaning they can move themselves; they can propel themselves. Female gametes tend to be larger and non-motile, meaning they can't propel themselves. Now we call the fusion of sperm and egg fertilization, and this will form a zygote. So we will get a zygote from fertilization, and that zygote is going to be diploid, meaning it will have two copies of each chromosome, one from each parent, thanks to those gametes each being haploid. Now some organisms reproduce through asexual reproduction. And this is when an offspring arises from a single organism, and it only receives genes from that parent. Now, there are a few ways this can occur, one is budding, which is when an organism will kind of grow off of its parent and break off when it becomes mature. You can see that happening in these yeast cells here. Yeast are, you know, eukaryotic organisms, and they reproduce by budding. You can see this parent cell has this bud growing off of it, so does this one. When these buds are mature, they will literally break off from their parent cells. This also occurs in the animal's hydra. They also reproduce through budding. Now fission is when an organism divides into two or more parts, and this is how, for example, bacteria reproduce. This is known as binary fission in bacteria. And as you can see, it's going to lead to two genetically identical organisms. Now, parthenogenesis is a very interesting type of asexual reproduction because it's when growth and development occur without fertilization. You see this in, for example, some species of lizards which are going to be all female, and will just grow and develop offspring in the app without any fertilization, and those offspring will, of course, be female.
So you might be wondering what's the point of sexual reproduction? Because it's more energy-intensive and it's less efficient than asexual reproduction. I mean, look at this little graphic behind me. Asexual reproduction, like we can see here, is going to lead to much greater increases in population size. So what's the advantage of doing it this way? You can't increase the population size nearly as efficiently. It also requires two organisms just to produce one new offspring, or, you know, in the case of some organisms, you'll produce multiple offspring. But you get my point. It requires two parents to get together to generate any offspring. So what's the point? Well, one explanation for this is known as the Red Queen hypothesis, and the idea is essentially genetic diversity. Organisms that reproduce asexually don't have a harder time introducing genetic diversity. Sexual reproduction naturally confers genetic diversity to the population. This is important because the Red Queen hypothesis says that organisms have to constantly adapt and evolve to compete with opposing organisms. This idea comes from a quote from Alice in Wonderland, from the character known as the Red Queen. When Alice is talking to her, she says that, you know, in where I come from, you know, not Wonderland, when you run you get somewhere. You know, when you move, you actually get to someplace. And the Red Queen says, "now here you see, it takes all the running you can do just to stay in the same place." And that is why this is termed the Red Queen hypothesis. It's because organisms have to constantly be running, right, adapting, and evolving, which is why sexual reproduction is favored because it introduces genetic diversity that promotes that adaptation and evolution. And this is necessary because they have to compete with all the other organisms around them that are also constantly adapting and evolving.
With that, let's go ahead and flip the page.