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00:00Sure, you can always talk to me.
00:09The talk?
00:10I don't think I'm ready for that level of responsibility.
00:16Dear Tim and Moby,
00:18Why do flowers need bees?
00:20From Josie.
00:21Seems like a pretty weird relationship, right Josie?
00:25Bees are pollinators.
00:27They travel from flower to flower, transferring pollen as they go.
00:31When pollen lands on the right part of a flower, that plant will make a seed, which can grow
00:36into a whole new plant.
00:38I don't know if you've ever thought about it this way, but that's sexual reproduction.
00:44Material from one parent coming together with material from another parent to create a new
00:50organism.
00:53Since plants can't exactly meet up, they need help from pollinators like bees.
00:58Well, some organisms can actually skip all that trouble.
01:02Like strawberries, a single plant can sprout a whole new plant all on its own.
01:07Creating an organism from a single parent is called asexual reproduction.
01:12And it's common among many plants, simple animals, and microbes.
01:16Earth's earliest lifeforms reproduce this way.
01:19It's simple.
01:20One parent splits into two separate but identical organisms.
01:24The child, or offspring, has all the same traits as its only parent.
01:29If the parent has extra large leaves, its offspring has them too.
01:33Traits like these are passed on through DNA, a molecule in cells.
01:38DNA carries all the instructions for building and maintaining an organism.
01:43In asexual reproduction, the offspring gets a full copy of its parent's DNA.
01:48So it has all the same traits that helped its parent survive.
01:53That's a question scientists wondered about for a long time.
01:56If single parenting works just fine, why do so many organisms reproduce sexually?
02:02It's a whole complicated thing that takes a ton of energy.
02:06Some organisms go to great lengths just for a chance of finding a mate.
02:10Compare that to strawberries, which just split off into identical new plants.
02:15Easy peasy.
02:16But there's a big problem with all that similarity.
02:19Weaknesses get passed down too.
02:22Say the parent needs a lot of water to survive.
02:25If a drought comes along, the parent's a goner, and its offspring are too.
02:29Bringing in a second parent makes all the difference.
02:32When there are two parents, the offspring are a mix of both their DNA.
02:37That's what sexual reproduction is really about—producing genetic variation, or differences
02:42between individuals' DNA.
02:45This comes in handy when the environment changes and life gets tough.
02:48Like, some individuals may be better at dealing with cold weather.
02:53So they're more likely to survive a severe winter.
02:57Well, the genetic variation isn't entirely random.
03:00DNA is organized in chunks called chromosomes.
03:05In humans, there are 46 of them, but some animals have as few as 6, and some plants
03:09have over a thousand.
03:11In sexually reproducing organisms, chromosomes are arranged in pairs, one from each parent.
03:17So our 46 chromosomes are actually 23 pairs, half from the female and half from the male.
03:23Each half is delivered in specialized cells called gametes.
03:28The female gamete is called an ovum.
03:31The male's is called a sperm, or pollen if it's a plant.
03:35Gametes are different from other cells in the body.
03:37They carry only one copy of each chromosome.
03:41That way, when a sperm and egg cell meet up during sex, the result is a full set of paired
03:47chromosomes.
03:48This process is called fertilization.
03:50Well, having two versions of each chromosome creates a lot of genetic variation.
03:57Chromosomes carry genes—segments of DNA that hold the instructions for different traits.
04:02In a plant, it could be the size of its fruit, or tolerance for hot temperatures.
04:07With two versions of each chromosome, there are two versions of each gene.
04:11The corresponding version of each gene are called alleles.
04:14Though both versions have instructions for the same trait, they may be written a little
04:18differently.
04:19Like, this gene determines the size of the plant's fruit, but one allele results in
04:24bigger fruit than the other.
04:26That's why people have some traits in common with their mother, and some with their father.
04:31You might end up with a cleft chin because you got the cleft chin allele from your dad,
04:35and freckles because you got the freckle allele from your mom.
04:39In humans, there are about 20,000 different genes, and each one has different alleles.
04:44So, the number of possible different combinations of alleles is virtually endless.
04:48In other words, sexual reproduction creates almost limitless variation.
04:54All these differences come in handy when environmental conditions change.
04:59A trait that didn't help an organism survive before might suddenly come in handy.
05:04Those who survive will have a better chance of reproducing and passing on their DNA.
05:09So favorable traits are more likely to show up in the next generation, and unfavorable
05:15traits will tend to disappear.
05:18Over time, favorable traits can spread through entire populations.
05:23This process of change is known as evolution, and it's fueled by the differences in genes.
05:29Earth's biodiversity, or variety of living things, depends on genetic variation.
05:35Without it, there would be fewer kinds of organisms, and they would be simpler, too.
05:39Earth would look very different.
05:41For one thing, we humans wouldn't be here.
05:43I don't know about you, but I personally enjoy having all the different kinds of beetles.
05:50Let me tell you a story about beetles.
05:52Wait, where are you going?
05:56I thought you wanted to talk.