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Natural Selection

Hank guides us through the process of natural selection, the key mechanism of evolution. Created by EcoGeek.

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  • piceratops ultimate style avatar for user Dayvyd
    How does a new species increase heterogeneity in its population? Is there a crossover period of sorts where the new species can interbreed with its parent species?
    (35 votes)
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    • leafers seed style avatar for user Nadine Schrode
      A new species doesn't interbreed with its parent species. The two never coexist. It takes way too long for a new species to arise and it's a gradual process.
      But, species often have several differing populations that have undergone changes due to different selection pressures. Those populations can still interbreed if they come across each other. That's called genetic drift. Some of these populations will accumulate more and more changes in adaption to their environment and will eventually be considered a new species.
      Think about different dog breeds. They are different "populations" but of the same species so they can still interbreed and cause variety in their offspring.
      Horses and donkeys are an interesting example because they seem to be different species but they can still interbreed. Their offspring is usually infertile. So it seems that these animals are on the border of having become two different species.
      (3 votes)
  • piceratops ultimate style avatar for user Jessamyn
    At 0.25, how do creationists explain the evolution of the peppered moths? I've heard about the "neither creation nor evolution can be observed scientifically" argument but this is evidently an example of evolution in the natural world...
    (11 votes)
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  • blobby green style avatar for user paul.kim
    What is the difference between adaptation and evolution?
    (10 votes)
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    • piceratops ultimate style avatar for user Just Keith
      In short, an adaptation is the effect, evolution is the cause.
      An adaptation is a trait that is useful in the life of an organism, such as resistance to a pathogen or the ability to break a nutshell. Evolution is the process by which natural variations are selected such as those that provide advantageous adaptations are naturally selected for.

      But, note that natural selection does not keep improving an adaptation: once the adaptation gets adequate, such that any further improvement will not provide additional reproductive advantage, then natural selection will not continue improving the trait -- it will just keep the trait from getting worse (as long as the trait is needed).

      Also, remember that adaptations are traits that appears in individuals. But, individuals do not evolve -- evolution works on breeding populations over generations, not on specific individuals.
      (8 votes)
  • leafers seedling style avatar for user Allison Hoss
    I live on the coast in North Carolina and we typically have mild winters, however this winter we had two ice storms. The trees froze and branches cracked and fell everywhere. Some trees, especially pines and willows, had their trunks split in half . Once it was over the damage was worse than a hurricane. Now that spring is here we have the most pollen I have ever seen. Is it possible that in an attempt to survive that they are producing more pollen than usual in reaction to the traumatic winter?
    (12 votes)
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    • winston baby style avatar for user Ivana - Science trainee
      I do not think that anthropogenic factor here is the main culprit.

      https://waojournal.biomedcentral.com/articles/10.1186/s40413-015-0073-0

      When it comes to allergies, it is hard to find out who the real culprit is.
      Just because diagnostics is better does not mean we get more pollen (hay fever) than before. Allergies was noticed back in history, but the concept is introduced at the beginning of the 21st century.

      On the other hand, pollution can take a role, just like the environmental stress, which stimulated plants to reproduce. But again, having little to almost zero or abundance of pollen would likely result in allergy if nearby are people who are allergic to pollen.
      (1 vote)
  • duskpin ultimate style avatar for user Azeema Marzook
    My family is so closely interrelated that if I make a family tree dating to my great-great grandparents (technically saying this from my mother's side), it will show both my mother's and father's side (My parents are also first cousins). My family has always had this extreme tendency to marry into the family, so is this why I'm short, skinny, and fevers are very common for me?
    (4 votes)
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    • female robot grace style avatar for user tyersome
      Disclaimer — I am not a medical genetics counselor and this is not an appropriate place to get medical advice!

      That being said, inbreeding is known to cause health problems and increases the chances that many offspring will be homozygous for some of the negative recessive alleles§ all humans carry.

      Impaired immune function is also likely — for example due to reduced diversity at the HLA locus.

      That being said, inbreeding can also result in eliminating some of the negative recessive alleles from a population (in this case your family), but at the expense of increased levels of illness and I would guess many miscarriages.

      So, the good news is if you live to adulthood and choose to have a child with someone who isn't a close relative, your child might actually be somewhat less likely to suffer an inherited genetic disorder ...

      §Note: Not all recessive alleles have negative effects on an individual and not all dominant alleles are beneficial, but there are some recessive alleles that have very negative effects when they become homozygous. These effects are more-or-less completely hidden in heterozygotes.
      (3 votes)
  • leafers ultimate style avatar for user Louis Poirot
    How does a new species increase heterogeneity in its population? Is there a crossover period of sorts where the new species can interbreed with its parent species?
    (3 votes)
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    • leafers seed style avatar for user Nadine Schrode
      Guys, I think you're looking at it too narrowly.
      One species doesn't just turn into another. Individuals of a species undergo changes from generation to generation and depending on the environment different changes might be maintained in different populations of the same species. But the longer those populations are separated and allowed to change according to the different environments the more different they become. And only after many many many generations might those populations be so different that they can't be considered the same species anymore (they can't interbreed anymore). in the here and now we can clearly see the differences of two "sister-species" but the original species never co-exists with the new species, because it's a gradual, long-term process that can often only be labeled with terminology like "species" afterwards.

      But to answer your question variation is constantly created in all organisms by random mutations and by genetic drift (which is when two populations in a species, that have already undergone some changes, interbreed)
      (4 votes)
  • piceratops ultimate style avatar for user Avro
    So in the context of humans there were many different species? but our species of humans survived dues to variations from Neanderthals?
    (4 votes)
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  • leafers seedling style avatar for user Allison Hoss
    Hank has mentioned several times that angiosperms arrived around the time of dinosaur extinction, and also that there was a prehistoric plant type that now is used for energy in the form of coal. Could it be possible that the angiosperms were in competition with these forest plants (can't remember their name) and wiped them out therefore destroying food for herbivores and leading to dinosaur extinction?
    (1 vote)
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    • winston default style avatar for user infinitely_infinite
      It could be! However, the accepted theory (a giant stone speeding at extreme speeds smacking the bigger rock we live on) has better evidence (deposits of a mineral rare on our rock but common on other rocks around the world) and more importantly, dinosaurs would have probably eaten the angiosperms due to lack of cycads .
      (1 vote)
  • aqualine ultimate style avatar for user Kali
    Is there a limit to how many times an organism can evolve?
    (2 votes)
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  • leafers ultimate style avatar for user Reader247
    How many times can something evolve?We are/were learning about it and I asked this question, but my teacher couldn't answer this. Is there even a real cap on how many times you can evolve?Is there one on how long it takes?
    EDIT:Did Darwin get inspiration from Gregor Mendel?
    (2 votes)
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    • leafers ultimate style avatar for user Andrew
      Great questions! Your first question depends a lot on what you mean by something evolving more than once. Certain concepts can definitely evolve more than once. For instance, flight has evolved at least three times that I can think of. Birds, bats, and insects have all developed flight even though they're in completely different lineages. I believe that a similar thing could also be said of vision.

      As for an entire species evolving more than once, we do not observe anything like that in nature. Two groups of unrelated organisms could in theory stumble upon the same genetic pattern, but the odds of that happening would be extremely low. In fact, it would be pretty much a statistical miracle.

      Unfortunately, Darwin did not know about Mendel. Had he understood the implications of Mendel's findings, it would have impacted his work for the better. He would have gotten a glimpse at the mechanisms of heritability and how they fit into his model of evolution.
      It's a shame that he didn't get to see that.

      There is an old legend to the effect that Darwin had copies of Mendel's work in his office, but that he never got around to reading them. I'm not sure if this has ever been substantiated by evidence though.
      (2 votes)

Video transcript

- Hi, I'm Hank, and I'm a human. But let's pretend for a moment that I'm a moth. And not just any moth. A peppered moth. Now let's pretend that I'm living in London in the early 1800s right as the Industrial Revolution is starting. Life is swell. My light-colored body lets me blend in with the light-colored likens in tree bark, which means that birds have a hard time seeing me and which means that I get to live. But it's starting to get noticeably darker around here with all these coal-powered factories spewing soot into the air, and suddenly all the trees have gone from looking like this to looking like this. So thanks to the soot-covered everything, I've got problems. But you know who doesn't have problems? My brother. He looks like this. Yeah, he has a different form of the gene that affects pigmentation. Moths like him represent about 2% of all the peppered moths at the start of the Industrial Revolution, but by 1895, it'll be 95%. Why? Well, you're probably already guessing, as the environment gets dirtier, darker moths will be eaten less often and therefore will have more opportunities to make baby moths, while the white ones will get eaten more so overtime the black color trait will become more common. As for me. (upbeat music) My friends, it's a wonderful example of natural selection, the process by which certain inherited traits make it easier for some individuals to thrive and multiply, changing the genetic makeup of populations over time. For this revelation, which remains one of the most important revelations in biology, we have to thank Charles Darwin, who first identified this process in his revolutionary 1859 book, On the Origin of Species by Natural Selection. Now, lots of factors play a role in how species change over time, including mutation, migration, random changes in how frequently some allele show up, a process known as genetic drift. But natural selection is the most powerful and most important cause of evolutionary change, which is why today we're going to talk about the principles behind it and the different ways in which it works. Darwin came to understand the process of natural selection because he spent his adult life, even most of his childhood, obsessed with observing nature. He studied barnacles, earth worms, birds, rocks, tortoises, fossils, fish, insects, and to some extent even his own family. And I'll get back to that in a bit. But it was during Darwin's famous voyage on the HMS Beagle in the 1830s surveying expedition around the world that he began to formulate this theory. Darwin was able to study all kinds of organisms and he kept amazing journals. Looking back on his notes he hit upon a couple of particularly important factors in species survival. One of them was the many examples of adaptions he noticed on his journey. The ways in which organisms seem to be ideally shaped to enhance their survival and reproduction in specific environments. Maybe the most famous example of these were the variations of beaks Darwin observed among the finches in the remote Galapagos Islands off the coast of South America. He observed more than a dozen closely related finch species all of which were quite similar to mainland finch species, but each island species had different shape and size beaks that were adapted to the food available specifically on each island. If there were hard seeds, the beaks were thick. If there were insects, the beaks were skinny and pointed. If there were cactus fruit, the beaks were sharp to puncture the fruit's skin. These superior inherited traits led to Darwin to another idea. The finches' increased fitness for their environment, that is their relative ability to survive and create offspring. Explaining the effects of adaptation and relative fitness would become central to Darwin's idea of natural selection. And today, we often define natural selection and describe how it drives evolutionary change by four basic principles based on Darwin's observations. So, first principle is that different members of a population have all kinds of individual variations. These characteristics, whether they're body size, hair color, blood type, facial markings, metabolisms, reflexes, they're called phenotypes. The second is that many of these variations are heritable and can be passed on to offspring. If a trait happens to be favorable, it does future generations no good if it can't be passed on. Third, and this one tends to get glossed over a lot even though it's probably the most interesting, is Darwin's observation that populations can often have way more offspring than resources like food and water can support. This leads to what Darwin called the struggle for existence. He was inspired here by the work of Thomas Malthus, an economist who wrote that when human populations get too big, we get things like plague and famine and wars and then only some of us survive and continue to reproduce. If you missed the SciShow Infusion that we did on human overpopulation today and Malthus's predictions, you should check it out now. This finally leads to the last principle of natural selection which is that given all of this competition for resources, heritable traits that affect individuals fitness can lead to variations in their survival and reproductive rates. It's just another way of saying that those of favorable traits are more likely to come out on top and will be more successful with their baby making. So, in order to wrap all these principles together, in order for natural selection to take place, a population has to have variations, some of which are heritable. And when a variation makes an organism more competitive, that variation will tend to be selected. Like with the peppered moth, it survived because there was a variation within the species, the dark coloration, which was heritable and in turn allowed every moth that inherited that trait to better survive the hungry birds of London. But notice how this works. A single variation in a single organism is only the very beginning of the process. The key is that individuals don't devolve. Instead, natural selection produces evolutionary change because it changes the genetic composition of entire populations. And that occurs through interactions between individuals and their environment. (happy piano music) Let's get back to Darwin for a minute. In 1870, Darwin wrote to his neighbor and parliamentarian, John Lubbock, requesting that a question be added to England's census regarding the frequency of cousins marrying and the health of their offspring. His request was denied but the question was something that weighted heavily on Darwin's mind 'cause he was married to Emma Wedgwood, who happened to be his first cousin. Her grandfather was Josiah Wedgwood, founder of the company that remains famous for its pottery and china, and he was also Darwin's grandfather. In fact, much of Darwin's family tree was complicated. His marriage to Emma was far from the first Wedgwood-Darwin pairing. Darwin's maternal grandparents and mother were also Wedgwoods and there were several other marriages between cousins in the family, though not always between those two families. So, Darwin and to a greater extent his children carried more genetic material of Wedgwood origin than Darwinian. And this cost some problems, the likes of which Darwin was all too aware thanks to his own scientific research. Darwin of course spent time studying the effects of crossbreeding and inbreeding in plants and animals, noting that consanguineous pairs often resulted in weaker and sickly descendance, and the same was true of his family. Emma and Charles had 10 children, three of whom died in childhood from infectious disease, which is more likely to be contracted by those with high levels of inbreeding, and while none of Darwin's seven other children had any deformities, he noted that they were not very robust and that three of them were unable to have children of their own. Likely, another effect of inbreeding. Now, so far, we've been talking about natural selection in terms of physical characteristics like beak shape or coloration, but it's important to understand that it's not just an organism's physical form, or its phenotype, that's changing, but its essential genetic form, or its genotype. The heritable variations we've been talking about are a function of the alleles that organisms are carrying around. And as organisms become more successful, evolutionarily speaking, by surviving in larger numbers for longer and having more kids, that means that the alleles that mark their variation become more frequent. But these changes can come about in different ways, and to understand how, let's walk through the different modes of selection. The mode we've been talking about for much of this episode is an example of directional selection, which is when a favored trait is at one extreme end of the range of traits, like from short to tall or white to black or blind to having super night goggle vision. Over time, this leads to distinct changes in the frequency of that expressed trait in a population, when a single phenotype is favored. So, our peppered moth is an example of a population whose trait distribution is shifting toward one extreme, almost all whitish moths, to the other extreme, almost all blackish. Another awesome example is giraffes' necks. They've gotten really long over time because there was selection pressure against short necks which couldn't reach all of those delicious leaves. But there's also stabilizing selection which selects against extreme phenotypes and instead favors the majority that are well adapted to an environment. An example that's often used is a human's birth weight. Very small babies have a harder time defending themselves from infections and staying warm, but very large babies are too large to deliver naturally. Because of this, the survival rate for babies has historically been higher for those in middle-weight range, which helps stabilize the average birth weight. At least until cesarean sections became as common as bad tattoos. So, what happens when the environment favors extreme traits at both ends of the spectrum whilst selecting against common traits? That's disruptive selection. Now, examples of this are rare but scientists think that they found an instance of it in 2008 in a lake full of tiny crustaceans called Daphnia. The population was hit with an epidemic of a yeast parasite and after about a half a dozen generations, a variance had emergence in how the Daphnia responded to the parasite. Some became less susceptible to the yeast but were smaller and had fewer organism, the others actually became more susceptible but were bigger and able to reproduce more, at least while they were still alive. So there were two traits that were being selected for, both in extremes and both to the exclusion of each other. Susceptibility and fecundity. If you got one, you didn't get the other. Also an interesting example of selection being driven by a parasite. Now, while these are the main ways that selective pressures can affect populations, those pressures can also come from factors other than environmental ones like food supply or predators or parasites. There's also sexual selection, another concept introduced by Darwin and described in the Origin of Species as depending not on a struggle for existence but a struggle between individuals of the same sex, generally the males, for the possession of the other sex. Basically, for individuals to maximize their fitness, they not only need to survive but they also need to reproduce more, and they can do that one of two ways. One, they can make themselves attractive to the opposite sex. Or two, they can go for the upper hand by intimidating, deterring, or defeating the same sex rivals. The first of these strategies is how we ended up with this. I mean, the peacock tail isn't exactly camouflage. But the more impressive the tail, the better the chances a male will find a mate and will pass its genes to the next generation. Sad looking peacock tails will diminish over generations, making it a good example of directional sexual selection. The other strategy involves fighting, or at least looking like you want to fight for the privilege of mating, which tends to select for bigger or stronger or meaner-looking mates. And, finally, thanks to us humans, there are also unnatural forms of selection, and we call that artificial selection. People have been artificially selecting plants and animals for thousands of years and Darwin spent a lot of time in the Origin of Species talking about breeding of pigeons and of cattle and of plants to demonstrate the principles of selection. We encourage the selection of some traits and discourage others, that's how we got grains that produce all those nutrients. Which is how we managed to turn the gray wolf into domesticated dogs that can look like this or like that. Two of my favorite examples of artificial selection. Now, these are different breeds of dogs. Oh, where are you going? No, no. But they're still both dogs, they're the same species. Technically, a corgi and a greyhound could get together and have a baby dog, though it would be a weird-looking dog. But what happens when selection makes populations so different that they can't even be the same species anymore? Well, that's what we're gonna talk about next episode on Crash Course Biology. How one species can turn into another species.