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Video transcript

hi I'm Hank and I'm a human but let's pretend for a moment that I am 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 lichens and 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 looking like this so thanks to this it 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 two percent 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 over time the black colored trait will become more common as for me my friends it's a wonderful example of natural selection the process by which certain inherited traits make it easier for some of the visuals 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 and how frequently some alleles 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 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 earthworms 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 and the 1830s a 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 adaptations he noticed on his journey 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 shaped and sized 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 puncture the fruit scan these superior inherited traits led Arwen to another idea the finches increased fitness for their environment that is the relative ability to survive and create offspring expending 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 the first principle is that different members of a population have all kinds of individual variations these characteristics whether their 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 favourable 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 his 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 in 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 in Malthus his 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 with 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 an order for natural selection to take place a population has to have variations some of which are heritable and what a variation makes an organism more competitive that variation will tend to be selected like with the peppered moth it survives 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 evolve instead natural selection produces evolutionary change because it chain is the genetic composition of entire populations and that occurs through interactions between individuals and their environments 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 weighed heavily on Darwin's mind as 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 in 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 wedge woods and there were several other marriages between cousins in the family they're 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 caused 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 conseguí despairs often resulted in weaker and sickly descendants and the same was true of his family Emma and Charles had ten 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 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 organisms physical form or its phenotype that's changing but it's 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 long 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 populations trait distribution 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 middleweight range which help 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 while 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 Daphne the population was hit with an epidemic of a yeast parasite after about a half dozen generations variants had emerged and how the Daphnia responded to the parasite some became less susceptible to the yeast but were smaller and had a fewer offspring 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 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 all these are the main ways that selective pressures can affect populations those pressures can also come from factors other than like food supply or predators and 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 maximise their fitness they not only need to survive but they also need to reproduce more and they can do that one or 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 big cocktail isn't exactly camouflage but the more impressive the tailed better the chance is a male 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 and the Origin of Species talking about breeding of pigeons and of cattle and of plants to demonstrate the principles of selection we incurred the selection of some traits and discourage others it's how we got grains that produce all those nutrients which is how we manage 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 you going no no but there's still both dogs through the same species technically a corgi and a greyhound could get together and have a baby dog though it would be 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 going to talk about next episode on crash course biology how one species can turn into another species
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