Natural Selection

Hank: Hi, I'm Hank, and I'm a human. Let's pretend for a moment that I'm a moth. Not just any moth, a peppered moth. 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, which means that I get to live. 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. Thanks to the soot-covered everything I've got problems. You know who doesn't have problems? My brother. He looks like this. 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 over time the black colored trait will become more common. As for me ... (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 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 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, 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. It was during Darwin's famous voyage on the HMS Beagle in 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 adaptions he noticed on his journey; the ways in which organisms seemed 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. 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 to puncture the fruit skin. These superior inherited traits led 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. 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 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' 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 individual's 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. 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. 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 changes the genetic composition of entire populations. That occurs through interactions between individuals and their environment. (boppy music) Let's get back to Darwin for a minute. In 1879, 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, because 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 Wedgwoods, and there were several other marriages between cousins in the family, though not always between those two families. Darwin, and to a greater extent his children, carried more genetic material of Wedgwood origin than Darwinian. 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 consanguineous pairs 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. While none of Darwin's other seven 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. 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. 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. 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's 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. 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. What happens when the environment favors extreme traits at both ends of the spectrum while selecting against common traits? That's disruptive selection. 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. After about a half dozen generations, a variance had emerged in how the daphnia responded to the parasites. Some became less susceptible to the yeast but were smaller and had 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 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. 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 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 maximize 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 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. 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. These are different breeds of dogs. Where are you going? Nope, nope. 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. What happens when selection makes populations so different that they can't even be the same species anymore? That's what we're going to talk about next episode on Crash Course Biology, how one species can turn into another species.