adaptation: Any heritable characteristic of an organism that improves its ability to survive and reproduce in its environment

adaptation: Any heritable characteristic of an organism that improves its ability to survive and reproduce in its environment. Also used to describe the process of genetic change within a population, as influenced by natural selection.

allele: One of the alternative forms of a gene. For example, if a gene determines the seed color of peas, one allele of that gene may produce green seeds and another allele produce yellow seeds. In a diploid cell there are usually two alleles of any one gene (one from each parent). Within a population there may be many different alleles of a gene; each has a unique nucleotide sequence.

adaptive landscape: A graph of the average fitness of a population in relation to the frequencies of genotypes in it. Peaks on the landscape correspond to genotypic frequencies at which the average fitness is high, valleys to genotypic frequencies at which the average fitness is low. Also called a fitness surface. deme: a deme is a term for a local population of organisms of one species that actively interbreed with one another and share a distinct gene pool. When demes are isolated for a very long time they can become distinct subspecies or species. The term deme is mainly used in evolutionary biology and is often used as a synonym for population.

anthropology: The discipline which is trying to understand what it means to be human.

anthropology subfields: 1) Ethnography, which studies human behavioral practices and the construction of meaning within human society; 2) Linguistic anthropology, which takes advantage of the fact that language occupies a uniquely important role in humans--not only for how we interact with each other, but how we develop and how we understand the world around us; 3) Archaeology, which tries to take advantage of the fact that human societies have left a long history of material culture dating back to deep into ancient times, 4) Biological anthropology, which seeks to understand human variation by looking at the biological mechanisms which shape that variation and by taking an evolutionary approach to understanding how variation is patterned across time in humans.

epigenetics: The study of the process by which genetic information is translated into the substance and behavior of an organism: specifically, the study of the way in which the expression of heritable traits is modified by environmental influences or other mechanisms without a change to the DNA sequence.

evolution: Heritable change in a population through time.

evolution: Darwin defined this term as "descent with modification." It is the change in a lineage of populations between generations. In general terms, biological evolution is the process of change by which new species develop from preexisting species over time; in genetic terms, evolution can be defined as any change in the frequency of alleles in populations of organisms from generation to generation. gamete: Haploid reproductive cells that combine at fertilization to form the zygote, called sperm (or pollen) in males and eggs in females.

gene: A sequence of nucleotides coding for a protein (or, in some cases, part of a protein); a unit of heredity.

gene flow: The movement of genes into or through a population by interbreeding or by migration and interbreeding.

genetic drift: Changes in the frequencies of alleles in a population that occur by chance, rather than because of natural selection.

genotype: The set of two genes possessed by an individual at a given locus. More generally, the genetic profile of an individual.

heterozygote: An individual having two different alleles at a genetic locus. Compare with homozygote.

homozygote: An individual having two copies of the same allele at a genetic locus. Also sometimes applied to larger genetic entities, such as a whole chromosome; a homozygote is then an individual having two copies of the same chromosome.

morphology: The study of the form, shape, and structure of organisms.

mutation: The creation of new variation from one generation to the next.

mutation: A change in genetic material that results from an error in replication of DNA. Mutations can be beneficial, harmful, or neutral.

natural selection: The differential survival and reproduction of classes of organisms that differ from one another in one or more usually heritable characteristics. Through this process, the forms of organisms in a population that are best adapted to their local environment increase in frequency relative to less well-adapted forms over a number of generations. This difference in survival and reproduction is not due to chance.

phenotype: The physical or functional characteristics of an organism, produced by the interaction of genotype and environment during growth and development.

phenotypic characters: Individual traits that can be observed in an organism (including appearance and behavior) and that result from the interaction between the organism's genetic makeup and its environment.

Welcome to 207X

Welcome to anthropology 207X. My name is Adam Van Arsdale, Professor of Anthropology here at Wellesley College. This class is an introduction to human evolution. We will explore the human fossil record. We'll talk about how we develop knowledge from the human fossil record, and you'll get the opportunity to explore key fossil localities, to learn about the researchers who have shaped our understanding of human evolutionary history, and mainly to understand how and why we know the things we know about our evolutionary past. We share-- all of us--5 to 7 million years of evolutionary history. Our last common ancestor was the living apes, the chimpanzees and the bonobos, existed in Africa around that time. And since then, we have become who we are today. We've become bipedal, walking upright on two feet. We've developed the ability to use and construct stone tools that help us modify the environment around us. We've developed larger brains, increased sociality. We've become humans during that time period. This class will explore those major events-- how we came to be human and how we know the things we know about that process

Whats an anthropologist?This class is an anthropology class, which lends the question, what is anthropology, exactly? I usually describe anthropology as the discipline which is trying to understand what it means to be human. Now, of course, there are a lot of disciplines that try to answer this question. What I think distinguishes anthropology is that anthropology doesn't take any singular approach to trying to understand what it means to be human. Rather, it tries to incorporate information simultaneously from lots of different sources to understand what it means to be human. In the United States, we generally break down anthropology into four separate subfields dealing with different kinds of information and different kinds of methodological approaches. Ethnography, which studies human behavioral practices and the construction of meaning within human society. And ethnographers oftentimes work by embedding themselves within cultural groups for long periods of time to try and get an insider's view as to how and why populations do what they do. The second subfield is linguistic anthropology, which takes advantage of the fact that language occupies a uniquely important role in humans-- not only for how we interact with each other, but how we develop and how we understand the world around us. A third subfield is archaeology, which tries to take advantage of the fact that human societies have left a long history of material culture dating back to deep into ancient times. Now, unlike Indiana Jones and images you might think, archaeologists aren't interested just finding artifacts from the past. Rather, they're interest in reconstructing patterns of behavior in the past and how cultural practices have changed over time. Finally, there's biological anthropology, which is what I do. As a biological anthropologist, I seek to understand human variation by looking at the biological mechanisms which shape that variation and by taking an evolutionary approach to understanding how variation is patterned across time in humans. Now, taken this definition of anthropology, it's important to recognize that there's a lot of different ways of developing knowledge about the human condition-- about understanding what it means to be human. Part of being an anthropologist is recognizing that there are multiple ways to answer the same question, not all of them mutually exclusive. Some questions lend themselves more readily to biological explanations. Some of them lend themselves more readily to cultural explanations. But many, many questions lend themselves to both simultaneously, and to truly understand them in a deep manner, you need to be able to understand and integrate cultural as well as biological phenomena and how they interact. One of the reasons I like studying and teaching human evolution is that compared to studying evolution in general, studying human evolution poses particular challenges but also particular opportunities. Within the last two million years, human behavior and human cultural practices have become a really important factor in shaping how evolution acts on humans. In other words, they become very important factors for understanding how evolution operates and the diversity with which evolution operates. So human behavior and human cultural practices become a very important evolutionary issue. Within this class, we'll explore human evolution and talk about, especially in the second half of the class, how human behavior and the development of human culture become important factors in shaping how evolution operates in humans differently than other organisms, even our closely living relatives such as the great apes.

Biological anthropologyBiological anthropology itself could be further subdivided into a series of specialties depending on the kind of material that people work with and the questions that researchers ask. For example, primatologists study living populations of primates, as a way of better understanding the variation within primates today to better understand the environment out of which humans evolved in the past. They are geneticists, which study patterns of variation within living populations of humans, but also in ancient populations of humans as DNA extracted from fossils in the past. Human genesis help us look at the relationship between populations, the timing of events in our evolutionary past. There are human biologists who study patterns of adaptation in biological variation of living populations today, particularly as it relates to aspects of health and social change. And there are human ostealogists who study human physical remains, our bones, as a way of addressing questions about health and trauma and disease in the past. As a paleoanthropologist, I try to incorporate all of these perspectives in to understanding the human fossil record. Paleoanthropology is inherently multi-disciplinary. To develop knowledge in paleoanthropology and to develop as complete a picture as possible of human evolution requires us to rely not only on experts across those various sub fields of anthropology, but also information generated by geologists and other researchers within the field to help understand the past. The human fossil record can be viewed as limited in some ways, in the sense that there aren't that many fossils out there. But in reality, there's a wealth of data to dress and develop knowledge about our evolutionary past. Throughout this class, we'll introduce you to methods, approaches, and the material used by researchers across a variety of sub fields to understand the events, and circumstances around the major evolutionary changes in human history.

What is evolution?

In this class, we're going to be taking an evolutionary approach to understanding human variation in human evolution. Now, the first step in that process is expanding a little bit more on this idea of what is evolution? A basic definition of evolution is that evolution is heritable change in a population through time. Now, it's necessary to break down each of these components of this definition a little bit further in order to better understand what that means. Heritable change, with an emphasis on the heritable aspect, most commonly we use to refer to DNA-- the genetic molecule that's passed on from parents to offspring, and codes for the basic developmental plan as to how our bodies develop throughout life. The second component of this is the notion of a population. Change evolutionary change is not my change as an individual throughout my life. It's a change of the population that I'm part of. We don't think of evolution in terms of the level of the individual. We think of evolution as acting on populations. So how populations of individuals have heritable change. And the last notion is this concept of through time. Again, evolution is not my change from birth to death. That's something that we refer to as simply as development. Rather, evolution is how I differ from my parents. Or even more, how my children differ from their grandparents, my parents. So evolution occurs when the scale of generations, our point of focus is generally populations. And the component in which we're really looking at about are those changes which are heritable. Those changes which are passed on from one generation to the next. Now, there are a couple challenges with this as we move into the understanding of these questions in a fossil record. The first of all is that looking at fossils, we're not necessarily looking at populations. We're looking at individual specimens or perhaps samples of specimens. So one of the first challenges is translating fossil based data into population data. And thinking about that relationship, something we'll talk about more fully next week. This notion of through time also comes into place when we look at the fossil record. Fossils come from at times very specific points in time, but oftentimes from differing points in time. And actually understand exactly how much time separates fossils can be a challenge. But something we certainly need to incorporate into the knowledge that we develop out of a fossil record. The final aspect is this notion of heritable change. When we're looking at the fossil record or the associated geological record, we're not looking at an evidence of DNA for the most part. Though by the end of this class we'll certainly be talking a lot about inherited DNA. Instead what we're talking about is more of logical changes in fossils. Differences between skulls, differences between post cranial anatomy, between one specimen and another. We generally believe that differences in morphology correspond to underlying differences in genetic or heritable change. But this is an issue that we'll also further explore later on in the class. But evolution is heritable change in a population through time.

Darwin 1

Throughout 207X, we're going to be talking about evolution, human evolution. How humans have changed through our evolutionary past. When we think about evolution, inevitably we're drawn back to the life of Charles Darwin. We're very privileged here at Wellesley to have a first edition of on The Origin of Species, published in 1859. This book provides us with the earliest unified theory of evolutionary change. How not only biological creatures change through time, across generations. But how also new kinds of biological creatures, new species emerge. Now, these days change might seem a simple topic. It's even the topic or the theme of presidential elections. And yet it's important to think back to the world of the 18th and 19th century, where change was not such a simple topic. Now, Darwin was hardly the first person to think about the notion of change in the biological world. Indeed, Darwin's own work reflexes background in geology and the natural sciences. Even Darwin's grandfather Erasmus Darwin published a volume Zoonomia in 1803 on the notion of organic change in nature. Even going back further into the 18th century, we have many naturalist who influenced later evolutionary thinking including that of Darwin. Comte de Buffon published a 36 volume natural history of the world. Spanning nearly 40 years of his life, putting together a tremendous amount of information about variation in the natural world. Not only the diversity of creatures on the planet, but what kind of physical characters those creatures possess, and how they differ from one another. What's important to keep in mind is that these earlier works of natural history were largely focused on trying to explain the diversity of life on the planet as a way of understanding God's plan for the planet, as a way of, understanding the world in its diversity as created by God. What's important about thinking about Darwin's work on The Origin of Species is it provides us with a unified evolutionary theory. A scientific approach to understanding how biological creatures change through time, how they reproduce, multiply, how they come to fit their environment, and how new species emerge on the planet.

Darwin 2

In thinking about the significance of On the Origin of Species, it's important to recognize that the work provides not just one idea, but a lot of important ideas for how we think about evolution. The 20th century biologist Ernst Mayr identified five separate revolutionary ideas that come out of Darwin's work. And it's amazing that going back and reading this book more than 150 years after its publication, it still provides valuable insights into how we think about evolution today. The five theories, or the five revolutionary ideas that Mayr identified in Darwin's work include first and foremost the notion that evolution happens. The world changes. It's not a constant. And it changes according to systematic rules and practices. Now, this idea wasn't entirely revolutionary at the time. A number of authors had previously identified the notion of change in the natural world. But Darwin's idea was so well documented, so well described in On the Origin of Species, that very soon after its publication, the notion that evolution happens became widespread. The second idea that Darwin provides us with is the notion of common descent. The idea that humans are not the product of unique creation, but rather descend from a common ancestor correctly identified by others in the 19th century as the African apes. And that because of this, humans are not at the center of the natural world. This was a radical shift in thinking about how the biological world is structured and what place humans occupy within that structure. Humans are not the center. We're just another creature that has been evolving over time. The third idea we can get from Darwin's work, and this was heavily influenced by his training in geology, is that evolution is a process that happens gradually. It takes a long time for evolutionary changes to accumulate and contribute to the origin of new species. Now, this is an idea that itself remains questionable and controversial to some academics. But it's certainly the case that many of the processes of evolution that we'll observe are processes that require a long amount of time to observe and behold. And that's a point we'll come back to in just a moment. The fourth idea that we get from Darwin's work is the notion that natural selection is the primary evolutionary force that shapes an organism's fit with its environment. Now, one of the things we think about when we think about Darwin and natural selection is the role his experience as a naturalist on the ship the Beagle had in shaping his views on this topic. When Darwin graduated from college, prepared to go on to a life in the ministry, he instead had the opportunity to spend two years as a naturalist on the Beagle. This took him around the world. And in the end, the trip actually took five years. During this time, he had the opportunity to see geological formations and natural wildlife in a variety of different environments. And across these environments, he observed how different organisms- not only were they amazing in the kind of biological variation they displayed, but they fit their environment in a unique way that suggested the uniform process of change that shaped these organisms to fit that environment. Something that we think of today as adaptation. Now, it's interesting that Darwin's work actually was heavily influenced by not just his voyage on the Beagle, but his subsequent life and his examinations of not natural selection, but artificial selection as practiced by English farmers and pigeon breeders. The final idea we can get from Darwin's book is the notion that evolution is a population process. New species emerge not because a single individual emerges with new differences and new changes, but rather populations change over time. So the focus and the shift to populations as the primary unit of evolutionary change was critical and vital for the development of 20th century understandings of evolutionary sciences. In the course materials that follow this lecture, you'll find a link to a complete online edition of On the Origin of Species. Well worth your time to check out. And if you do check out that edition, you'll see that in the entirety of On the Origin of Species, there's just one single illustration seen here. Now, this illustration is what we would think of as a phylogeny today, or a hypothesis of evolutionary relationships. But it also highlights some of Darwin's key ideas. Looking at the illustration along the bottom, there are a set number of species in the past, and there are species in the present. And then connections between them. We see in this process change through time. Some lineages changing a great deal over time, other lineages not changing very much at all. We see the origin of new species. We see the extinction of species in the past. And we also see that variation at any given time point is very important in Darwin's thinking. So although simple, this illustration reveals a lot of Darwin's ideas.

Darwin 3

Saying that Darwin provides us with the first unified theory of evolutionary change, what I mean is that Darwin's theory of evolution fits the scientific standards we need to develop knowledge within a scientific context. Darwin, as I said, provides us with the first unified theory of evolution. By this I mean that he provides a scientific context in which we can begin to understand and develop evolutionary knowledge. His theory of evolution generates predictions that we can test through study of the natural world around us, as well as the fossil record left on the history of the planet. Evolutionary science is an interesting science, and a challenging science in the sense that it's something that's difficult to test in the lab. Evolution, as Darwin suggested, is oftentimes a gradual process that proceeds across generations. Long generations, thousands of years. Hundreds of thousands, even millions of years, making it challenging to test hypotheses. However, because evolution is something that has occurred throughout the entire world, constantly throughout our evolutionary past, it's something that's provided a tremendous amount of natural experiments that give us observations that we can test and used to generate knowledge about how evolution works. Now it's again important to think about the fact that in Darwin's own construction of the idea, he relied not just on his voyages in the Beagle, not just of his observations of creatures in their natural habitat, but extensively on studies of artificial selection. How actually generation after generation of English sheep or pigeons change across time. These studies in artificial selection helped provide Darwin with fundamental mechanism of how things change over time. How variation can be selected for. Now, in Darwin's studies with farmers and breeders from the English countryside, selection was something that was very much intentional. Specific traits were chosen and selected for. It's important to keep in mind that natural selection lacks that directionality. It lacks an intentionality. As illustrated by Darwin, the theory of natural selection involves random change through time. It's not that there's a plan to be elucidated. It's not that there's some preset way in which life is meant to be unfolded. But rather, there are natural processes which create random generation. And that certain kinds of variants, simply because of the benefits they provide those individuals, are more likely to be present in subsequent generations. In this context, Darwin was heavily influenced by the work of the population biologist, Malthus, who said basically that there are countless numbers of individuals who might be present on the planet. But only a few of those individuals are destined to survive and reproduce. This notion of the imbalance between the amount of variation that's present and actually what will be present in subsequent generations provided Darwin with the fundamental insight that he needed to come up with the idea of natural selection. Not every individual is going to survive and reproduce equally in future generations. Some individuals will be better at this than others. And the traits that determine how much better they are, are the ones that are more likely to be present in future generations. This was the fundamental insight of natural selection that Darwin developed. Now interestingly, parallel to Darwin's work, Alfred Russell Wallace, working in Southeast Asia, actually was making many of the same observations. And together, along with other 19th century anatomists and natural historians, it very quickly became clear that Darwin's thinking was correct. That evolution precedes in the natural world. That the pattern of variation we see in biological forms of life is a result of long term pattern change within biological systems. The result of evolutionary change.Mutation

The first evolutionary force we need to consider is mutation. In some ways, mutation is the most basic and fundamental force of evolutionary change. Mutation is the source of all new biological variation. Broadly speaking, mutation refers to the creation of new variation from one generation to the next. Again, we'll most commonly think about mutation in the context of changes to our DNA from one generation to the next, mutations which actually change the basic structure of our DNA and change that coding platform upon which life is developed further. But let's consider an example of how mutation works in practice. The basic operation of mutation is very simple. You could imagine some basic starting point, in this case, our blue disc right here. With each subsequent generation, that disc is going to replicate itself into the next generation. But it's not always going to replicate itself perfectly. In some cases, there might be novel mutations which develop. In this example here, we see that in one instance it's replicated itself perfectly. In another instance, we've added a new property. In this case, a small, red disc housed internally. And we can imagine this property moving forward across generations. Again, with each generation producing new kinds of variants and new kinds of variation. Carry this out to the end and even with, in this case, a very limited example with a few small changes, we see a lot of new properties have developed. A lot of new variation has developed within this sample. So mutation is the basic source of new variation. And in this case, and just, again, our four generations, we've added emergent properties in the case of new kinds of features. Here we see the internal properties. Here we see changes to the external properties. Here we see changes to, basically, the addition of a new feature altogether. And we can imagine this again in thinking about the fossil record as the development of new properties within fossil lineages as well. An important thing to keep in mind about mutation is that mutation occurs all the time across all generations. The basic rate of human mutation is generally quite low. Each individual contains only maybe a few dozen or perhaps 100 new mutations at the genetic level from their parents. However, the overall force of mutation can be quite strong if we consider the scale of the effect. Even if most mutations have essentially no effect and each individual houses only a few new mutations, if you have a population of 1,000 or 10,000 or a million or even a billion people, suddenly the amount of variation being produced each new generation is huge So while we might not expect mutations generate much within a single lineage over a short period of time, the fact that populations are large and extend across many, many, many generations gives mutation the chance to create huge amounts of variation for evolution to act upon.

Genetic drift

So if mutation produces new variation, the next step in understanding how evolution works is to think about what happens to that variation. Now as it turns out, most of the variation produced by mutation is lost, and it's lost through an evolutionary force that we refer to as genetic drift. Now, genetic drift is in some ways the simplest of the evolutionary forces, because it operates simply as a result of mathematical properties. The fact that populations in the real world exist in finite population size, they're not infinitely large. And because of that finite nature, they don't replicate themselves sperfectly from one generation to the nextAgain, we can imagine a theoretical population. In this, case we have a population of red and blue individuals. Again, illustrated here by basic disks on your screen. In this case, our starting point is we have an equal number of red or blue individuals in a population. Now, if each of these individuals in the next generation is going to produce an offspring so that the population remains constant in size over time, we could imagine each individual has a 50/50 chance of producing a red or a blue offspring. So at our starting point, the population has an equal number of individuals. And each individual has an equal likelihood of producing either a red or blue individual in the next generation. In other words, it's the same probability as tossing a coin and getting a head or a tail as your result. However, just like tossing a coin, you're not necessarily going to get a perfect 50-50 distribution. The larger the population is, or in the case of coins, the more time you toss a coin, the more likely you are to converge on that expectation of a true 50-50 outcome. The smaller the population, or the fewer coins you toss in our analogy, the more likely you are to have a significant deviation from that outcome. In an extreme example, we might imagine a population of just two individuals, illustrated here in front of you. Assuming this population doesn't change in size, the chance that the next generation there will be an equal number of individuals who are red and blue, in other words, one red and one blue individual, is 50%. In other words, this blue individual has a 50% chance of producing a blue individual, and this red individual has a 50% chance of producing a red individual. Likewise, this blue individual has a 50% chance of becoming red, and this red individual has a 50% chance of becoming blue. If we add up these properties, we'll find that this overall outcome of the population remaining stable through time remains 50%. In contrast, in this example, there's a 50% chance that either the red or the blue condition will be lost, simply because of that random probability of retaining that property. So in this extreme example, when the population size is limited to just two individuals, there's a 50% chance that one of our variants will be lost in the next generation. Over time, genetic drift acts in just this manner to eliminate variation. The ultimate fate of any new mutation that arises, any new variant that arises is either to be lost to genetic drift, or to become fixed in a population. And by fixed, we mean basically at or near 100% frequency in a population. This is how genetic drift leads to the loss of variation. The random sampling of finite populations over time. Its effect is greater when population size is smaller. Its effect is smaller when populations are larger, but it's always operating, and it's operating under a basic math mathematical principle, the idea that finite populations by probability error alone are unlikely to sample themselves perfectly from one generation to the next. In the exercises following this lecture, you explore this concept in more depth to better understand how genetic drift operates in populations of different size. For now, it's important to note that genetic drift leads to the loss of genetic variation, and acts particularly strongly when population size is small, and acts much less strongly when populations are very, very large or expanding very, very quickly.

Mutation-Drift Balance

At this point, we can introduce another fundamental concept for understanding variation. Again, if our hope in taking evolutionary approach to huma evolution is to understand how and why variation is distributed within and between human populations the way it is and across evolutionary time, we need to understand that balance of variation within populations. And mutation and drift give us at least a starting point to understand that balance. If mutation is introducing new variation into a population and genetic drift is eliminating variation in a population, over time these two forces are going to enter into an equilibrium. Where the amount of new variance introduced into a population via mutation and the amount of variation being lost via genetic drift come in to balance. This fundamental balance, mutation drift balance, gives us a baseline expectation as to how much variation we might expect in an individual population. The greater the rate of mutation, the more strongly that balance might be shifted into the favor of having a lot of variation within a population. The smaller the population size or the greater the effective genetic drift, the less variation we might expect to see in a population. So this fundamental balance between new variation entering into a population via mutation and variation leaving a population via genetic drift, give us a baseline expectation as to how much variation we might expect to see within a population.

Gene Flow

So far, we've talked about how evolutionary change operates within populations. But of course, populations are not alone. They're connected to other populations. So a variation that arises in population A might get passed on to population B. And the way that this happens is through gene flow, or the exchange of genes between populations. Gene flow reflects the structure of populations across space and time. Populations, again, overlap in their geography and interactions such that genes that might be exchanged or information might be exchanged between populations across long distances and across long periods of time. As an evolutionary force, gene flow homogenizes populations by sharing variation between populations. The more interchange or the more gene flow that occurs between populations, the more rapidly they'll become similar to each other. The less gene flow, the more forces such as mutation have the opportunity to make them more unique from each other as the differences that develop within them become more different over time. So gene flow acts to homogenize or make populations more similar over time. Primates and humans have many characteristic ways in which they exchange genes, or in which gene flow comes to structure the relationship between populations. For example, in chimpanzees, females when they reach a certain age tend to leave their natal group. They leave the group that they're born in and move into a different population, establishing this particular structure of gene flow across chimpanzee populations. In other primate groups, males are the individuals who leave a population. Humans traditionally display a huge range of variation. In some traditional societies, men leaving the group that they're born in; in others, women leaving the group that they're born in. And across the range of human variation, every myriad combination of patterns potentially existing. And if we think about the way humans operate today, of course, gene flow is incredibly complex, given the ability of individuals to travel and move across large periods of geographic distances as well as across cultural boundaries. One important point to note about gene flow is that it doesn't require individuals moving across long distances for genes to move across long distances. Much as a wave can move in from the center of the ocean without bringing in water from the ocean but rather transferring energy from the center of the ocean, human populations can also exchange genes without large movements of people. For example, when a wave comes in from the ocean, it's not bringing a lot of water from the center of the ocean. Rather, it's carrying energy. This energy is transferred from wave to wave, causing the water to move up and down as that energy moves towards the coast. In much the same way, you can imagine an individual mating with an individual in an adjacent population. And across several generations, the genes moving from population to population to population across a geographic distance without individuals from those populations moving across that distance. So the movement of genes or heritable information via gene flow doesn't require the movement of individuals across long distances. And it can occur simply by mating between adjacent populations across long periods of time, therefore leading to genes or other inherited information moving across geographic distances without any individuals moving long distances.

Natural selection

The final concept we have to consider, and arguably the most important, is natural selection. Now, natural selection is the most complex form of evolutionary change. Because it has the ability to shape patterns of variation in complex and varied ways. Natural selection was at the heart of Darwin's vision of evolution. Because it explained to Darwin the notion of the fit between an organism and the environment it occupies. In general, natural selection shapes the pattern of variation in populations. The simplest kind of pattern we might see-- for example, using the example that we looked at earlier to illustrate genetic drift-- is to imagine that the red variant in that theoretical population we'd examined is favored over the blue variant. And as result, the red variant over time becomes more frequent. And the blue variant is lost. Now, it's important to note that when we say the red variant is favored, what we mean is that individual or that property is becoming more frequent in a population. Because individuals who have that property-- in this case, individuals who are red-- are more likely to survive. They're more likely to reproduce. And they're more likely to pass their genes on to future generations. The language that we use to describe natural selection, including that word "selection," suggests that selection is an active force, that there's some outside force that's choosing this to be a favored variant and predicting that it's going to be more likely. But that actually reverses the course of action. In our example, red individuals are more frequent in future generations because they're better able to reproduce and survive. They aren't better able to reproduce and survive because they're red. This is a subtle but important distinction. Because it helps illustrate the difficulties of interpreting the why of evolution. There isn't some future plan that we're trying to unfold. Rather, the properties of the moment help shape the properties of the future. In this case, how selection is acting now determines what the properties of our population might look like in future generations. Now, when I talk about natural selection being complex, that's because it can shape variation in lots of different ways. Whereas mutation creates variation, genetic drift leads to the loss of variation, and gene flow serves to homogenize populations, natural selection can do a whole variety of things. It can create variation. It can limit variation. It can change the pattern of variation that we see between individuals. To understand why this is the case, we need to think about the biological variation with respect to three concepts-- phenotype, genotype, and environment. The genotype of an organism can be thought of as the total package of inherited material that an organism begins with. Again, we'll commonly think of this most likely as simply the DNA of an organism, though there might be other things included within the inherited package that an organism begins with. This basically though reflects the overall developmental plan that this organism has at birth and proceeds throughout its life. Phenotype, in contrast, is anything that we can observe, measure, or record about an organism. It reflects not just the genotype but how that genotype has developed through time. So you can think of the phenotype as the downstream outcome of genotypic development. But phenotype itself can be complex and can be construed and defined in lots of different ways depending on the questions we're interested in. It's important to draw this distinction between genotype and phenotype, because we generally think of natural selection as acting on the phenotype. It acts in the real world on the individual organism as that organism is operating within its environment. It doesn't necessarily act most commonly on the genotype itself, but on the expression of the genotype in the form of phenotype. So it might act on the height of an organism, or the color of an organism, or the shape of an organism, or whatever that downstream development of the genotype is. So we generally think of natural selection as shaping phenotypes, and thereby indirectly shaping genotypes. The final concept we need to consider is the environment. Now, the environment is also a complex term. We tend to think of it as the trees and the leaves and the sky and all the external physical forces outside of us. But really, the environment can be much broader than that. Now, when natural selection is acting on a phenotype, or when a genotype is developing into a phenotype, these are all occurring within a specific environment. And the fact that they're occurring within a specific environment means that those processes might change as the environment changes. So how natural selection operates is specific at any given time to a given phenotype within a given environment. If you change the environment, you might change how natural selection is acting on an organism. An example of how environmental change might affect how selection acts, we can look at humans and actually how we digest milk. So again, 10,000 years into our past, no humans, or very few humans, were probably able to digest milk products as an adult. Today, a very large portion of the human population can do this even though most mammals can't digest milk. So sometime in our last 10,000 years, there was an evolutionary shift that allowed humans to digest milk. And it turns out this change was an environmental change, one induced by cultural properties, in this case the domestication of livestock and the production of dairy-based food products, particularly the kinds of dairy-based food products that we can store and transport, like cheese. that suddenly made the environment for selection different in humans. Suddenly, when people began in certain populations to producing milk products, the ability to digest milk as an adult became a favored property. So mutations that allowed for that to develop within humans, mutations, changes in the genotype that changed the adult phenotype that allowed for the digestion of milk, suddenly became favored. These kinds of mutations might have existed in the past. But in the absence of an environment which included dairy products, there was no reason for that mutation to persist. And that mutation probably would have been lost through genetic drift. But in a changed environment, one in which suddenly there's a strong selective advantage to be able to digest those milk products, suddenly the environment favors the development of that trait. And now, we have mutations associated with lactose tolerance becoming more commonly present, especially in populations involved in dairy production. So in this example, we can see how environmental change--in this case, culturally directed environmental change in the form of dairy production-- can lead to changes in how natural selection acts to favor certain phenotypes, and therefore produce genetic change in human populations, and explains why in some populations, populations with a long cultural history of dairy production, lactose tolerance is more common these days than in other populations.

Adaptation

An important visual metaphor for thinking about how natural selection operates is something that we refer to as the adaptive landscape. This was a concept developed by Sewall Wright, a population geneticist and biologist in the middle of the 20th century. But it's important way of thinking about how phenotype, environment, and genotype are connected via natural selection. The basic idea is that any individual phenotype has a specific fitness associated with it, given a specific environment. Now, that fitness in this case refers to basically how likely an individual with that phenotype is to survive and reproduce into future generations or, basically, how fit they are for their environment, how likely they are to pass on their genes. So there are areas of high fitness within this landscape. There are areas of low fitness within this landscape. Now, given a certain population starting point-- we might have population scatter across right here, for example--natural selection is going to tend to drive populations towards areas of higher fitness. And any traits that develop, phenotypes that develop, because they become more frequent, because of a process like this, are what we refer to as adaptations. So an adaptation is a trait that's become highly frequent in a population because natural selection has made it more frequent within a population. It's a trait which confers high fitness. In other words, it reflects that evolutionary fit that Darwin observed between organisms and their environment. Now there are a few important concepts to illustrate out of this. The first is that evolution doesn't necessarily optimize an organism's phenotype. Rather, it optimizes within a local context. So if we imagine this initial starting point of this population as being someplace different, for example, say, here, we would expect natural selection to drive it to the locally highest fitness peak. Or perhaps, maybe through other actions, driving it up towards this way. But neither of these peaks are as fit as this initial one that we illustrated first. The reason being that evolution can only operate on the variation that's present within a population. Another important point to illustrate out of this illustration is that that relationship between phenotype and genotype is important. Again, genotype develops into phenotype. If we have changes within that process, that can also alter how natural selection operates on a phenotype and at what level natural selection is operating on a phenotype. So this notion of development, this idea that phenotype is the developmental outcome of genotype, is the concept that we're going to refer to repeatedly throughout the semester. Finally, it's important to note that natural selection is not random. Natural selection acts in a specific direction. It takes a population from its starting point and moves it to areas of higher fitness, assuming those areas are available to it. This is different than the other forces of evolution that we've talked about. Mutation and genetic drift are both random in specific context of that word. Mutation is random in the sense that a mutation that's more likely to produce higher fitness is not necessarily more likely to emerge. Once that trait is emerged, that high fitness mutation might be more likely to persist. But mutation itself is random with respect to outcome. Things that are advantageous aren't more likely to come about. The same is true with genetic drift. Genetic drift is also random. Genetic drift is driven by the frequency of a trait within a given population, not necessarily by how likely or how advantageous that trait is within that given population. Another thing to note about this also is that as the environment changes, we would expect the actions of evolution to change as well. If this environment changes from time one to time two-- for example, if we imagine this fitness peak shifting-- we would expect evolutionary change in how natural selection is acting to shift according to that environmental change. So the action of natural selection is specific to a given time and place. As the properties of that time and place change, as the environment changes, as the phenotype of individuals within a given population change, or as the genotype of a population changes, we would expect natural selection to change in how it acts. One additional point is that natural selection is not synonymous with evolution. Natural selection might be the most memorable and notable form of evolutionary change, but it's just one force of evolutionary change. Not everything is an adaptation. Not every characteristic we look at was there because it was selected for. Many things that develop in our phenotype develop because of other kinds of developmental and environmental constraints. They come about because of other evolutionary processes-- things like gene flow, things like genetic drift, things like mutation-- and not because of the action of natural selection. So while natural selection was foremost in Darwin's mind when he developed the ideas of evolution, natural selection is not the only force of evolutionary change, and natural selection is not synonymous with evolution. The exercises that follow this section well help reinforce these concepts-- mutation, which increases the amount of genetic variation in populations; genetic drift, which leads to the loss of variation in populations; gene flow, which serves to structure how variation is patterned across populations; and natural selection, which does pretty much whatever it wants in shaping how variation is situated within populations-- to help you better understand these concepts and how they relate to human evolutionary history.