The Science of Everything Podcast - Episode 21: Introduction to Evolution

Episode Date: July 12, 2011

An introduction to the basic concepts of biological evolution, including an overview of Charles Darwin’s contributions, a discussion of heredity, a summary of all the major evolutionary mechanisms, and a review of all the major lines of evidence supporting evolutionary theory.

You're listening to The Science of Everything podcast, episode 21. Introduction to Evolution. And I'm your host, James Fodor. So, in today's episode, we're going to look at the basic concepts of evolutionary theory, including an overview of the contributions of Charles Darwin, a discussion of hereditary, and then we'll look at a summary of the main mechanisms by which evolution occurs, including natural selection, sexual selection, genetic drift, and so on. And finally, I'll go through the major lines of evidence that support evolutionary theory and the fact that common descent has occurred, because there are many separate lines of evidence in support of this, such that the evidence is quite overwhelming.

Okay, so let's get into it. First of all, we'll look at just a basic definition. What is evolution? Evolution, also known as organic or biological evolution, is simply the change over time in which, one or more inherited traits found in a population of organisms. So these inherited traits could be anatomical traits, biochemical or behavioral characteristics. So whenever these things change over time in a population of organisms, evolution is said to have occurred. Evolution can be a sort of a short-term phenomenon, which leading to relatively minor changes in a population. That's referred to as micro-evolution. For example, a bacteria might develop an ability to metabolize a, a new chemical or develop a new enzyme or a species of animal might develop a new behavioral

characteristic or something like that, that would be microevolution. Macroevolution is a longer-term phenomenon whereby basically one species evolves into another, a completely different species. That's also referred to as speciation. So, for example, the evolution of prokaryotes into eukaryotes, and then eukaryotes into multicellular organisms, then into plants and animals, from reptiles to mammals and so on, all of that would be macroevolution. Now, I just want to provide a brief background as to the history of evolutionary thought, because many people associated it with Charles Darwin, and of course he was very important, but he was not the first person to come up with the idea of evolution. So the

actual concept of evolution, basically that of simpler organisms developing over time into more complicated organisms actually dates back to some writings of the ancient Greeks and Romans. There are various scholars and philosophers at that time sort of proposed that general idea, but it never went anywhere in particular. Throughout most of the Middle Ages and the early modern period, conventional Western thought was that God had created all organisms in their current form and that they sort of formed a hierarchy of being whereby you had the plants at the bottom and then simple organisms, and then it went right up to the top two man, the pinnacle of creation,

and that they had been created in those forms, and that they sort of formed essential categories, and they were quite distinct and had been created that way by God. So you can see that that's quite a different concept to evolution. It was not until the late 18th century that Western sort of scientific biology started to take the idea of evolution seriously. The idea of evolution sort of evolved from evolutionary cosmology and the sort of mechanistic conception of the universe that had developed throughout the Enlightenment

whereby the universe was created by God, but after the point of creation, it's just sort of evolved or moved forward through the operation of natural laws. And when that was applied to biology, you could see how it would be consistent with the idea of species, becoming more complicated and varying

and therefore evolving over time. This was also the time when paleontology was just coming on the scene. They were discovering fossils, fossils had been known for thousands of years, but it was just around this time that they were first sort of treated scientifically, and people started to really take seriously the idea that these were creatures that had existed in the past, but no longer were on the earth. And that therefore led to the idea that life has changed over time, therefore that's consistent with evolution.

In the early 19th century, a guy by the name of Jean-Baptiste LeMarc proposed the theory of transmutation of species, which was kind of the first fully formed scientific theory of evolution. His idea was that sort of similar to Darwin's in that species changed over time to become better adapted to their environment. However, his mechanism of action was that it was called the theory of acquired characteristics. The classic example is that the giraffe got its long neck because early short-necked sort of the ancestors of giraffes would reach up to try and get at the leaves on higher branches of trees. and in doing so they would stretch their necks a little bit, and so make them a bit longer. And then that slightly stretched neck would be passed on to the offspring

of those original giraffe ancestors, who then would in turn stretch their necks a bit further, and that would be passed on. And so each generation would stretch their necks a little bit more to try and reach the higher branches to access food, and in the process of that, the neck of the giraffe gradually became longer and longer. So the key point there is that the characteristic that is passed on

is acquired during the life of the organism. That was Lamarck's theory of evolution. We now know that that is wrong, basically because a characteristic, or most characteristics that are acquired during the life of an organism cannot be passed on to offspring. And so then there's, sort of each generation has to start at baseline again. And so there's no cumulative increase in, in this case, neck length or any other characteristics. So that idea would not work. But of course, at that time, Lamarck proposed it there was modern understanding of genetics and hereditary didn't exist, so he couldn't have known that. It was not until the mid-19th century that Charles Darwin and Alfred Ross Wallace published their idea of the new theory of evolution based on natural selection.

And Darwin also introduced the idea of common descent with a branching tree of life and sort of synthesized his theory from a whole range of different evidence based on animal husbandry, biogeography, geology and so on. And it was a bit controversial at the time, but it was fairly quickly accepted by the scientific community because it had so much support behind it, and it's been particularly well established since the modern understanding of genetics and how hereditary works developed over the course of the early 20th century, which confirmed and sort of consolidated his ideas. Darwin proposed his idea of evolution by natural selection before current understandings of genetics and hereditary were around, but he had other evidence to support the

theory of evolution. Since then, understandings of genetics, DNA, and hereditary have progress substantially and those findings have confirmed and expanded our understandings of Darwin's original theory of natural selection. Now one thing that I really want to emphasize is the difference between evolution and natural selection. Evolution is a concept that predates Darwin as I've just explained, you know, dates back to basically ancient times and then increased interest in the 18th century Jean-Baptiste Lamar had his own theory and then really expanded with Darwin. But evolution simply

refers to the change in organisms over time. Natural selection is one particular way in which evolution can occur. There are other ways that evolution can occur. Natural selection is just one way, probably the most important way, but one way nevertheless. Darwin came up with the idea of natural selection himself. That is his unique contribution and also providing evidence for natural selection and explaining how natural selection can lead to evolution. But he did not come up with the idea of evolution. He came up with the idea of natural selection, and I'll talk more about what natural selection actually is a little bit later. Before we do that, however, I want to talk about hereditary, basically just some basics of

inheritance and genetics that you'll need to understand. Evolution in organisms occurs only through changes in heritable traits, that is traits that can be passed down from one generation to the next. Any changes that are not heritable may be beneficial for a particular generation and can happen for various reasons, but cannot lead to evolution. Basically by definition, because evolution is the change in organisms over time, that is overgenerational time. So if a change cannot be passed down, then it can't lead to that long-term change. Okay, so if evolution has to occur through heritable traits, then that leads to the question, what traits are heritable and how are they inherited? As you probably know, inherited traits are controlled by genes, and the complete set

of genes within an organism is called its genotype. So it's genotype. So it's genotype. The genotype of an organism is basically all the genes in that organism. Different organisms will have different genotypes. Organisms of the same species will have almost the same genotype because, you know, they're pretty much the same. But each individual is slightly different with mutations and other things, different alleles, and I'll talk about that in a second. So they have their own unique genotype. So when people say something to the effect of humans and chimpanzees have 99% same DNA,

basically what they mean is they have 99% the same genotype. So only one in a hundred genes or base pairs of DNA, depending on how it's measured, varies between chimpanzees and humans. Of course, that's a little bit misleading because humans don't have the exact same genotype either, as I just explained. Organisms within the species will have different genotypes too, but that difference will be even smaller than the difference between species. Okay, so that's the genotype, complete set of genes. The set of observable traits that make up the structure and behavior of an organism are called its phenotype. That's spelled with a pH. So there's phenotype and genotype. They're two different concepts.

Because genes generally determine the structure and behavior of an organism, because genes code for proteins, and then those proteins are used to basically do stuff in the body. And if you have listened to my previous podcast on biochemistry basics, you'll understand what I'm talking about. So genes generally determine the phenotype of an organism. So genotype generally determines the phenotype. However, there are other factors that affect phenotype as well. For example, environmental factors. So, genotype and phenotype are not the same thing, but they are generally closely related.

Genotype is basically your DNA, your genes, phenotype is your structure, your behavior, all the stuff that you can see in an organism. Now, genotype, because it's dependent upon genes, is completely heritable. So basically, each in sexual organisms, you'll have 50% of your mother's genotype and 50% of your father's genotype. Phenotype, however, as I just said, is also dependent upon the environment, and so is not heretical. You'll generally, people generally look like their parents, for example, but that's not because they're inheriting the phenotype of their parents. They're inheriting the genotype of their parents, which then determines phenotype. And because the genotypes, you inherit your

parent's genotype, you'll also tend to look like them, but there'll be differences as well. And those differences will be partly based on environmental differences. For example, if you eat a lot and become fat, then that's part of your genotype. Your parents may not be fat because they didn't eat as much. That's an environmental differences, so that's part of phenotype, not genotype. However, if you inherited a gene from your parents which predisposes you to an eating disorder or whatever, then, or a slow metabolism, then that may have contributed to your becoming overweight, and so that gene itself is part of your genotype, whereas the being overweight is part of your phenotype. Now, as I mentioned before, Lamarckian evolution cannot work, because

Lamarckian evolution was based upon differences in phenotype, for example, the stretched neck of the giraffe, being passed on to offspring, but we know that that cannot occur. You can't pass on phenotype, because phenotype is partly based on environmental characteristics, and there's no way that phenotype is transferred directly, at least, to offspring. Genotype, however, is transferred because genes are inherited from parents, and Darwin's theory of evolution relies upon the inheritance of genotype by inheritance of genes, and so that's why Darwin's theory of evolution works, and Lamarck's theory of evolution does not. As I sort of mentioned in the Biochemistry Basics episode, genes are made up of basically molecules of DNA, and the order of the bases in the molecule of DNA determines the

genes or the information that is encoded in that DNA, and therefore in that gene. And it is these actual molecules of DNA, which are copied and then passed on to subsequent generations. And so you receive some DNA from your mom and some DNA from your dad. These DNA molecules then come together to form your genotype. A gene is just a sequence, a section or a sequence of DNA that codes for something, generally a protein, sometimes also RNA or some regulatory function. Generally, though, it's a protein. So that's what we call a gene. Basically, it's a bit of DNA that does something by encoding the information to make something. Now, DNA sequences vary between individuals. And that's one of the keys to evolution, because if you did not have this

variation, there would be no evolution would be impossible. I'll talk more about that later. But the fact that how do DNA sequences vary between individuals? Well, basically, the idea is that two individuals might have basically the same gene. Well, it is the same gene. Maybe it's a gene for eye color or a gene for determining height or anything really, or to make a particular enzyme. But if just a few base pairs within that sequence of DNA, within that gene,

are different between these individuals, then the product that's made as a result of the DNA, be a protein or RNA or whatever, will be slightly different. Now, sometimes that won't make any functional difference, and sometimes it makes a big functional difference. But either way, there's a difference in the gene. It could only be a few base pairs, it could be a large number of base pairs. Changing those base pairs changes what the DNA codes for, therefore changes the protein or RNA that's produced as a result of the DNA. Now, if most of the gene is the same, but there's just a difference in a few base pairs,

then the different forms of this gene are called alleles. That's A-L-L-E-L-E-S, Alleles. Alleles are generally the thing that we keep track of in terms of a genetic very reaction within populations. Because as I said, generally the individuals within the same species have basically the same DNA. They've basically the same genes, basically the same genotype, but there are some differences. Generally, these differences come in the form of alleles. So they have all the same genes, they're all there, but there are different forms of the genes. And some individuals have one form, some have another form. So classed examples of that within humans, there's a gene that

controls whether you can roll your tongue or not, well, an allele of a gene that controls that, there's an allele of a gene that controls eye color, well actually a few alleles, hair color, skin color, there are alleles of genes that control this. So basically everyone has these genes, but some people have slightly different versions than others, and then that leads to different traits like eye color, skin color, hair color, height, and so on. So it's these different frequencies of alleles within populations that permits genetic variation and therefore permits evolution to occur. When the allele frequency in the population changes over time, we say that evolution has occurred because the genotype of the population has changed on average

and therefore probably the phenotype has also changed. Okay, so that's the basics of hereditary. Basically, we went over the concept of phenotype, genotype, genes, heritability, DNA sequence, alleles and genes. So just bear those concepts in mind because they'll come up again in the future. Now I want to move on to how evolution actually occurs. So there are, basically three steps that we need for evolution to occur. These are basically necessary and sufficient conditions, meaning that you need all three of them for evolution to occur, and if you have all three, evolution pretty much has to occur, kind of by definition.

So these three criteria are variation, that is genetic variation within a population, also called genotypic variation, selection of some individuals within that population to breed, or to produce offspring, and inheritance of that genetic variation. If you have variation within a population but it's not heritable, you won't get evolution. If you have variation within a population but there's no selection so that everyone just gets to

breed or to pass on their genes as much as everyone else, then there is also no evolution. And also, even if you have selection and inheritance, if there's no variation, if everyone's exactly the same, there's no way you can get evolution either. So you have to have the variation, the selection, and the inheritance for evolution to work. The first question then is how do we get the same? the variation in the first place. That's kind of the first thing we need, the variation. Variation basically comes from mutations in DNA or genetic mutations. These can be caused by a number of, a number of different mechanisms. But basically it means that a, remember, a gene is composed of

a sequence of DNA base pairs. It could be an A, a G, a C, or whatever. When one of those bases is substituted for another one, so for example, a G turns into an A, then that's a mutation. The gene has changed. Basically, that will have formed a new allel, a slightly different version of that gene. There are also other ways mutations can occur. For example, a gene can be copied into a gene em, so it occurs twice when before

there was only one of it. Part of it could be copied and added on, or it could be moved from one chromosome to another, but the most common form of mutation, or the one we normally talk about is just one base pair changing into another one, which then changes what the gene codes for and then changes what is produced. So that may mean the enzyme that it produces as different, or the

or the structural protein is a bit different and so on. And it might not sound like that just changing one base pair in a DNA molecule is going to make much of a difference. And often it doesn't make much of a difference. But sometimes it has a big difference. For example, sometimes changing the structure of a protein just by one amino acid will mean that it's a, it now, instead of bending this way, it bends a different way, or it doesn't bend at all. It just stands straight or something like that. And that different shape means that it can't do whatever it was supposed to do before.

Sickle cell anemia is an example of this because the hemoglobin protein that goes into red blood cells, one amino acid in that protein is changed just that it can't form the right shape that it did before and therefore blood cannot carry oxygen properly, you're likely to die if you have just that one amino acid in the wrong spot. So single mutations like that can actually be important, or sometimes a single mutation might not be important, but if you get enough of them in the gene, then it starts to have effects. Most mutations are harmful, so they, it means that you produce a protein or RNA or whatever that's less useful than before that can't do its job or doesn't do it as well. But sometimes, just by pure chance, you get a mutation that's beneficial.

So you get amino acid changed here that then produces a protein that actually binds a bit better or that carries oxygen a bit better or that makes the cell, that it increases the structural integrity of the cell, or whatever, it could be anything. Another way that you can... So that's one way you can get genetic variation, just through mutation, leading to new alleles, which can then be selected for. Another way, as I said, you can copy a gene and paste it somewhere else in the genome, or on some other chromosome, and then if you have two copies of the gene, you only really need one of them to produce the proteins that you need. The other one then is sort of a backup which then is able to engage in further evolution or experimentation.

So, for example, you could change a few base pairs here and a few there, and if it stuffs it up, if it can no longer, if it's no longer functional, then it doesn't really matter because you've got the other copy. And most of the time that will be the case. most of the time the changes that occur, the copied version of the gene will be pretty useless, will make it unnecessary, will make it non-functional, and so it doesn't do any good. But that doesn't really matter. It just sits there in the genome, doesn't really hurt anything. Occasionally, however, these mutations that occur will actually be beneficial.

And so by having that extra copy, you allow that extra chance for sort of tinkering around by evolution, just random mutations to occur, and sometimes they happen to stumble upon configurations or sequences of nucleic acids which are actually better at doing something than the original gene, and therefore that's produced genetic variation within the population. Genetic mutations, by the way, can be produced by copying errors. So when cells divide, they copy their DNA, to make multiple copies of it, one going to the parent cell, one going to the daughter cell, and that process is pretty good, but it's not perfect, so errors creep in.

When those errors occur, then that leads to mutate. Radiation, for example, ultraviolet radiation can also produce mutations as can certain viruses and mutagenic chemicals. So there are a wide variety of ways for genetic mutation to be generated in that way. But the basic idea is you have a gene, some amino acids get changed, that produces a new allele of the gene, and therefore that generates genetic diversity within the population. So that's the first thing we need for evolution, that genetic diversity. The second thing we need is selection from amongst that genetic diversity. What I mean by that is that the selection refers to

the fact that not everyone or not every individual within that population is able to reproduce as much or as successfully as every other individual. So if every individual within the population produce, say, two offspring, and then each of those two offspring produced two offspring and so on, then this genetic variation wouldn't make any difference. It would just stay the same. And the population wouldn't change over time. You know, if you had 50% tall individuals and 50% short individuals and tall and short individuals all had the same number of offspring, then you would still, you know, a thousand years later, a million years later, have 50% each short, 50% tall. Even though there's variation, there's no selection, there's no differential success

in reproduction between amongst those different variants or the different alleles of the genes, and therefore there's no evolution. In order to get evolution to occur, we have to have selection. So some individuals have to be more successful in reproducing than others, and that's the selection factor that basically something is selecting some individuals to be more successful than others. And there's a wide variety of things that can do that selecting. Natural selection, as I mentioned, is just one mechanism that can do the selecting. There are other things, too. A number of other mechanisms that can act as that selecting mechanism. But natural selection is important because it's probably the most important one. It's also the one that Charles Darwin,

as I said, came up with himself. Now, evolution is often sort of paraphrased as survival of the fittest. That is sort of misleading, but it's also sort of true. As long as you understand that fitness in biology has a very precise meaning. It doesn't just mean running the fastest or being the biggest and strongest or whatever. Fitness in biology or an evolution of biology actually means, it refers to the contribution that your genes make to the genotype in the next generation. So another way of saying that is how good are you at passing your genes on to the next generation? That in turn will depend upon how many offspring you have and how many of those offspring survive. If you have lots of offspring but they all die before they reproduce, then you're not going to be very fit.

Alternatively, if all your offspring survive, but you only have two offspring or hardly any offspring, you're also not going to be very fit. So the fitter you are, then the more offspring you have and the more of them survive, and therefore the more of your genes are passed on to the next generation, and then the next generation after that and so on. Evolution can only occur because not every individual in a population has the same fitness. Some individuals are fitter than others. And fitness is really a relative concept.

It doesn't matter how many offspring you have or how many copies of your genes you pass on to the next generation. All it matters is how many you pass on relative to your competition to other individuals within your population. Obviously, that's the case because 10 kids is a lot for a human to have, but 10 kids is hardly any for an insect to have. So it just depends on the population. So we've just said that depending on their fitness, some individuals will have more offspring than others, and therefore pass on their genes better than others. But what determines an individual's fitness? Well, the idea is that the variation that existed within the population,

that genetic variation, the different frequencies of alleles, some of those alleles, some of that variation, will produce more fit individuals, and some will produce less fit individuals. So the alleles or the genetic variants that are more fit will have more offspring, they'll pass more of their genes onto the next generation. Those that are less fit will pass less of their genes onto the next generation. therefore have fewer offspring and few of those will survive.

Now, the reason that this genetic variation and then the selection, the reason all that leads to evolution, is because if an individual with a certain allele or a certain characteristic, let's say tall people, if tall people have more kids, more offspring than short people, then in the next generation, there'll be more tall people than short people. So say originally you have 50-50, 50-50 tall people to short people. But suppose that tall people have slightly more kids, than short people. So in the second generation, we'll have maybe 51% tall and 49% short. And then in the

next generation, maybe it's 53% tall and 47% short. And that will increase, as long as tall people continue to have more children or more offspring than short people, another way of saying that would be as long as tall people are fitter than the short people, the proportion of tall people in the population will gradually increase until, unless something changes. In this example, it will continue until tall people make up 100% of the population. And the reason for that is simply because they're having more offspring. They're passing more of their genetics onto the next generation. And so by having more offspring than alternative individuals with different genotypes,

their type of genotype, their variety of people or that variety of organism, will become more prevalent in the population. It's really a very simple concept at the base, because it simply means that if you have multiple variants of something, whichever variant replicates more rapidly, or more frequently than the other version, that version, the one that replicates more rapidly, will become more prevalent.

It will form a larger portion of the population, simply because it is replicating more quickly. This is why some people criticise evolution as being a tautology, because it almost is in this particular aspect of it, in that if you reproduce more rapidly or have more offspring, then you'll form a larger part of the population. It's just a simple logical deduction, really. The example I gave was whereby a torporal,

people increase in proportion until they form 100% of the population is called directional selection. Basically, where evolution selects or natural selection selects for organisms with a particular trait at a particular end of the distribution. So an example of this is the giraffe necks. In the initial ancestral population of drafts you would have had sort of a distribution. Some drafts would have been had a shorter necks, some would have had longer necks, most would have sort of had in the middle. However, because those are giraffes at the taller end of the distribution will have an advantage in terms of how much food they can get, they will also tend to have more offspring than other giraffes, and therefore the giraffes over

time will tend to become taller, have longer necks, as the whatever genes in their population are responsible for producing longer necks become more and more frequent in that population, or those alleles of whatever genes determine neck size, become more frequent in the population. That is directional selection because it tends to produce traits that that are more extreme in one direction or the other. It could make the animals bigger or smaller or more aggressive or more passive or whatever. There are two other types of selection, however, disruptive selection and stabilizing selection. Disruptive selection is one that

selects for extreme traits in both directions. So in this example it could select for short drafts and tall drafts but not middle-sized drafts. Big sizes of Darwin's finches that he examined to come up with his theory of natural selection is an example of this because Because if you have a large beak, then you're good at cracking larger nuts, and you can feed on those. But large beks are not terribly good for smaller nuts and seeds because they're, you'd be too clumsy with them. Smaller beaks are more nimble and better at opening the small nuts, but they're not large enough to crack their bigger nuts. So it's beneficial to have either large or small beaks, but it's not beneficial to have medium-sized beaks because you're kind of not good in anything.

And that's just one example of disruptive selection. So in that case, natural selection would select large beaked animals and small big animals, but not medium-sized big animals. Stabilizing selection is basically the opposite of a disruptive selection. It tends to select for individuals in the middle of the population, the middle of the distribution, and select against the extreme values. An example of that is birth weights in babies, because babies, human babies, that is, babies that have particularly high and particularly low birth weights are much more likely to die in infancy than babies with. moderate birthways and so if many large and small babies die then few of those babies bearing those alleles grow up to pass on their genes and therefore those genes or alleles leading to large and small babies tend to become

less frequent in the population so that's sort of the the basic idea of natural selection just to recap there is variation within the population with different individuals having different alleles of the underlying genes some of those alleles produce differences in phenotype differences in behavior or morphology or whatever, that in turn lead to differential success in reproduction and survival. Those individuals that have alleles that lead to better survival and reproduction pass on more of those genes to the next generation than those individuals with poorly adapted genes or alleles. And therefore, those well-adapted alleles become more frequent

in the population than the poorly adapted alleles. Therefore, over time, the population tends to change or evolve such that it now is comprised entirely or mostly or entirely of individuals with this particular allele or therefore this particular phenotype. So that is natural selection. And it's called natural selection because the selecting factor is the natural environment. It could be the weather or climate in the particular location that the animal lives in, the height of the trees or predators that exist. All factors like that in the natural environment select out individuals. So populations or species under natural selection tend to become more adapted to their environment over time because individuals who are more adapted to the

environment have more offspring and those that are poorly adapted become have fewer offspring and therefore become less frequent in the population. Now as I said natural selection is only one mechanism that can lead to evolution. There are other mechanisms too and I want to go through those just briefly now. One that is very close to natural selection is artificial selection. In this case rather than nature or environmental conditions selecting out which individuals get to breed, humans select which individuals get to breed. And humans can select based on whatever trait they want. We can select, for example, the largest rats,

breed the largest rats in one generation, then pick the largest rats within the second generation, just breed those together and so on. And in doing so, we can create rats that are progressively larger and larger. That's evolution caused by artificial selection. Humans have decided which factors lead to reproductive success. And the domestication of animals, for example, cows, sheep, dogs, and also crops like bananas, wheat, soybeans, pretty much anything like that,

all of those have been changed significantly from their natural state or from how they originally existed by human artificial selection, selecting for animals that are more docile that produce larger yields that are easier to deal with and so on. So that's another mechanism of evolution, artificial selection. A third mechanism is biased mutation. Remember I said that mutation, DNA mutation or genetic mutation, is basically the cause of genetic variation within a population.

That in itself would not tend to lead to any evolution because mutations tend to be random in terms of what is mutated and how it's mutated. But sometimes some mutations are more likely than others. However, sometimes some mutations are more likely to occur than others. For example, maybe it's more likely that a genucleotide will mutate to an A than to something else. If that is the case, if some mutations are more likely than others,

then over time that population will tend to evolve, into one that has more of those, where more of those mutations have occurred. That's not a particularly interesting mechanism of evolution, in my opinion, but it's still important. Okay, a fourth mechanism by which evolution can occur is sexual selection. This is often sort of given as a subset of natural selection. Basically, in this case it means that the environment itself, like predators or weather or whatever, does not determine who gets to reproduce more often. It's actually one of the sexes within that population exercising a preference in terms of who they're going to mate with.

Generally, this is the females who exercise this preference for reasons I'll explain in the later podcast. But female mating preferences, for example, a classic example of this is the peacock. You know how they have the extremely elaborate and colorful feathers. Those feathers don't serve any purpose other than attracting females. Female peacocks, who are called peahens, are attracted to males who have really large, really bright and colorful feathers. And so they are brighter and more colorful than bigger your feathers are, the more likely you are to get a mate, reproduce, and therefore pass on those genes or alleles that produced those large and culpable feathers to the next generation.

And that is how peacock feathers evolved. And there are a number of different, many different examples of these traits. For example, antlers on deer and other animals like that. They do serve partly a combat purpose, but they also serve an ornamental purpose of just being something that females prefer. And you might wonder why females in the given species would prefer a trait like this, like big feathers that don't do anything or antlers that don't really do anything. Why would that preference evolve in the first place? Well, basically, there are

two potential reasons that have been given for this. One is simply that it was by accident. It's like a side effect of something else. Maybe the females evolved a behavior that was beneficial for hunting or for their young or whatever, and a side effect of that was simply that they preferred big feathers or whatever, or big antlers. Another possibility is that the ornaments that are produced by the males, like the feathers or the antlers, are not useful in and of themselves, but they are a sign or a signal of the fitness of the individual. For example, if you can afford to produce very large, colourful feathers, that takes a lot of energy. And so if you can do that, that must mean that you're able to access food and escape predators and so on. If you were not

able to do that, then you wouldn't be able to produce the big feathers. And so the feathers or the antlers or whatever else is a sign of fitness of the man. males and therefore females would prefer to mate with those males because they would produce more, their children would be more fit. And of course the females, it's in their interest to mate with the best males possible in order to maximize the number of genes they pass on to the next generation. And so that's a second possibility as to why a sexual selection would occur. But either way, if one sex in a species prefers to mate with the other sex, with members of the other sex who exhibit a particular property, be it large antlers or feathers or something else, or birdsong is another one, then that trait will be selected for in the population and will become more prevalent. Okay, yet another mechanism of evolution is called genetic drift.

Genetic drift is basically changes in allele frequency that occur simply because of random chance or sampling error. It's particularly relevant to small population, because basically the smaller population you have, the more likely that if you take a random sample of that population and then breathe that random sample and allow them to pass on their genes to the next generation, the more likely it is that that sample is unrepresentative of the population. For example, to take an extreme example, if you just have two individuals, one tall and one short,

then whichever one you pick, it's going to be 100% tall or 100% short, and that's not representative of the population, of the initial distribution of the population. However, if you have a million individuals and you picked down 10,000 of them, then, well, actually, even if you had a million individuals and just picked one,

that still would be very unrepresented of the population. But if you picked 1,000, individuals, then it's very likely that that sample will have 50-50 tall and short because you've picked such a large sample. Genetic drift can occur simply by if, say, only 50% of animals in a population get to breed or get to pass on their genes to the next generation, then you're selecting a sample from that initial population to then form the next generation of said population. If that 50% that you've selected, or 10% or whatever number it is, is fully representative of the initial population, so, you know, if the initial population was

half tall, half short, and the sample is also half tall and half short in terms of who gets to breed, then that's fine. The second generation will have the same properties as the original one, and there's no evolution will occur. If tall individuals are more likely to breed than short individuals, because they're more adapted to their environment, or because females prefer to mate with tall individuals, or because humans are selecting them through artificial selection, all of those things will cause tall individuals to become more prevalent in the population as well. However, it's also possible that simply by chance more tall individuals will mate or pass on their genes than short individuals, and so the second generation will have a larger number of tall individuals

than short individuals, particularly that continues over time, that will result in evolution, and that mechanism of evolution is called genetic drift. And as I said, it's particularly relevant in small populations because it's more likely, basically, that you'll have a sampling error in terms of who gets to reproduce. Final mechanism of evolution is gene flow, which basically is just changes in allele frequencies by migration into or out of a population. An example of this might be different races of humans, if you want to call them races, humans with different skin colors moving into a new country or a new continent, and therefore interbreeding with the existing population leading to an overall change in skin color. So originally you had very light-skinned

Europeans, dark-skinned Africans, and maybe if they migrate to the same place, America, for example, and then interbreed with each other, over time, perhaps the population will change such that most people have a medium skinny hue. That's just an example, but gene flow just means changes in ill frequencies because people or organisms are physically moving from one population to another. So those are all the different mechanisms by which evolution can occur, natural selection, artificial selection, bias mutation, sexual selection, genetic drift, and gene flow. I was going to go through some of the evidences for evolution, but I have basically run out of time, so what I'll do is I'll put those into a separate podcast, so I'll leave it there for the moment.

I'll just go over a recap of the basic concepts, though. We talked about what is evolution and hereditary, and then I discussed how evolution occurs. So basically, evolution is the change in inherited traits, including anatomical, biochemical, or behavioral characteristics in a population over time. Evolution occurs because of variation within a population. If you have no variation, you cannot have evolution. That variation is in turn caused by genetic mutations through ultraviolet radiation or chemicals or copying errors and viruses and other things like that produce variations or mutations within genes leading to different alleles of that gene, which then in turn leads to different phenotypic traits of organisms. For example,

maybe there's a mutation which leads to a gene producing a protein that's a slightly different shape, that in turn leads to the organism being taller or faster or slower or whatever. These phenotypic differences in turn lead to differences in fitness. That is the ability of organisms to pass on their genetic material or their genotype to the next generation. So maybe these taller individuals, as a result of this differently shaped protein, they've become taller. Maybe these taller individuals can access food better or eat more easily than shorter individuals, and so they are more likely to reproduce than shorter individuals. And that is called a selection effect whereby a certain characteristic within the initial population is made more likely to breed or to reproduce than other characteristics,

individuals with other characteristics. These taller individuals, because they have more offspring, the next generation of animals tends to have a larger proportion of taller individuals than the initial generation, and that will occur in subsequent generations, therefore causing the population to change or evolve over time towards having taller individuals. And then maybe a new mutation comes along and makes even taller individuals, or a producer. some other change. And over time, over very long periods of time, that can produce substantial changes in organisms. Basically, evolution's had about four billion years to go from the very earliest protocells to, you know, human beings and whales and other marvels that we see today.

So evolution has had a very, very long time for this to occur. Evolution is not the same as natural selection. That's another thing I want to emphasize. Evolution is just the change in organisms over time. Natural selection is one mechanism that can select out amongst the variations within a population to choose which organisms reproduce and which do not. Natural selection was what Charles Darwin came up with, and that's why he's so famous. Charles Darwin did not come up with the idea of evolution. Evolution itself can be caused by numerous things apart from natural selection, including artificial selection, biased mutations, sexual selection, genetic drift, and gene flow.

So I think that's about all we need to cover today. There's a lot more to say about evolution, but those are the basic concepts. I think in that second episode I'll talk about the evidences for evolution and then maybe some common misconceptions of evolution and arguments against evolution and the rebuttals to those. So I hope you enjoyed this episode. Please send me an email with feedback and comments and anything else. My email address is Fods12 at gmail.com.

And any reviews that you might like to post on iTunes or anywhere else would be most appreciated as well. I think I have two reviews on iTunes at the moment. Three would be good, four would be even better. So please do that. and spread the word about the podcast by telling other people about me and how much you've learned from my episodes. And until next time, goodbye.