The Science of Everything Podcast - Episode 86: The Evidence for Evolution

Episode Date: August 12, 2017

An outline of the various lines of evidence in favour of the theory of evolution by natural selection, including a discussion of natural and artificial examples of natural selection in action, as well as a review of the evidence from the fossil record, comparative anatomy, biogeography, and molecular evidence. Recommended pre-listening is Episode 21: Introduction to Evolution.

You're listening to The Science of Everything podcast, episode 86. The Evidence for Evolution. I'm your host, James Fodor. So in this episode, we're going to carry on from the episode I did way back in, I think, 2011, introduction to evolution. That's episode 21, which is a prerequisite for this show, where I talked about some of the basic concepts of evolution, how it works, natural selection, the origins of the,

idea, etc. And then I promised, at some point in the future, to do a follow-up show discussing the evidence for evolution. And this is that show. I finally got around to producing this episode. So hopefully you'll enjoy it. I'm going to discuss a number of the major different lines of evidence for evolution, including some natural and artificial examples of evolution, particularly through induced natural selection in the artificial cases. We'll talk about the fossil record and look at some transitional fossils. I'll talk about comparative anatomy and the molecular evidence for evolution and biogeography. So as I said before, recommended pre-listening is episode 21, introduction to evolution,

and episode 34, 35 on DNA structure and function may be helpful for some of the stuff on the molecular evidence, although that's probably not as necessary. But certainly I'll be assuming that you have some idea about evolution and what it is in this episode. So recommend you go back and listen to episode 21 if you don't have that. So let's make a start, and we will begin by talking a little bit about what I mean by evidence for evolution and what constitutes evidence for evolution. So the idea here is that a piece of evidence or an observation or something

counts as evidence in favor of the theory of evolution, if it is readily accounted for by the theory of evolution. but not by rival or alternative hypotheses or theories. So in particular, we're going to be looking at evidence for common descent over alternative hypotheses that do not hold that animals or plants share a common ancestry. So evidence for common descent will be an important focus of this episode, as well as evidence for change of animal forms over time, over the alternative hypothesis that animal forms or plant forms have remained unchanged over large geological time.

scales. And finally, evidence that allelic variations, the variations in the allele frequencies in different populations that are postulated by evolution, evidence that these both exist and change over time as evolution says they should, whereas again, alternate theories wouldn't necessarily predict any such changes. So those are the three main things that we're going to look at today. Evidence for common descent, evidence for changing forms over time, and evidence for the role of these allelic variations in population. And we'll look at mostly animals rather than plants, as well as a little bit of stuff on microbes. So because there are three main aspects of evolution that we can look for evidence for,

the different strands of evidence are differentially useful for that. For example, the fossil record is mostly useful for showing the change of animal forms over time, whereas molecular evidence is better for demonstrating common ancestry or common descent. But you'll see more about that as I discuss each of the types of evidence in turn. But before we get to that, I just want to discuss a few natural and then a few artificial examples of evolution, particularly of evolution by natural selection. That is, these are cases where we have in historical time, sometimes in experimental time, observed the change of animals as the result of some particular pressure,

selective pressure that's been brought to bear on them. So essentially, this is direct observation of evolution by natural selection at work. Obviously, we have the ability to directly observe very large scale changes in animal forms, like the emergence of mammals from reptiles or proto-reptiles, for example. But on the smaller scale, we are able, in a number of cases, to directly observe the phenotypic change associated with evolution by natural selection. So one good example of this is various strains of nylon-eating bacteria that are capable of digesting various byproducts of nylon-6.

Now, nylon-6 is a synthetic fiber, and the general consensus of scientists is that this capacity to synthesize nylonase, that is the enzyme that these bacteria needs. in order to break down nylon 6 is the result of a single mutation that then survived and was passed on because it improved the fitness of this particular type of bacteria in providing it with an additional food source. So this is a clear example where a mutation has produced a differential phenotype, that is a new protein that is able to convey a reproductive or survival advantage on the organism by allowing it to access an additional food source.

Another very famous example of evolution and action are the peppered moths from Britain. Now, this story has a bit of a history to it. So the basic idea is that before the 19th century, the only type of peppered moths that were known were light-colored. And these were a type of moths that lived, either resting on light-colored trees or the light-colored lichens that lived on the trees. But over the course of the 19th century, it was observed that a new dark-colored variant of the peppered moth became increasingly what was first observed in 1811, and then became increasingly predominant over the light-colored variety, until almost all of the observed peppered moths were of the dark variety by the end of the 19th century. And this was directly associated with the Industrial Revolution,

and particularly around the city of Manchester, which produced a large number of pollutants, which resulted in the death of many of these lichens and a covering of black, dark-colored soot over the trees. So the idea was that the light-colored moths were increasingly easy to spot by predators, whereas the black or darker-colored moths blended into the soot-covered trees much better,

and therefore had a survival advantage. So at the time this was observed, it was postulated, but not directly confirmed that this was the case. A biologist by the name of Bernard Kettlewill was the first to directly investigate this in the 1950s, so sometime later. Now, these original experiments that he conducted, which confirmed essentially the classical idea

that this darker form of camouflage was successful in reducing predation, particularly by birds, was subject to a fever. bit of criticism from the 60s through the 70s, 80s and 90s. And so there was some controversy about some of his research methods and the validity of conclusions. But more recently, his findings were very carefully and rigorously, essentially, replicated

by another scholar called Michael Majeris, who conducted a seven-year experiment, which was apparently one of the most elaborate that's been done in this sort of area, which, without going into all of the detail, essentially vindicated, you know, Kettle Wool's original results while fixing some of the methodological problems. But the basic original story about the peppered moths does seem to be more or less accurate. Originally, they were light-colored. Pollution rendered their light coloration of poor fitness because it meant that they were easy to see by predators, and so therefore there was a selective pressure that over time

increased the population of dark peppered moths over the light peppered moths. And that's since been reversed in a large degree since the pollution. levels in those parts of Britain have reduced since the late 19th century. So that's a very clear, visual, macroscopic example of natural selection in action. Another very widespread and sort of visceral example of natural selection is the development and spread of antibiotic resistant bacteria, which is a huge problem around the world. Whenever a new antibiotic is developed, usually within a few years or a few decades, one or more strains of bacteria that are resistant to the antibiotic develop.

And this is becoming an increasing concern about whether we're going to run out of antibiotics because even the strongest antibiotics these days are showing up strains that are resistant to them. And obviously the selective mechanism is clear here because if you treat a person or a surface or whatever with an antibiotic, then you are selecting for whatever organisms might exist in that in that population that have any ability to resist that antibiotic. So any mutations that occurred that produce a protein that helped them resist, the antibiotic will be strongly selected for, and thus will have the emergence of antibiotic-resistant bacteria,

whereas when before the antibiotic was deadly to all of the bacteria. So that's a very clear and straightforward case. Another example of evolution occurring in a natural environment, or, well, sort of a natural environment, but at least not directly caused by humans, not intentionally caused by humans, are radiotrophic fungi that have been found in growing inside and around the Chernobyl nuclear power plant since the disaster there in 1986. So this is fairly recent research, I understand, that's looking at a number of different types of molds that have been found to actually utilize the gamma radiation from the radioactivity,

residues surrounding the power plant in order to produce energy to help with growth and also through some means I'm not exactly sure how survive in extremely high or much higher than the usual background radiation levels where other organisms wouldn't be able to so it appears that some forms of fungus have been able to adapt again through through selective pressure to this new environment provided by the the Chernobyl nuclear plant is disaster. So again, a fascinating example of natural selection in action in a natural environment. And there are many other examples, of course, these are just a few of them that I've picked out, the few particularly prominent ones. Now, let's move from natural examples of evolution

in action to artificial examples of selection and action. These are cases of artificial selection where humans have deliberately intervened to change the organisms. In many of the previous cases, humans were involved, but they weren't deliberately trying to change the organisms. That's the difference between these cases. So there have been a number of long-run experiments designed to selectively breed particular types of animals, either bacteria or even larger organisms in some case. So one's long-running study by Professor Theodore Garland has been working on an experiment for, I think a couple of decades now, in which they have selectively bred mice to higher spontaneous activity rates on

running wheels. And they've been able to evolve mice that run about three times as many revolutions of the wheel per day compared to the unselected control groups. This is over a period of several dozen generations. That's quite an interesting example. Another interesting long-term experiment involves E. coli evolution experiments run by Richard Lensky, and he's been tracking genetic changes from a number of almost identical original populations of E. coli since the late 80s. And one discovery that he has made a couple of years ago is that one of the populations evolved to be able to utilize citric acid as a source of energy, which previously wasn't able to do. So this is the evolution of, again, the enzymes to be able to metabolize

a completely new energy source that previously wasn't able to do. So this is somewhat similar to the radiotrophic fungi and the non-not-eating bacteria cases in the natural environment, except this was done in a lab. And in general, really, any case of a domesticated organism, be it wheat, bananas, pigs, horses, cows, dogs, silkworms, chickens, maize, cabbage, all sorts of different vegetables, grains, and animals that have been domesticated have been substantially changed over their original ancestral forms through a process of artificial selection over many generations. over many generations, many centuries, even many millennia in some cases,

and even if the people who were engaged in this activity didn't really know what they would, didn't really understand the processes underlying what they were doing, they were really engaged in a process of artificial selection in that better animals or better plants were selectively bred so as to produce offspring that had a higher yield or that had more meat on them or whatever it be, depending on the case.

And these are all examples of natural selection in action, although in this case artificial selection, but the main point of selective pressures leaning to phenotypic changes over the generations. That's evolution in action. So there are quite a number of cases from the natural world and from the artificial world of evolution, particularly natural selection in action, to produce change over time. Obviously, they're not nearly as large scale as the sort of very substantial changes, you know, birds evolving from dinosaurs and mammals evolving from reptiles and things like that, but these happen over a much longer geological time scales,

not the few years to dozens of years to maybe at most thousands of years that we've had for these historical cases that we can document. But nevertheless, they illustrate the same processes in operation that then over-extrapolated over much longer time spans can produce much larger phenotypic change. So let's look at some of the evidence supporting that extrapolation. of these processes from relatively short time scales to much longer time scales and therefore much larger phenotypic changes. First, let's look at the fossil record. This is probably what people think of most readily when they think about sort of evidence for evolution. Although

personally I don't think it's the most important, but it is certainly an important piece of evidence for evolution. So a fossil is the remains or traces of an organism from a past time period that's become either embedded in a rock or essentially become lithified, that is, the bones have turned to rock through a process of mineral replacement. There's a number of mechanisms that this can happen. But basically, a fossil, a fossil is generally a rock or an impression in a rock that records information about a past organism. Discovery of these fossils can provide information, therefore, about organisms that lived in the past. The further you dig down into the ground and therefore access older and older sedimentary layers where these fossils are found,

the further in the past you can discover information about what organisms lived then and what they were like. This sort of information has become much easier to extract in the last 50, 60 years or so since the advent of radiometric dating, which has enabled us to provide actual year dates of the age of many of these different sedimentary layers, thereby enabling us to paint a much richer picture of the temporal change of organisms in different places around the world at different times and how the fossils map to the different periods in the geological time scale. So at a very coarse level of analysis,

one thing that's observed in the fossil record is that the types of fossils that we find change substantively at different times in the geological past. We don't see the same organisms in very recent, strata as we do in somewhat older strata and we don't see the same organisms there as we do in even older strata and so on. The types of organisms that we see change as we go further back into geological history and they change quite substantially as you go further and further back. Furthermore, the very oldest fossils contain very few types of fossilized organisms and they all have

a very simple structure, whereas younger rocks contain a greater variety of fossils, often with much greater structure. And that's a general pattern. The very old rocks have no fossils in them, and then as you move closer to the present through the geological time scale, you find more and more types of animals and more and more complex animals in the fossil record.

Now, this is not exactly a linear increase because there are periods of mass extinction and other things happening, but overall, this is a very clearly observed trend. So this is a clear evidence of changing animal forms over time. It just is not the case that the animal forms that we see on the earth

are the same as animal forms that were on the earth 50 million years ago, 100 million years ago, 200 million years ago. More specifically than that, we have in the fossil record found a number of cases, in fact, many dozens of cases of so-called transitional fossils or transitional organisms,

which are sometimes called missing links in the popular lingo, but that's not really a very accurate term. transitional forms is better because the idea is that a transitional fossil exhibits both ancestral and derived phenotypic aspects. That is, it shows some traits of modern organisms, or more recently evolved organisms, and some traits of older organisms, or those that date back further into the genealogical past. So, for example, Ticatalic is a transitional fossil that exhibits some of the phenotypes of fish and of tetrapods.

Effectively, it's a walking fish. Archaeopteryx is a transitional form between dinosaurs and birds. Essentially, it's a dinosaur with wings and feathers, so it shares phenotypic traits, both with dinosaurs and with birds. Ambulocetis, effectively, is a whale that still has webbed hands and feet, and still fully quadrupedal, so it's a transitional form between the land mammalian ancestors of whales and contemporary whales. Eupidotophis is a snake that has two small hind legs and is a transitional form between lizards and snakes.

Thrinaxidon is a reptile mammalian transitional fossil, which had fur and warm blood like contemporary mammals, but also laid eggs like reptiles. These are just a few examples of the transitional fossils that have been observed, or more to the point, these are the organisms that have been inferred on the basis of discovering these fossils, because these are the organisms. organisms don't exist anymore, but the fossils of them can be found a particular strata. And using these transitional forms, you can then build essentially a tree of life. Look at what organisms exist today and what organisms existed at this point in time, and do we have transitional forms between them, and what organisms existed even further back in time, and do we have transitional forms between those, and fit the pieces together in a puzzle? And essentially this is what evolutionary biologists, paleontologists, this is what they do.

This is there, one of the big jobs, essentially, to construct this tree of life, and it's still very much a work in progress. But the basic building blocks are there, and the pieces fit together, and they make sense. We don't observe rabbits in the pre-Cambrian, for example, pre-Cambrian being a time before any vertebrates existed. You don't just find random rabbit fossils there. That would completely break this story of the changing forms of life over time and present a huge problem for evolutionary theory.

but such things are not found. It's true that we don't have all of the answers, and all of the time there are fossils found that are hard to fit in or that raise questions about previous classifications, but in general terms, the overall structure of the tree of life makes sense with respect to finding the transitional forms

that we expect to find, fitting them together in a sequence that makes sense, and having the more derived forms found in later strata and the more ancestral forms in earlier, older strata and so forth. So overall, the fossil record provides strong evidence for the change of animal forms over time, and in a change in an order that makes sense, in terms of progressing from ancestral forms to more modern forms over time. This doesn't by itself directly show that the theory of evolution is correct, that is evolution by natural selection, according to Darwin's

hypothesis, but it does clearly show that animal forms have changed over time. Now moving on from the fossil record to look at comparative anatomy. And this is a particularly strong evidence of common descent. Remember, common descent is just the idea that different, contemporarily different types of animals originally shared a common ancestor from which they subsequently diverged. And personally, I think that this is one of the strongest lines of evidence in favor of evolution by natural selection. And I believe it's one of the main lines of evidence that Darwin used, because in his time, the fossil record wasn't nearly as well established as it is today. He also obviously didn't have any molecular evidence to draw upon. So comparative anatomy was one of the main forms of evidence that he relied

upon to develop the theory. And I still think to this day it's one of the most compelling lines of evidence. So the basic idea is that you look at the anatomy of different types of animals and make comparisons looking in particularly for structures that are what's called homologous. Homologous structures are those that are similar. in some way, but importantly, derived from a common evolutionary ancestor. Now obviously you can't tell if a structure is homologous just by looking at it because just looking at it doesn't tell you whether the organisms derive from a common ancestor. But the point is that through this sort of analysis of these,

comparative analysis of the anatomy of different types of animals, we can make inferences, which then allows to establish that structures are homologous. So in particular, the example that I want to look at here, one of the main examples is the pentadactal limb. Essentially, that just means a five-fingered limb, which is found in all classes of tetrapods. So that means amphibians, reptiles, birds, mammals. Now, throughout all of these classes, which is a very wide range of different types of animals, the limb has essentially the same structure at the skeletal level. There's a proximal bone, so that's the bone closest to the body, the humorous, two more distal ones, the radius and the

the ulna, then a series of carpals, which are in humans, wrist bones, followed by a number of metacarpals and phalanches, so palm bones and the digits. So in humans, this corresponds basically to your arm. Now, the point is, although on outward appearances, the limbs of many different tetrapods are completely different. If you look at the skeletal structure, the number of bones, their relationship to each other, and even many aspects of their shape and and connectivity, details like that, that anatomists, of course, study in depth, are very similar across the different types of animals. So in monkeys, four limbs are elongated, and they form a grasping hand used for climbing and swinging

trees, essentially. That's what humans have, more or less. Pigs lose the first digit, while the second and fifth digits are reduced, so that the two remaining digits are longer and stouter than the rest, and form a hoof for supporting the rest of the body, but the other bones are largely intact. horses are somewhat similar. The forelimbs are adapted for strength and support. The mole has a pair of short spade-like forelims for burrowing,

so they've adapted to use their forms for burrowing rather than, say, pigs for supporting the body, monkeys, grasping trees. Anteaters use an enlarged third digit for tearing into ant and termite nest, so they use that for feeding. Cetaceans, so that includes whales and dolphins. The four limbs become flippers used for steering and maintaining equilibrium during swimming. In bats, the four limbs are very highly modified

and effectively evolved, not effectively, have evolved into wings. So four digits become elongated and sort of form, while the first digit forms a sort of a hook, which helps to keep the wing intact and also used in hanging upside down. And there's many other examples throughout the tetrapods of how the pentadactal limb has been adapted

to the particular lifestyle. style and survival needs of the organism in question. But in all cases, there's a close analogy of the bones and their connectivity that clearly show that there's a common linkage between these. The relative size has changed and the function for which the pedidactal limb is used has been co-opted, but the connection between these animals is just very clear if you look at these, look at a comparison. and I think very strong evidence of common descent of all of these animals.

Another line of evidence from comparative anatomy concerns vestigial structures. These are body parts that correspond to body parts that ancestral species held, but have become smaller and simpler over evolutionary time, essentially because they're not used anymore. They've become degenerated, but are still around, even if in degenerate form. the existence of these vestigial structures is also clear evidence, I think, of the common ancestry, common descent of animals possessing these with animals that have the full form of the structures, because there's no other clear reason as to why these structures would exist,

were they not vestigial from an ancestral form. So, a classic example is the pelvic girdles in whales. Whales have a pelvic girdle and a number of bones attached to that, which essentially are the remnants of hind legs that their ancestors had when they walked on land. These bones, if you look at the skeleton of a whale, are completely detached from the rest of the skeleton. They serve no function at all. The only reason they exist is because their evolutionary holdouts or vestigial structures from ancestors. If you just designed whales from the get-go, there's no reason you'd have a pelvic girdle,

because they don't do anything. That's a particularly stark example, but there are many others as well. So hind wings of certain flies and mosquitoes, which don't serve any purpose, but are evolutionary holdovers. Wings of flightless birds like ostriches, again, which are residual structures from ancestors which could fly. Reminent eyes or eye structures from animals that have lost sight, such as blind cavefish, mole rats, and some types of snakes and spiders.

So these have residual eye structures that are non-functional. Another interesting example is the extreme detour of what's called the recurrent laryngeal nerve, which basically runs from the brain down to the heart and up to the larynx. Now, this is a very odd path to take because the nerve could just run essentially straight from the brain to the larynx, and it would not be a very long trip. And originally, this is essentially what it did, because originally in Fish Like Ancestors of Modern tetrapods, the nerve would have travelled from the brain past the heart into the gills, as it does in modern fish. However, over the course of evolution, the tetrapod descendants of

these early fish, the neck extended out, and the brain moved further away from the heart, while the heart itself moved lower in the body. So essentially, the laryngeal nerve was caught on the wrong side of the heart and had to sort of make a U-turn around to get back to the larynx. And the most extreme case of this is giraffes, which have an extremely long, extremely elongated neck compared to their ancestors. So the laryngeal nerve actually extends over four meters down from the brain around the heart and up, back most of the way up the neck again to the larynx. This is a bizarre way of connecting the brain to the larynx. But it makes perfect sense under natural selection and the process of evolution, because it's inherited this trait from, its ancestors. Over time, the length of the nerve has been extended, but there's been no sort of

redesign to redirect the laryngeal nerve in a more sensible direction. There could have been, I mean, that's an evolution, that's a mutation that could have happened. It just didn't, and evidently the selective advantages of doing that have not been sufficient to select out for that mutation, so it hasn't occurred. And therefore, we have this residual structure. which is very hard to understand outside the theory of evolution, or common descent from animals that had a much closer connection between the brain and the larynx, or in other words, where this nerve didn't have to transverse such a long distance as it does

in modern animals, particularly like the giraffe. So a third type of evidence from comparative anatomy concerns embryological evidence, and I won't talk too much about this because it's a bit more controversial, But nevertheless, the basic idea is that remnants of ancestral traits often appear and then disappear at different stages of the embryological development process, which is indicative of the fact that what we would today regard as quite distantly related organisms, and nevertheless share a common ancestry, which then is sort of manifested in different stages of the embryological development process. This has been noted, for example, if you look at the embryo, of different types of animals from reptiles through to mammals and amphibians and so on.

The embryos look a lot more similar than the grown organisms do. Now, it's important to understand that this is not the same as an argument about ontogeny, recapitulating phylogeny, which is a phrase that some of you might have heard. I don't want to talk about that in detail, but that idea was specifically that the process of development sort of directly followed the evolutionary stages. of that organism. And that is not an accepted idea anymore. It was postulated earlier in the 20th century and is since being rejected. That's not what I'm talking about here. I'm just talking about the fact that there are similarities evident in the embryos and in the development cycles

of many organisms that are not evident in the adult organisms. And these similarities are indicative of shared ancestry. In particular, you can look at things like the development and generation of a yolk sack, which even humans have, even though, of course, we don't lay eggs and our ancestors have not for a very long time. Another example is the appearance of gill-like structures, the pharyngeal arch in the vertebrate embryo during development. Now, in fish, these arches become gills, while in humans they become the pharynx. So quite different end structure, but during a particular stage in development, they're quite closely analogous. And again, this is a similar examples of what we saw before of initially shared ancestral structures that have been

subsequently co-opted for different purposes by different organisms, depending on the selective pressures and the environments inhabited by that organism in question. So all of these cases of the embryological evidence, vestigial structures, and a comparison of organs like the pentadactal limb clearly points to the fact that animals that today we would regard as separate species, but even different orders and classes, nevertheless, share a common ancestry at some point a long time ago in the past, which is consistent with the theory of evolution, which is exactly what is predicted by the theory of evolution.

It is inconsistent with other proposals, which say that different types of animals do not share a common ancestry. They would not predict any of these types of evidence, and therefore these evidences from comparative anatomy, I think, provide strong evidence in favor of the theory. of evolution by natural selection over these alternatives on which these evidences would not make any sense from which they would not follow. Now I want to talk a bit about the molecular evidence for evolution. This is particularly strong evidence both of common descent and also of the

development of molecular mutations over evolutionary time. So this is the allylic variant that I mentioned before. So all known extant organisms have more or less the same fundamental biochemical organizations. So they all have genetic information encoded by DNA, and they all transcribe that into RNA, and then use that to make proteins by the ribosome. The ribosome is very similar structure. Well, the structure is pretty much the same. In terms of the sequence, it's very similar all throughout life. The genetic code is pretty much the same for every organism. There's a few small variations, but basically it's the same. This is the way in which the order of nucleic acids in the DNA is translated to correspond to the order in which amino acids

are added when producing a protein. Go back to previous episodes on these sorts of topics, if you're unclear what I mean by that. But the code, the translation between DNA and protein essentially is essentially exactly the same all throughout life. And there's no reason necessarily we should observe these sorts of commonalities. If different forms, of life had different origins. There's no particular reason the genetic code would have to be the same, because, I mean, you could have any genetic code, really. The fact that they are all pretty much exactly the same is strong evidence that all life is related to each other. But we can get even more fine grain than that and look at particular DNA sequences and allow and compare the DNA sequences of different organisms. And what we find is that organisms that were closely grouped with each other by traditional tax army also share, similar DNA sequences. So humans have a most similar to chimpanzees and then still quite similar to gorillas, a bit further removed from bonobos, more closely related to other mammals than we are to reptiles, more closely related to reptiles than we are to invertebrates and so on. So we find a

pretty close mapping between genetic sequence comparison and sort of phenotypic traits. Now that in itself is perhaps not evidence for evolution because you might just say, one might say that molecular similarity or DNA sequence similarity is just replicating phenotypic similarity. After all, if the phenotype is determined by the genotype, then this is what you would expect, phenotypically similar organisms to also be genotypically similar. That's not necessarily always true, though, because there are many different ways, in many cases there are many different ways of achieving the same phenotype with different genetic codes, either because of the fact that the genetic code is redundant,

so there are different ways that you can get to the same protein, different ways that you can have different sequence, but get to the same protein in the end, and there are different proteins that you can have which have similar functions. So it's not necessarily true that similar phenotype would necessarily result in similar genotype if it weren't for the fact that organisms share a common ancestry. But there's also the fact, and I think this is the most persuasive fact, that parts of the genetic sequence that don't code for proteins, and in particular that are known to have been inserted in there, into the genetic code, without any particular advantage, survival advantage to the organism.

So, for example, endogenous retroviruses. Essentially, these are viruses that insert themselves into the genome and then just stay there and are passed on to the next generation from those that receive the original infection. So the viral genome just sits in the genome and is subject to mutation over time, but it's not performing any function, it's not really contributing to phenotype in any way. So there's no reason why, say, the sequences of endogenous retroviruses or other similar non-coding sequences should be similar in different organisms, even if those organisms have a similar phenotype, because it's not contributing to phenotype.

The only reason you would expect to see similarity in these non-coding sequences is because of common ancestry. That is, sometime back in evolutionary history, an ancestral organism received, was infected by this retrovirus, for example, or had some other mutation in non-coding DNA, which then was passed onto its offspring. And later, at some point later on,

its offspring split into different species, and there was subsequent differential evolution in each of the descending lines. so that the further back in history, the speciation event occurred, the more succeeding time there has been for divergence in these non-coding regions. Therefore, you would expect to see, under this hypothesis, you would expect to see non-coding regions of DNA have strong similarity in closely related organisms and less similarity in more distantly related organisms. And this is indeed precisely what we observe. and there are a number of cases of these that have been looked at in cats, for example, and comparing humans and chimps looking at endogenous retroviruses in particular

to look at how much divergence there has been between the copies of these retroviruses that we have versus the chimps have, etc. So again, the basic idea there is that, under the theory of evolution by natural selection, the more closely related to species are, the more similar their genetic sequence should be, both coding and non-coding sequences of DNA, because both are passed on and subject to mutations over time.

You expect the cony sequences to be more strictly conserved over time because they perform a function, and non-coding non-regulatory sequences to not be as well conserved because they don't necessarily perform a function. Some non-coding sequences do perform important regulatory functions, but there are also plenty of non-coding sequences of DNA that don't perform any function. like these retroviruses that have been stuck in there, for example.

So that's the ones I'm talking about here. So under evolution by natural selection, you expect these to be most similar in humans and chimpanzees, less similar in humans of bonobos and less similar again humans and dogs and so on. Whereas absent the theory of evolution by natural selection, whilst perhaps we would expect phenotypically similar organisms to share coding DNA, there's no reason to suspect or expect why phenotypically similar organisms

would also share non-coding DNA. because the non-coding DNA doesn't contribute to phenotype. And therefore, the only reason that it would be similar across these organisms is if the organisms were actually related to each other, not just because they look similar to each other. Okay, so that's the molecular evidence for evolution. There's much more that can be said there, but I think I've illustrated some of the main points.

The final line of evidence that I want to discuss briefly is biogeography, and this is evidence for changing forms of animals over time, of life forms over time, and also of common descent. So the basic idea of biogeography is that we can compare the types of fossils that are found not just in different periods of time but also in different parts of the world and then compare that to the processes of continental drift and other geographic changes that have occurred over the past and sort of make maps of where organisms are found

and compare that to the constructed evolutionary histories and see if they match up. So one thing that you would expect if evolution by natural selection is a correct theory is that populations that have been isolated, say, on an island for long periods of time, should have more time to diverge and therefore become more different than organisms on the mainland, say. And indeed, this is what we observe. We find many examples of species that are endemic to just particular islands, especially something like a flightless bird at Moe in New Zealand or lemurs in Madagascar, Komodo Dragon of Komodo.

We only find these on particular islands or chains of islands and not elsewhere. Now, there's no particular reason that you would expect that unless there has been a change over time, such that originally, you know, the ancestors of the organisms that came there were found multiple places, but then there was a separation of landmasses or the island broke off or whatever, and then there was differential selection over time so that the mainland and island populations changed over time, and hence forming the distinctive island populations that are not found island species, not found anywhere else. Likewise, we also find that different continental regions have particular endemic types of animals that are not found elsewhere or that are different elsewhere. So Africa has

old world monkeys, apes, elephants, leopards, giraffes, where South America has New World monkeys, cougars, jaguars, sloths, and llamas. And there's an analogy between those different types of organism, you know, between old world and new world monkeys, between leopards and jaguars and so forth. There's a similarity between the ecological niche that these different organisms fill. And so if the emergence of animal types was purely determined by the climate or the ecological niche, it's not clear why there is such a regional difference, say, between Old World and New World monkeys. Why don't you find only one type of monkey in Africa and in comparable regions of South America? There's no explanation for that, except under the theory of evolution,

which explains why you have different types of monkeys. the different places because a long time ago the continents were together when the ancestors of monkeys evolved they spread throughout regions that are now Africa and now South America but then those regions divided from each other and then the two branches were separated from each other and one lot evolved into Oldwood and the other into New World Monkeys and there are many other examples of this as well so if you look at succulent plants and cacti in the deserts of North and South American compared to Australia there differences in populations there

Another example from Australia, the different native animal fauna of Australia compared to similar latitude and geographical regions of South Africa and South America. You have marsupials like kangaroos in Australia, whereas they're not really found anywhere else. Again, there's no clear reason for that other than the fact that Australia has been isolated from the rest of the world for a long time, and therefore there's been more evolutionary time for its native form to become more distinctive. Penguins are another example. Penguins are only found at the South Pole, despite very similar climactic conditions of the North Pole.

Again, the geographical separation combined with evolutionary pressures over time provides a national explanation for why this is the case. And again, there are many other examples that I haven't discussed here, but just to give a broad picture of the type of evidence that can be provided by bioregography, Basically, the geographic spread of different organisms is consistent with what we would expect

if there has been both change of form over time and also common descent from initial shared ancestry. It's not consistent with what we would expect to see if the same forms of animals have existed throughout geological time and if there has not been common descent of different animal forms. It's not clear why we would observe the patterns that we do. So, in conclusion, the evidences for evolution, range across a wide variety of different branches of biology and include natural examples of selection from historical case studies, artificial selection from laboratory and other artificially selected examples, the fossil record, comparative anatomy, molecular evidence from

molecular biology and biochemistry, and biogeography, the patterns of distribution of animals across the world. So hopefully you found this episode interesting. If so, please leave the podcast a favorable review on iTunes or another podcast aggregator of your choice. You can send me an email. My address is FODS12, FODDS12, at gmail.com. Feel free to send me feedback, suggestions of future topics, or just say hello. I always like to hear from my listeners.

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