Darwin's evidence for natural selection - an Encore Presentation

Welcome to the BioAudio podcast. If you are a past listener, you'll probably recognize this episode. I'm reposting the first three episodes of season one, because they also set the stage for season two. If you are new to the podcast, we're starting from the beginning. If you've been here before, we're having a quick review. So please listen to our second conversation with Dr. Dave Home on the evidence used by Darwin to support his case for natural selection as the mechanism of evolution. Today on the BioAudio podcast, we are talking about the various lines of evidence for evolution, and I'm joined by an old colleague to discuss this. I'm not today's home. I'm a paleontologist/duology/something or other at a Queen Mary University of London. Okay, so let's start first with the idea of survival of the fittest. What is fitness? What do people mean when they say that? In a modern context, fitness is effectively how well adapted you are to the environment in which you are attempting to survive. Reproductive fitness is a very common one, which is very often measured as how many offspring you have, because that's a nice easy thing to measure in the field. That has really come from that survival of the fittest quote, which lots of people really hate, but it's also not a bad one. But of course, the problem is people think of it fittest in terms of a more kind of general exercise level of your quick and can run and jump and carry, whereas it's about much more of biophysiology, basically, or ecophysiology, almost for a better word. Are you going to survive and have offspring? And the more surviving you are and the more offspring you have that inherit your characteristic? So that's ultimately what he's getting at in that sense. Okay, so survival of the fittest may be a really simplified statement. But it's also not a bad one. People really don't like it, and I think it's not bad at all. But then fitness in an evolutionary sense is then not about physical fitness, but about your ability to survive and reproduce so that you can pass on the adaptive traits that you have. Being the fastest cheater ever and able to capture every single antelope you ever chase, but being sterile is zero fitness. You have no survivors. One thing students often seem to mistake is this idea that natural selection can somehow create a new trait. It's not that way at all. You've got variability within a population and natural selection acting more as an editor where the environment simply selects for those best fit to survive and reproduce. What of this did Darwin understand? He was clearly operating without an understanding of genetics, but what bit did he get right? Darwin is writing in a pre-genetic world, but yeah, mutations happen for all kinds of reasons, and novel combinations of genes will appear through sexual reproduction, and those can cause variations in traits or the appearance of new traits, region of the disappearance of old traits. Depending on what's going on in the environment, sometimes those traits do better than others, and those animals are more like to survive, they're more likely to have offspring and they're rostling more likely to survive. It's still very hard sometimes to wrap your head around the concept that all traits are ultimately the result of the accumulation of small mutations, either individually or in groups over time, but they do appear at random first. It's only afterward that the environment can select for this spread of those traits through fitness or their removal. That's largely what's going on, no animal, no organism that I'm aware of is capable of controlling its mutation. So there's a number of lines of evidence that we now talk about as those things that Darwin used to argue for, natural selection, and the most obvious source of information for him was the biogeographical observations he made while he was traveling around the world on the beagle. Can you tell us a little bit about how biogeography plays into this as his first line of evidence that he collected? Wallace is the person who wrote the book on biogeography because of the stuff in Southeast Asia. One of the arguments that Darwin was making is if a creator is going out there to make organisms, and as was conceived at the time, to make them perfect, they should be perfectly adapted to where they are. Why the hell do we need antititors in South America and odd vox in Africa and Pangolins in Asia? So it's almost the sheer scale of biodiversity that there was all this ecological redundancy. You've got animals occupying the same ecological niche that are wildly different in different parts of the world. There's got an awful lot of time these times from making antiters and beefls and big cats and seaweeds and millions and millions and millions of species. When it's really very clear that actually you could make one fox and that fox would probably be fined in North America and it would probably be fined in Europe and it would probably be fined in Asia too and therefore maybe one fox might be suitable. People didn't like ironically a creator who was that kind of involved in that festidious and becomes a problem because if you're working in Europe, particularly if you're working in Britain, we've got a very de-porperate post to ice age foreigner. We don't actually have that many species knocking around. Whereas you start getting into the tropics, you just start just going just how many there are and then once you start realizing that there's you know five or ten thousand of species just in the UK of beetle and that we might be looking at the hundreds of thousands or millions worldwide. Javier's Halzain's fate, fate, famous, you know, what can you tell about the mind of a creator? He has an inordinate fondness for beetles. You know, this didn't sit well with the clergy actually, the idea that you know that that sixth or fifth or sixth day of creation, whichever when it was where he sat down and made the beetles, he put an awful lot more time into that than some of the other things that he did. So is that the sort of raw biogeographical argument then that you didn't need this level of redundancy around the world and out traveling he sees these separate communities of different but ecologically similar animals and it just doesn't make any sense to have designed it that way. You know, and the Galapagos were a prime example of that. Why is there a different cactus finch on each of six islands that are barely a kilometer apart and they're all the same? Where the hell wouldn't you make one cactus finch and dump it on all five islands or all five or six islands? Well, you know, and that's really what he's getting at. Whereas, of course, and this does kind of then support his ideas, a local adaptation to those subtly different conditions separated by long periods of time suddenly makes a lot more sense. So yeah, biogeography was a good argument, but I definitely say it's far from his strongest. Okay, a stronger argument then. How about homology? This idea that structures that have highly variable functions can have almost identical underlying structures. The classic example being the pentadactyl limb. This idea that many vertebrates have an identical underlying bone structure regardless of the function of how they use their arms. If a creator is creating all these dozens of species and, you know, has a perfect infinite omniscient mind, why do whales and bats and foxes and horses or at least ancestry or embryologically or, you know, and lizards and on and on and on all have the same sets of bones in their arms. Fundamentally, you have a humorous upper arm bone, radius on the lower arm bones, you have carpools in the rest, metacarp was in the hand and phalanches of the fingers and you have five fingers and usually with the same number of bones in each finger as well, only a couple of differences. Why would you need to do that? Surely there are other patterns of bones that work well, potentially work even better. Surely there are other opportunities to have different numbers of fingers or more carpools or three bones in the lower arm instead of two or put two bones at the top and then one or who knows what, but they're all the same, all the same. So this argument boils down to then there are so many other viable patterns. Why is there only one? And if there's only one, the more likely explanation is that there's a common origin to that pattern which has diversified rather than somebody just caught photocopied and created a bunch of identical patterns. And you mentioned embryological development. What was the argument about embryological development? Well, a similar kind of thing in that we're seeing, you can look to embryos when they are less developed in the adult forms and they've not yet reached their final stages and what you've seen to find is that they tend to look very similar to each other and be actually sometimes they show features which don't appear in the adults. And of course most notably you get things like gill slits in all kinds of tetrapods which lack them as adult, but suggest that previously they effectively were swimming animals and that you're seeing part of their evolutionary history through their develop. It really is as simple as that. You know if foxes were created to live on land and run around and breathe air and don't really go swimming and certainly don't live in the sea and breathe water, I think they got gills in their embryological history. And why they got gills that are basically identical to those or fish who absolutely do finish up as animals that swim around in the sea and breathe water through their gills. And it does rather look like creating that would have been a very very odd thing to do for a perfect omniscient being, but inheriting that from a swimming ancestor would be a very reasonable thing that it has hung on somehow. Mentioning gills slits is a good one because this introduces the other line of evidence which is the existence of vestigial structures. Yeah so those are things which are still turning up effectively functionless structures in adult or kind of organism. In a modern context, they're rather complicated because of course so many things have been co-opted in evolutionary history. It's hard to know what might really truly be vestigial. So a classic argument was the whale pelvis because the legs have gone and the pelvis is just this tiny bit of bones floating in the body, but some of the muscles attached to them, the further some of the tail fluke, now is that them then being secondarily co-opted to a function or was that a function all along that it would, it's difficult. But again, that's a modern argument. If you look at the argument at the time, that was pretty convincing. Like whales don't have legs, they swim with their tail, why have they still got a partial pelvis or stubs or leg bones. And things like, you know, the appendix in humans, you know, things like this which were long thought to have no obvious function. Again, a creator who creates perfect organisms shouldn't be creating them with superfluous structures, let alone structures which are present in Darwin's putative ancestors or relatives. So you can kind of see where that argument is coming from. Another really good one that's often quite easy for students understand goes back to Kuvier and his fossil strata. And there's two very obvious hypotheses about the order with which you find fossils in the stratigraphy. And one of them supports natural selection and one of them doesn't. Yeah, so essentially if there is no evolution in history and everything was created simultaneously and everything has been around as long as everything else, even if you allow for some extinction here and there, you would be effectively expecting to find everything everywhere or for it to be effectively random. So you should be as likely to find a simple worm or a moss as you are a flowering plant or a fish or a rabbit or anything else. On the other hand, if things started off simple and got progressively more complicated through evolution, you should find mosses and worms first and then you should be finding fish and grasses later and then you should be finding really complicated things like birds later still. And yeah, curious enough, that's what we find. And I mean, that's an interesting one because the fossil record was incredibly sparse at the time. You know, Darwin writes a chapter in origin called Problems with the Theory and one of them was, yeah, the fossil record is absolutely rotten and we've got almost nothing in it. And I say that they're absolutely everything is an ancestor of absolutely everything else. So the fossil record should be absolutely heaving with these intermediate forms we haven't got any. You know, it was clear within years, you know, the earliest kind of stratigraphy being done in like, you know, the late 1700s that, yeah, you could date fossil strata by the organisms that are in them. And moreover, you see certain things appear in certain orders. There's a reason that the mesozoic was called for a very long time, the age of reptiles because that's where you found reptiles and the senozoic is called the age of mammals because that's where you found mammals. Not scattered randomly in any way shape or form when you dug into the older stuff, you tended to find worthy things and, you know, brachyapods and muscles. And when you dug into later stuff, you found fish and into later stuff, still we found reptiles and later stuff, still we found mammals and birds. And therefore, that's, you know, a sequence. So the fact that the fossil record is very non-random appears in a really good sequence, which matches with our hypothesis about the origin of different groups and that you didn't need to date the layer to find the fossils. You could use the fossils to date the layer is pretty good evidence that this is a not a random arrangement of things, even when they had really very tiny number of fossils. They could see this. I mean, ironically, it's easier with less stuff because, of course, now we do have mammals going deep into the mesozoic and say there are loads and loads and loads and loads of birds in the age of reptiles. But when you've only dug up a few dozen, you find the most common things and you're pulling out please you saw the nictusaws and dinosaurs. Well, that's clearly very, very reptile indeed. So one of the final things that Darwin would have argued for or should have argued for, which might have been so easy to explain to everybody in sort of European British society was artificial selection, that they could fundamentally go out and make stuff that looked different over a very short period of time. What evidence would he have had at that point for artificial selection? Yeah, so I mean, I think this is by far the way his strongest argument is that, you know, because artificial selection is, you know, a perfect summation of natural selection, the gentry in particular, but, you know, even the average man on the street would have understood dog breeding and horse breeding and pigeon breeding and not just farm animals, what was he had, you know, things like greyhound dog racing and, you know, bullfighting and this kind of stuff was going on. And so yeah, you can look at those fundamentals and they're still there. Characteristics are inherited. If you breed two white horses together, you'll probably get a white offspring. If you've got two orange cats and breed together, you'll probably get an orange cat. They can see that. Those characteristics are inherited, but also there's variation because that litter of kittens, there might still be a black one or a tortoise shell one or ones a bit bigger or ones got slightly longer legs or turned up ears or whatever it is. And then selection happens because we go, well, we're breeding a race horse and so we'll keep the fastest one and let that one have the most offspring and that slow one is the one that, you know, is sent to the farm or the dog food factory. But just imagine that it's not you picking it. It's a predator that's picking who's the fastest and who's the slowest and you'll come to exactly the same conclusion. And so that I think is, yeah, white's the strongest argument because it has the all the elements of natural selection and the only thing you need to switch in your mind is imagine if they weren't racing against other horses, they were racing a lion, but the outcome would still be the same. The fastest one would do better and the slowest one would be eaten and removed. That's it. That's all I'm saying. And yeah, that I think is incredibly easy to grasp if you were landed gentry, you had a farm by definition, you know, you were a landowner, you were probably interested in things like this at the lower end, you know, racing pigeons and dog fighting and stuff like this and just having pet cats was popular. And there's lots of flour breeding and stuff like this going on as well. Supposedly at one point when Darwin pre-origin of species or even when he's suggested to his publisher, he wanted to write this book. Supposedly the publisher's comment wrote back as, have you considered writing a book about pigeons? Pigeons are very popular at the moment. That's it. You know, books on breeding pigeons to get weird pigeons was absolutely a thing. Darwin again was doing that stuff himself director. He was engaged in active observation by spending his family's money building pigeon lots and showing guests all his different pigeons that he was breeding and really boring some of them by all accounts. But, but pigeon breeding was something that at that time, if you are an English gentleman, you probably were engaged in or at least discussed like a sporting event. Oh, absolutely. But again, also at all levels of society, you know, Darwin, you know, kind of got himself introduced to people who were doing this and a lot of these people were poor people at the lowest end of society. And he would go and hang around in pubs and kind of wait to be introduced because that's not the sort of thing you did as gentry. So you had to get introduced to them to be able to talk to them about pigeon breeding to try and get that first hand knowledge because of course the flip side is you're right. The the gentry were very much interested in pigeon breeding, but they do it in that they would have people who was job it was to look after the pigeons and would go, well, do you like these ones? Yes. Okay, well, I'll go away and breed them and then come back six months later with some more pigeons. But they're not in there every day looking at every individual and doing that. Darwin was doing that himself because he was doing things like counting the number of feathers on the tail and measuring the lengths of the wings and stuff like this. So the one thing that Darwin knew nothing about of course is DNA and he he really didn't understand how inheritance worked. He got the idea that did work and that you could pass on a trait and some of his biggest blunders were in not understanding DNA. Yeah, so he had a this idea of gemules which were somehow your kind of total characteristics would come together to form some kind of gemule particle. These would come together and the next generation would be a blended mix of the two parents. He persisted with this despite the fact that it was obviously wrong and so people at the time pointed this out to him and said, well, look, if if you've got black out in animals and by chance you've got what we now call a mutation, but whichever mechanism and now one of them is white. If blending happens, the next generation is going to be gray. It's going to be half black and half white and the next generation is going to be three quarters black and one quarter white and the next one is going to be seven eights black and one eights white. Your perfect white organism in its snow camouflage which has magically appeared gets itself blended out of existence. It doesn't get a chance to be selected for because its offspring will always lose the trait that has just appeared. It doesn't work. He needed something where you had the opportunity for that new trait to survive despite blending, which is of course exactly what Mendel found with dominant and recessive, but Darwin didn't have it. But I think he was looking too ideally at, well, if you've got a tall mother and a short father, you're probably somewhere in the middle and he's not wrong when, of course, there are lots of genes that play in all the rest of it for simple absolute traits. Blending obviously actively gets rid of something when it should be actively being promoted. It is ironic that one of the things that was his biggest blunder is actually also now a great source of evidence for evolution and at least a common origin of life. DNA itself is evidence. What is amazing to consider and what I often explain as a geneticist to my classes is that the code used by DNA to translate DNA into an RNA, which then becomes an amino acid to build a protein, is effectively a universal code across all of life. And you wouldn't assume that. The code is effectively random. DNA is arranged into little groups of three letters. Those three letters are in 64 different combinations and they code for 22 different amino acids. Those amino acids are the building blocks of all proteins. But the code underlying that, those groups of three has nothing special about it. It's effectively random. And if this was a designed system by a creator or there were multiple origins of life, there would be no underlying assumption you would use the same code everywhere. And in fact, there are errors in that code. It's not a perfect system, but it is effectively universal. And this is one of the biggest arguments for a single origin to life. Well, and yeah, and then homology of things like, you know, Hockstein's, which are so, you know, ludicrously constrained, you can swap a human one into a drosophilor and still produce a perfect fly. I mean, that, again, why would you ever make that? Students have asked me if the lack of variation in the code has ever been argued as evidence against evolution. And in fact, it's probably the best evidence for the strictest natural selection you can find. Any variation. They're insanely constrained. Yeah, because it just ruins you and you never do that. Yeah, if you have a single variant on that genetic code that specifies for the wrong amino acid, chances are that embryo doesn't develop. And so it's actually that that it's one of the rare cases where the lack of variation is one of the strongest arguments in favor of really strict natural selection on a code. Once that code evolved and worked, it never changed again. Or there's very, very rare changes in the code and some odd bits of life. But, but basically it's a universal code. But also, but, you know, there's one of those things where you you can almost flip that around and go, okay, well, if you are using it as a creation argument, that they're all exactly the same. Why are they still a handful of different ones? Those should never have appeared. You know, if you really are photocopying, your 100 million species, none of them should ever vary, but there are a couple. And then they vary for certain regions because there's something like an extremophile, which case that makes sense that you've had to shift it because of the system. So, yeah, it's I think you're caught by either prong if you attempt to argue that that's a creation argument. But the universality of the genetic code is probably one of the greatest pieces of evidence for a single common origin to life from which everything else diversified because that code is effectively unchanged across billions of years. Okay, so is there anything else you want to add? I think just to remember that, you know, Darwin himself wasn't perfect. You know, the origin phenomenally important though it was, natural selection was over time. If Darwin hadn't got to it, someone else was going to sooner or later because all those strands were coming together. Wallace was clearly onto it. His grandfather was sort of onto it. The mark was discussing other ideas. Lyle was coming through. Hooker was obsessed with it. You know, lots and lots of Huxley, lots and lots of other people were getting into these ideas. Someone was going to crack it sooner or later and probably come up, once they cracked it, come up with the right kind of evidence to support it. But yeah, you know, the origin wasn't just some moment where Darwin went plink and everything was so because it wasn't because some of his arguments were a bit chunky. Some of the data basically didn't exist. Lots of people didn't believe him for various reasons and even when he was right, yeah, he had a mechanism of inheritance, which was completely wrong, and he continued to argue through his entire life. He thought that acquired characteristics was still a thing. Yeah, Darwin didn't have this moment of clarity and perfection followed. It was part of the story of science, but even that goes down to the fact that things develop over time. Yeah, he got his mechanism of inheritance wrong, but he had no real way of testing it. That this was a time where biological science didn't exist the way it does now. There really wasn't a way of going into the field and looking at finches and saying which of these is fittest. You can't do that in the kind of time span of a Victorian era scientist. That's just not how things worked. If it couldn't be done in a few days in the lab, it wasn't really done. Darwin was running experiments like those famous worm experiments and growing various seeds and breeding various orchids and they took years and they were diligent experiments and he had friends and family and other scientists helping him out with them, but it was still really, really small bed to the way we'd run an experiment. So yeah, I think the kind of two sides of it is on the one hand, yeah, he got things wrong. Things could have been better. He caused problems for himself and others with mistakes that he made, etc. On the other hand, he was working at a time when certainly within biology, science as a formal field basically didn't exist in a way that it kind of did for chemistry and physics and maths and other things. And therefore it's arguably a more phenomenal achievement that he got as much right as he did and just how much stuff has been born out in the future. Did you always set out to be an evolutionary biologist or a paleontologist? Did you come to this profession through an unusual series of events? I came to be a dinosaur specialist through a series of unusual events, but no, from the age of dot I was obsessed by animals. It was the only thing I was ever interested in, the only thing I ever wanted was animal toys and animal books and I wanted to go to the farm and go to the zoo and go to the natural history museum and that's what I liked to do and basically nothing else and not that much has changed. And so yeah, it was always an ambition to be a biologist of some form and yeah, and I pursued that fairly relentlessly and as with, you know, pretty much all academics, a healthy dose of luck and I fell into paleontology, that was a complete chance, worked at biology and everything that I could and did for as long as I could and did a zoology degree. After that, it's just a question of what PhD turned up first. And having visited many a museum and zoo with you, I can vouch for your obsession with the infinite details of animals. How did you become a paleontologist? I did a degree, I did my zoology degree, a specialized animal behavior because that was really interested in, I thought I had a PhD lined up in biomechanics of fish, which is something I'd been working on from my undergraduate dissertation. It fell through that last minute, that left me a bit stumped. I went and did masters on taxonomy and diversity and phylogenetic methods and stuff like this. And at the end of that, by chance, I kind of ended up doing a project on dinosaurs, which ultimately got published and then I was hunting around for PhDs and one came up on terraces of flying reptiles and it's kind of like, oh, well, I'll do that, that's fine, you know, I like them as well. And then at the end of it kind of rose, oh, I'm kind of a paleontologist now because I've just been doing fossils for three to a bit years. Do you want to make a pitch for terrible lizards? I have a podcast called Terrible Lizards where we talk about dinosaurs and there's some bits we talk about evolution and general paleontology and how to write papers and scientific conferences and stuff. But there's 70, 80 hours of content there through six or seven series already. And I've got a blog on a monitor and other stuff. If you find DaveHone.co.uk, there's more of me than you could ever wish to see, read and listen to and find my books and other stuff as well. Thanks this week to Dr. Dave Hone for his discussion with me on Darwin's evidence for natural selection. This has been a presentation of the BioAudio podcast. I started BioAudio as a live Q&A session with a class when they had questions that were outside my area of expertise. Over the course of a few years, live sessions became some recorded sessions and then a hosted interview and then subaudio files for the class. At the request of some of my students I made them public as a podcast so they could more easily listen on their phones. I was not prepared for how enthusiastic the class was and a few episodes soon became a dozen and then enough to provide a free alternative to traditional textbook readings. The goal is to learn through interviewing experts and former students and to make an alternative free and more inclusive resource. We are not perfect, but we're learning as we go. If you have enjoyed this episode, particularly if you are a student, leave a review on Apple, Spotify or wherever you get your podcasts. Follow me on X at Dr_Bat_Girl on Macedon at [email protected] or Blue Sky at ProfBatGirl.BSKY.Social where I post new episodes and new news from my research lab. I hope you've enjoyed this presentation of the BioAudio podcast.