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Okay, but what's the deal with Darwin's finches? · Okay, But... Birds

• 0:00
  Coming up on this episode of Okay, But Birds.
• 0:03
  What birds actually did matter for Darwin?
• 0:05
  It wasn't the finches.
• 0:07
  It was... Oh, boy.
• 0:09
  You're putting me on the spot here, Scott.
• 0:11
  I'm thinking about chickens and things like that, that he wrote about domestic species.
• 0:15
  I'm still thinking about the Galapagos.
• 0:16
  But yeah, chickens did matter a lot.
• 0:18
  So did pigeons.
• 0:19
  I mean, we should call them Darwin's pigeons.
• 0:22
  We should call them Darwin's pigeons.
• 0:54
  You step off the plane on Baltra and feel the heat coming off the tarmac as you descend the stairs.
• 0:59
  A breeze brings the faint scent of Palo Santo trees.
• 1:02
  The island is flat and brown and dotted in cactus.
• 1:05
  As you walk towards the airport, you might spot a lava lizard, maybe a land iguana, but you'll definitely see some birds, really special ones.
• 1:13
  I've been lucky enough to make that landing several times.
• 1:15
  First as a postdoc at the Cornell Lab of Ornithology, then as a lecturer aboard a National Geographic ship.
• 1:20
  And since then, taking CU students down to Ecuador.
• 1:23
  The Galapagos has had my imagination since I was a kid.
• 1:26
  And every time I visit, I discover something new.
• 1:29
  So consider this episode a bit of a field trip and consider me your chaperone.
• 1:33
  And I want to be clear about what kind.
• 1:35
  I'm not a regular professor.
• 1:36
  I'm a cool professor, which is exactly the thing a regular professor would say, or so I'm told.
• 1:43
  Quick orientation.
• 1:44
  The Galapagos Islands sit about 600 miles or 1,000 kilometers off the coast of Ecuador, and they have never been attached to anything.
• 1:50
  They're volcanic, formed at different times over a hotspot in the seafloor.
• 1:54
  So every island started as bare lava.
• 1:57
  Every living thing out on the islands had to cross an ocean to get there, and the islands are wildly different from each other, each like a different level of Super Mario.
• 2:05
  An empty place with nothing but opportunity, if you can survive.
• 2:08
  which is the setup for one of the most famous stories in biology.
• 2:12
  A young Charles Darwin shows up in 1835, spends five weeks here, and leaves with something that will eventually change how we understand life on Earth.
• 2:20
  All right, so here's the roster.
• 2:22
  18 species, sorted by how they eat.
• 2:25
  Six ground finches going after food down low.
• 2:27
  All seed eaters except one specialist who behaves more like a horror movie villain, but you'll have to wait for the full wild scoop on that one.
• 2:35
  Three cactus finches specialize for prickly pear.
• 2:38
  Five tree finches up in the canopy, including the mangrove finch, very rare, and the woodpecker finch, a user of tools.
• 2:46
  One vegetarian finch, which eats leaves, obviously.
• 2:50
  Two warbler finches and the Cocos finch, off on its own island, a thousand kilometers away.
• 2:55
  Eighteenth species from one ancestor in about a million years, which is faster than it might sound.
• 3:01
  Now, they're called Darwin's finches, so you assume they were the big light bulb for Darwin.
• 3:05
  Spoiler, they were not.
• 3:07
  Perhaps not surprisingly, I'm a bit of a birder, and every time I'm out there, I still find myself squinting at a relatively drab bird on a branch going, all right, small ground finch or medium ground finch?
• 3:17
  Because that's the thing.
• 3:19
  The males are all black.
• 3:20
  The females and immature birds are all the same streaky brown and gray.
• 3:24
  The only real thing that differentiates what you're looking at is...
• 3:27
  The beak.
• 3:29
  A specific phenotype.
• 3:30
  You're going to hear that word phenotype a few more times this episode, which is just the scientific term for something you can actually see and measure.
• 3:37
  Not the genes themselves, but what those genes produce.
• 3:39
  Size, shape, color, behavior.
• 3:42
  Your height is a phenotype.
• 3:43
  A beak is a phenotype.
• 3:45
  And out here, the beak is the main character, because it's the tool each Darwin's finch uses to uniquely survive.
• 3:51
  Which brings us to the topic of today's episode.
• 3:53
  Okay, but what's the deal with Darwin's finches?
• 3:56
  To explore this topic, we'll talk to my friend and colleague, Dr. Eric Enbody, an assistant professor in the Department of Computational Biology at Cornell University.
• 4:04
  After the break, Eric walks us through how one bird became 18 so fast.
• 4:08
  We get into what a surprisingly small population of birds on a single volcanic rock have taught us about evolution in real time.
• 4:14
  And we get to that bird I wouldn't name.
• 4:16
  Stay tuned.
• 4:23
  Well, welcome back to the podcast, everyone.
• 4:25
  I'm really excited to have Eric with us today to talk all about Darwin's finches.
• 4:28
  Thanks for joining us, Eric.
• 4:29
  Yeah, thanks for inviting me.
• 4:30
  It's great to be here.
• 4:31
  Of course.
• 4:32
  Well, I thought we could start off with the fact that these little brown, gray, black birds are the poster child for evolution in many textbooks, but most people probably couldn't pick one out of a lineup.
• 4:43
  Yeah, this is a great place to start.
• 4:45
  So these finches, they live in the Galapagos Islands, and there's currently thought to be about 18 species of Galapagos finches.
• 4:53
  And, you know, as you pointed out, there really are a pretty drab and plain bird.
• 4:57
  You know, most species are either some variation of brown or all black.
• 5:00
  There's a couple of highland taxa that have, you know, at least a little bit of green and reddish in their color.
• 5:05
  So, you know, more traditional of some other mainland species.
• 5:09
  But overall, I mean, they really are not the most inspiring visual group.
• 5:13
  And so it is, you know, pretty amazing that they've become, you know, somehow one of the most recognizable species.
• 5:18
  groups of birds on the planet, at least in terms of if you ask someone if they know what they are, it's, you know, oftentimes they have at some point.
• 5:25
  And, you know, really, it's kind of an interesting history for how they got to this point.
• 5:29
  You know, the name, you know, of the group of Darwin's finches, you know, of course, comes from this idea that, you know, observations that Darwin had of these finches had some impact on how he thought about evolution and his theory of natural selection.
• 5:42
  You know, somewhat from a historical perspective, interesting to see that, you know, if you go and you read origin, the origin of species, you know, the finches aren't even brought up at that point.
• 5:51
  And so, you know, really it's one of these things where, you know, retrospectively, you know, we think that Darwin probably looked back at the finches and thought, you know, this actually really fits pretty well, this idea.
• 6:00
  But really when he was in the Galapagos and collecting finches, you know, he was somewhat puzzled by them.
• 6:05
  And that's really because, you know, different islands have different species.
• 6:10
  different assemblages of species of finches.
• 6:13
  If you were Darwin going around and collecting them and looking at them, it was probably a bit mysterious.
• 6:18
  The plumage was pretty much the same, but maybe bill size and shape looked a little bit different depending on where you were.
• 6:24
  And so it's sort of different than other mainland groups of birds that have generally different plumage overall.
• 6:30
  And so it's this variation in beak and body size that has really fascinated biologists for a long time.
• 6:36
  One ancestor becoming 18 in a million years is pretty fast.
• 6:41
  I mean, if you don't think about how fast evolution can happen, trust us that 18 species in a million years is really fast.
• 6:49
  So how does that happen?
• 6:50
  How does a single ancestor split that fast?
• 6:53
  And with these finches, we've been saying they're kind of drab looking, but their beak shape is quite variable, right?
• 7:01
  Yeah, that's exactly right.
• 7:02
  And if you were to sort of lay out these 18 species, that's the first thing that you would notice is that these beak and body size, or beak size and beak shape vary considerably across different species.
• 7:13
  And they also differ a little bit in body size.
• 7:15
  And so, you know, it's...
• 7:17
  Despite the fact that it wasn't written a lot by Darwin, there's been, you know...
• 7:20
  probably 150 years of research of different scientists going and studying these birds to try to understand that exact question you're asking.
• 7:27
  How can speciation proceed so rapidly and generate such diversification, such variation, particularly in these beak traits?
• 7:36
  And it's generally thought that really ecological opportunity is one of the main reasons that this group of birds diversified so rapidly.
• 7:44
  So if you sort of go back in time and imagine a desolate Galapagos landscape, it's eventually populated with plants.
• 7:50
  And if some birds get lost there and encounter these plants, then over time they may adapt to these food resources.
• 7:56
  And so over the long term, as different species look,
• 8:00
  encounter different islands and maybe there's subtle variations in the food availability, then they may adapt to these different environmental conditions that leads to these changes in beak morphologies.
• 8:09
  Yeah, for sure.
• 8:10
  So some of these islands are just volcanic rock with almost nothing on them, whereas other islands have elevation.
• 8:18
  And so they actually have wetter forests and moss and things, but none of it feels that tropical when you're there, right?
• 8:23
  Yeah.
• 8:24
  The islands are small enough you can go from those rock, barren rock parts of like Baltra up into the highlands of Santa Cruz in 20 minutes.
• 8:32
  And once you get up there, it can be foggy, it can be wet.
• 8:36
  And the trees, they've got moss on them.
• 8:38
  It really feels a little bit like a cloud forest.
• 8:41
  It does, yeah.
• 8:42
  And so that sort of microtransition in environments is just extreme in a way that I haven't seen other places.
• 8:48
  It's quite remarkable.
• 8:48
  It's incredible.
• 8:49
  And then to boot, those trees aren't even trees.
• 8:51
  Well, they're trees, but they're daisies, which is insane.
• 8:54
  So daisies that grew into trees.
• 8:58
  Quick recap on what birds actually did matter for Darwin.
• 9:02
  It wasn't the finches.
• 9:05
  It was... Oh, boy.
• 9:07
  You're putting me on the spot here, Scott.
• 9:10
  I'm thinking about chickens and things like that, that he wrote about domestic species.
• 9:13
  I'm still thinking about the Galapagos, but yeah, chickens did matter a lot.
• 9:16
  So did pigeons.
• 9:18
  We should call them Darwin's pigeons.
• 9:20
  We should call them Darwin's pigeons.
• 9:21
  The mockingbirds.
• 9:23
  So on the islands out there, there's just basically one species of mockingbird per island.
• 9:28
  There's one that's widespread.
• 9:30
  And then there's a few that are only on certain islands, like the Espanola Mockingbird, the Floriana Mockingbird, which is critically endangered, and San Cristobal.
• 9:42
  Sancristal.
• 9:43
  Maybe.
• 9:44
  That has an endemic species?
• 9:45
  Cool.
• 9:46
  Yeah.
• 9:46
  So I think that because there was only one mockingbird per island, it was easier for Darwin to contextualize that, okay, these are all the same kind of bird, but they look slightly different on different islands and they have different behaviors.
• 9:59
  The Espanola mockingbirds act completely nuts compared to some of the other ones.
• 10:04
  Well, and I think one thing that was confusing about the finches to Darwin is that, you know, unlike the mockingbirds that have this sort of geographic pattern, which, you know, I think that geographic variation was early on, you know, sort of an inspiration for how he thinks about, you know, species descending from another, right?
• 10:19
  Going from the mainland to the islands and between islands.
• 10:22
  But the finches, many of them actually do co-occur on islands where multiple species are present on one single island.
• 10:28
  Right.
• 10:29
  And that's perhaps puzzling from sort of the simplistic early thinking about divergence across islands.
• 10:35
  But then it also provides opportunity to see this really interesting thing about the finches, which is that if there are multiple species, you might have small, medium, and large ground finches.
• 10:43
  And if they're occurring on a single island, they tend to eat different foods on that island.
• 10:47
  So smaller species eating the smaller seeds, larger species eating larger speeds.
• 10:51
  And so they segregate with the ecology of the island that they're on.
• 10:55
  Daphne Major is this island that, again, if you've never been to the Galapagos, but you're a student of biology, you've probably heard of Daphne Major.
• 11:03
  We all have different visions of it before we see it, but it's just like kind of the top of an old volcano, right?
• 11:09
  It looks, this has this caldera, very circular, very cool.
• 11:12
  And that's where Peter and Rosemary Grant really documented how Darwin's finch beaks physically change after drought.
• 11:20
  And the cool thing about this work was that it,
• 11:22
  You know, they had banded the whole population.
• 11:24
  It was all in real time, which, you know, I don't think prior to their work and some other projects that we thought evolution could be witnessed to this fast.
• 11:32
  But just over a handful of years, they found that evolution was changing the shape of these Finch bills.
• 11:39
  Tell us about Daphne Major.
• 11:40
  I've actually never been there.
• 11:41
  I've only ever had the fortune of seeing it from the airport, but I never got to go over there because only people with permits get to do that.
• 11:49
  You're absolutely right that when you see it kind of coming out of the ocean, it's just this giant volcanic rock.
• 11:54
  And the cliffs on it as you approach it are incredibly steep.
• 11:58
  And so it's this kind of imposing island.
• 12:02
  The first thing you notice is that it's really kind of a seabird island.
• 12:04
  There's just boobies everywhere.
• 12:10
  Frigate birds everywhere.
• 12:11
  Tropic birds are screaming, flying around.
• 12:13
  And so it's this sort of beautiful marine island.
• 12:16
  And then once you start to look a little closer, you realize that there's this whole population of finches there.
• 12:21
  But really the dominant acoustic and visual and even scent of the island is very much the seabird colony.
• 12:27
  Seabirds easily dominate the scent profile of any island that they're on.
• 12:31
  That's for sure.
• 12:33
  Exactly, exactly.
• 12:35
  And the work that you're referring to, this early work showing the rapid pace at which evolution can proceed, I think can be contextualized when we think about this environment of the island.
• 12:47
  In the middle of the day, it's just brutally hot, right?
• 12:49
  In the dry season, there's hardly anything that's green.
• 12:53
  Maybe cactuses are flowering, but the environment itself is extreme.
• 12:56
  It's just really, you're out in the desert and there's not even water on the island.
• 13:00
  For humans, there's barely even shade, right?
• 13:03
  There's like one cave we can sit in in the middle of the day and get a little bit of shade.
• 13:07
  There really is only that one, right?
• 13:09
  There really is only that one.
• 13:11
  Crazy.
• 13:12
  And so, you know, the birds are living in this, right?
• 13:14
  They're constantly exposed to this extreme environment.
• 13:18
  And one of the earliest studies done on Daphne in terms of tracking individuals was a few years after the grants arrived on the island, and there was a major drought that completely knocked out the population.
• 13:33
  or there's an extreme drought that resulted in really low production of small seeds from plants on the island.
• 13:40
  And the consequence of this is that because so few small seeds were available, and we knew that the finch beak was related to what type of food they eat, right?
• 13:48
  So larger birds eating larger seeds, smaller birds eating smaller seeds.
• 13:52
  For the medium ground finches on the island, those individuals that were smaller experienced really high mortality.
• 13:58
  Something like one out of nearly 400 offspring.
• 14:00
  offspring the year before the drought survived until after the drought, right?
• 14:04
  This is an extreme mortality event.
• 14:06
  Extreme selection, man.
• 14:08
  It really is.
• 14:09
  And the outcome of this is that after this drought, where these smaller birds experience such high mortality rates, the population was actually larger only a couple of years on after this drought event.
• 14:19
  So that's really what's thought to be one of the strongest examples of natural selection being monitored and observed in really real human timescales.
• 14:27
  So we're talking about beaks a lot.
• 14:29
  These birds don't look that different.
• 14:31
  They do have quite different beaks.
• 14:34
  And we have seen, you've talked about how the grants documented rapid change in beak size over this drought period.
• 14:41
  So selection acts on beak shape.
• 14:42
  But there's other things influencing beak shape in this group, which is kind of...
• 14:47
  I don't know, a lot of what you've been focusing on.
• 14:48
  So for folks, these Darwin's finches hybridize.
• 14:52
  We've talked about hybridization before.
• 14:54
  It's when two different species interbreed to produce these admixed offspring.
• 14:58
  And when Darwin's finches hybridize, those hybrids tend to be fertile and can themselves reproduce.
• 15:04
  So these genes related to beak shape can jump around into other species.
• 15:09
  And so tell us about what you found there, the influence potentially of hybridization on how rapidly...
• 15:16
  beak shape can change in this group of birds?
• 15:19
  We can think about Daphne Major as another place to sort of think about this question of hybridization and its role.
• 15:25
  Because if you go early on to the long-term study on Daphne Major, you had these species that were generally not hybridizing with each other, right?
• 15:33
  You had medium ground finches and cactus finches, and they tended not to breed with each other.
• 15:37
  But over time, after a couple of decades, they were able to breed with each other.
• 15:42
  the researchers there started to observe the different species breeding with each other.
• 15:45
  And as you mentioned, remarkably, they do produce offspring that are, you know, successful and fertile and able to go on and reproduce on their own.
• 15:53
  And so there's not, you know, a sort of
• 15:56
  there's not any consequence that prevents these individuals from breeding with each other.
• 16:00
  And so when we think about the group as a whole and this broader question of how can you have such rapid divergence across the archipelago, it's sort of in contrast to this observation that these species can hybridize successfully then, right?
• 16:14
  It seems like if they can diverge rapidly and they can breed amongst themselves easily, then you might imagine that species barriers would break down really quickly.
• 16:22
  Yeah, you just end up with one thing that looks like a finch.
• 16:27
  One species of finch.
• 16:29
  And there are taxonomists who believe that maybe there is but one species of finch.
• 16:32
  Yeah, we can get into whether you think there are 18 or one in a minute.
• 16:40
  So you sequenced, was it like 4,000 individuals and found that a big piece of the variation related to beak size and shape lives in just like a handful of genes.
• 16:51
  So just a few genomic regions control all these really cool morphological differences in beak size and shape.
• 16:57
  Yeah.
• 16:59
  So a lot of the work that we do is really interested in, you know, what are the genetic variants that are responsible for the variation that we see in beak and body size across these different species?
• 17:08
  Yeah.
• 17:09
  And there's kind of two main outcomes of the research that we've done in terms of thinking about this genotype-phenotype relationship, which is the first is that once we really narrow it down and identify genetic variants that are associated with differences in the size of the beak or the shape of the beak, if you go and you look at how old are these genetic variants, what we find is that they're often really quite old.
• 17:29
  them almost as old as the radiation itself.
• 17:31
  So we're talking about genetic variants that are in the order of 600, 700,000 years old.
• 17:37
  The whole entire diversification of finches happened in about a million years.
• 17:41
  And so a lot of the genetic variation that we think is really important for beak and body size variation is actually quite old ancestral variation.
• 17:49
  And then when we go and we track that through different populations through time, it looks like hybridization has moved those genetic variants between different
• 17:56
  different species and between different populations.
• 17:58
  And that might be one of the ways that you can get such rapid changes in phenotype is through this process of hybridization and the movement of genes from one species into another, which is really pretty remarkable and certainly surprising.
• 18:12
  And it's this classical sense of thinking about species splitting and not coming back together and so forth.
• 18:17
  Yeah, totally.
• 18:17
  And I think in general, in like the popular press and literature, well, maybe I won't call it literature, but in pop science, you know, things like X-Men and Pokemon lead folks to believe that mutations lead to evolution.
• 18:33
  You know, that one hopeful monster, one positive beneficial mutation is the thing that leads to new variation.
• 18:41
  But it's important to remember that most mutations, so changes in
• 18:44
  the genetic code that happened randomly are not good.
• 18:48
  They're mostly deleterious and selection usually gets rid of them.
• 18:51
  So hybridization is then potentially this way to take little chunks of a genome that have already been tested by selection, put them in another genetic background, and then get this selection then acting on that new variation, which is a much more rapid way because mutations also don't happen that frequently.
• 19:07
  It's not like genomes are mutating at many mutations per generation.
• 19:13
  Yeah, so it's a neat way to think about how you could have rapid evolution.
• 19:18
  And the more places we look, it seems like the Finches is probably the best example of it.
• 19:22
  But there are other situations where you have these rapid diversification events that seem, you know, you seem to see signatures of hybridization as well, which is pretty.
• 19:31
  pretty cool.
• 19:32
  What you're saying also kind of brings me to the second observation that we were able to make when we look across trying to match the genotypes, the phenotypes of thousands of individuals, finches that are present on Daphne major, which is that many of the genetic variants that we find have really large effects on the overall size and shape of the beak.
• 19:52
  And to put this into some amount of context, if you look at humans and human height especially, it's one of the
• 19:58
  studied biological traits in any animal.
• 20:01
  If you look at the genotypes or the genetic variants that are involved in human height, any single gene variant in the genome only explains something like 1% of variation or typically much less than 1% of variation.
• 20:16
  Whereas we've got gene variants in finches that explain as much as 30% or nearly 40% of variation in beak size, which is just a hugely dramatic impact on the phenotype in the finches.
• 20:27
  Yeah, it's pretty amazing.
• 20:28
  I mean, again, you know, we tend to think like, oh, a gene makes the thing look a certain way and it's a very strong genotype-phenotype association.
• 20:37
  And what you're describing is the broader truth that that's almost never the case.
• 20:42
  That most, you know...
• 20:44
  Ways, you know, eye color is way more complicated in humans than we were led to believe.
• 20:48
  Hair color as well.
• 20:49
  And then, yeah, height.
• 20:51
  Hundreds and hundreds and hundreds of genes influence height variation.
• 20:54
  Plus, the environment influences the way those genes operate.
• 20:57
  So it's this very difficult to solve genetic issue, like if you wanted to understand the genetic basis of height variation in humans.
• 21:05
  But then in these finches, yeah, it's just a handful of genes explain all of this variation in a really satisfying way.
• 21:12
  I mean, and that's why it's this textbook example, because it's clear and teachable and there's still so much to learn.
• 21:19
  I mean, what's in the textbook isn't even caught up to what you've been finding.
• 21:22
  The Galapagos Islands sit in this really interesting position where they're partially influenced by the Humboldt Current.
• 21:27
  So there's cold water, but then they're at the equator.
• 21:30
  You know, they cross Isabella, crosses the equator, right?
• 21:33
  So the northern tip of Isabella Island is the only place in the world where you can find penguins north of the equator, which is pretty cool.
• 21:41
  So they're in this tropical region, but influenced by cold water.
• 21:44
  And that combination means they have pretty extreme climatic variability.
• 21:49
  You know, in El Nino years, it can be very rainy there and very warm ocean.
• 21:53
  And in La Nina years, cold ocean, very, very dry.
• 21:57
  But the climate is becoming more variable.
• 21:59
  So, you know, what do you think we can learn from the Finches about the influences of continued climate variation?
• 22:06
  Do you think that like these more frequent and more severe El Ninos are going to rapidly change the composition of populations out there?
• 22:15
  What can we learn from them in the context of climate change?
• 22:19
  Yeah, I think it's a fantastic question.
• 22:21
  It's a lot of what really interests me in terms of thinking about the future of doing research in the system.
• 22:25
  And we have this amazing example in the Galapagos where the Galapagos itself is relatively contained.
• 22:31
  This Daphne major island we've been talking about, there's only about a thousand birds on it.
• 22:35
  And so we know a lot already about the genes and how they match to the phenotype.
• 22:39
  And we know that the environment has a really large effect on the trajectory of these populations.
• 22:45
  And so it's really an amazing, in many ways, an amazing model for being able to study how are animals responding to and adapting to environmental conditions.
• 22:54
  Just really in the broader context, a handful of birds are teaching us all these amazing things about evolution and how species react to selection, right?
• 23:03
  Yeah, that's one thing that's remarkable about Daphne is that because the population is small enough that we can actually conceivably go out and band every bird and then track it through its lifetime.
• 23:13
  And that is probably about the maximum number of birds.
• 23:15
  That's about how many we think there are, say, in 2026.
• 23:19
  But if you go back in time, the population was actually much smaller.
• 23:22
  And it's interesting to think about how this population has expanded and if it has anything to do with these changes to hybridization that we talked about earlier.
• 23:29
  elsewhere in the episode, especially as their phenotypes have shifted, if that might have some impact on how much more, how the population size has changed over time.
• 23:37
  Do you have like specific plans in the context of like temperature change or precipitation variation and the finches?
• 23:46
  Yeah, it's a great question.
• 23:47
  And I think I'd bring us back to hybridization if I can, just a moment.
• 23:52
  I don't mind talking about hybridization.
• 23:53
  I've spent my entire career thinking about it, so go for it.
• 23:57
  I figure as much.
• 24:00
  I mentioned that if you go back in time on this experiment on Daphne Major, the Finnish populations weren't hybridizing at all.
• 24:07
  And now if you go in the modern day in 2026 and you go to Daphne,
• 24:12
  The finches are barely recognizable to what they looked like 30 and 40, 50 years ago because there's been such extensive hybridization between species, particularly between the cactus finches and the medium ground finches, that the phenotypes have shifted considerably, largely as a consequence of this hybridization.
• 24:31
  And one of the major changes is that the cactus finches on the island, their beaks have become much blunter and larger and deeper and also overall quite a bit more variable.
• 24:40
  Sometimes you catch cactus finch with this massive beak and other times with a much smaller beak.
• 24:44
  And they have this exceptional feeding habit where they feed on these apuncha cactus that are on the island.
• 24:50
  They actually rip the stamens off flowers and then eat them.
• 24:53
  They eat pollen.
• 24:54
  Their face is just covered in pollen all the time.
• 24:56
  They're kind of ugly, but in a beautiful, bizarre way.
• 25:01
  And so they're really this amazing species, but they've changed so dramatically in recent years that it raises this question of when we have things like the Super El Nino event coming this year, how is that going to affect this cactus finch population who's changed its phenotype or whose phenotype has changed so dramatically over the past decades?
• 25:21
  Are these extreme events that we're going to see and potentially see much more frequently, what are those effects going to have on these populations?
• 25:27
  Yeah.
• 25:27
  And at the moment, the two species phenotypically look quite similar as they've hybridized.
• 25:32
  But there's this open question then of, are other processes going to push them apart into the future?
• 25:37
  And so I think those aspects make it a really intriguing and interesting system to continue to study right now.
• 25:42
  Yeah, for sure.
• 25:43
  And you have such a history to build off of.
• 25:45
  You can sequence genomes back in time and going forward.
• 25:48
  And yeah, if the super El Niño is bad for the cactuses, you might see selection pushing the populations apart again, potentially.
• 25:56
  It will be very interesting to see, although it is sobering to think about the frequency with which these really severe climatic events are happening, especially as much of the world is currently on fire.
• 26:08
  Darwin's finches are not finches.
• 26:12
  Explain.
• 26:14
  That is true.
• 26:16
  Why are they called finches?
• 26:17
  Who named them Darwin's finches originally?
• 26:19
  Originally, at least Darwin was pretty uncertain on exactly what type of birds these finches were that he was encountering.
• 26:26
  Some were blackbirds even.
• 26:27
  Exactly, yeah.
• 26:29
  He had a number of sort of out there ideas about what type of bird they might be.
• 26:33
  When his specimens were formally looked at by a taxonomist, it was suggested they might be some type of finch based on the overall appearance and the sizes of the beaks.
• 26:45
  And it's really not until sometime probably about
• 26:48
  25 years ago now, when we actually had, with molecular sequencing technology, sequenced finch DNA and found that they're actually much more closely related to tanagers than they are to, say, other finches in the world or sparrows or other things they might look like.
• 27:02
  When you go to the finches, the plumage is relatively constant in closely related species, and it's the bill that's quite different.
• 27:07
  And so that provides, you know, an exceptional opportunity to really think about the factors that shape the evolution of a specific trait.
• 27:14
  And then also fundamentally, that's the thing that differs between species.
• 27:17
  And so you can study that really carefully.
• 27:19
  And so there's a lot of ways in which this limited, say, drabness of these birds has also provided, you know, awesome opportunities.
• 27:26
  And it's part of what has made them so famous going back to the early researchers puzzling over these minute variations.
• 27:33
  Yeah, for sure.
• 27:34
  For sure.
• 27:34
  No, they're incredible birds and it's okay that they're not colorful.
• 27:41
  I do wish if they really were a descendant of a paradise tanger or the finches that descended from a beautiful, colorful bird, then maybe when I go to the Galapagos and I go into the tourist shops, they would have finches for sale because none of the shops in the Galapagos have any finch material at all.
• 27:56
  They've got blue-footed boobies, they've got tortoises, but there's no finches on any of the items, which always makes me kind of sad because it's such a famous app.
• 28:05
  I know, it makes me sad too.
• 28:06
  One time I was in the airport and there was a woman who had just gotten off of a cruise ship and she was about to fly home.
• 28:11
  And she was clearly very annoyed by the birds trying to steal food off of her plate.
• 28:14
  And she was shooing them away and saying, get rid of these stupid birds.
• 28:17
  And there was this little flock of Darwin's finches.
• 28:20
  And I was like, girl, why don't, ugh.
• 28:22
  Like you're on the Galapagos.
• 28:23
  I wish you could recognize that these birds are these incredible things that we've learned so much about evolutionary biology from, but she just viewed them as a nuisance.
• 28:35
  The species question often comes up with Darwin's finches.
• 28:38
  Some people think there's just one because the speed with which this has all happened, the fact that they hybridize, if you stick to a strict biological species concept that requires reproductive isolation, there's just one.
• 28:52
  But there's something like 18.
• 28:55
  What are your thoughts on the utility of even defining how many species of Darwin's finch there are?
• 29:01
  Yeah, well, I think I'll take the classic evolutionary biologist out here and suggest that it's less about how many species are there and how can we learn from the uncertainty of what species are in this system.
• 29:14
  Excellent answer.
• 29:15
  Yes, I like it.
• 29:17
  But the reality is that, you know, if you look at sort of attempts to, say, lump everything into one, you know, say the one great finch is sort of maybe one of the names I've seen for it, is that, you know, the argument is essentially they can hybridize.
• 29:29
  And so they breed with each other freely, which sort of goes against this classical biological species concept.
• 29:35
  But at the same time, what long-term studies have shown us is that species tend to sing a song that they learned from their father.
• 29:42
  A male would sing the song that he learned from his father.
• 29:45
  And then females would learn the song also of their father and choose a mate based on that.
• 29:49
  And so what long-term studies teach us is that in general, species boundaries are maintained by the song that they sing.
• 29:56
  And these songs are culturally inherited, right?
• 29:58
  It's not genetic.
• 29:59
  It's kind of amazing and complete contrast to all these things we were talking about, geeks.
• 30:03
  And so there are, generally speaking, clusters of individuals that do differ in phenotype and they sing songs and they mate assortively amongst themselves.
• 30:11
  And the fact that there's some uncertainty in some gray areas is really what makes them so amazing for studying and thinking about the process itself.
• 30:17
  Totally.
• 30:17
  That's why we study them.
• 30:18
  That's why I study hybrid zones.
• 30:20
  It allows you to get at the mechanisms underlying either the generation or the maintenance of diversity.
• 30:25
  And certainly there's enough diversity out there that you could consider a lot of it to be species level diversity.
• 30:30
  Very cool.
• 30:31
  So in my introduction, I teased that there's a pretty weird finch out on the Galapagos that we haven't talked about yet.
• 30:37
  Strong selection can lead to pretty odd stuff.
• 30:39
  And could you tell us about maybe the weirdest finch out there and what it does?
• 30:44
  Yeah.
• 30:45
  You're probably referring to the vampire finches out on Darwin and Wolf Island.
• 30:48
  I am referring to the vampire finches, yes.
• 30:51
  Well, and these, you know, we talked early on about differences between islands.
• 30:54
  And Darwin and Wolf are these islands that are way, way out there.
• 30:57
  I mean, it takes way longer to get to these tiny satellite islands in the northern Galapagos.
• 31:02
  And there, there's a species of finch called the vampire finch.
• 31:05
  And they have a particularly unusual feeding behavior that has given them this name.
• 31:11
  And it's that they'll actually drink the blood of boobies and other seabirds on the island.
• 31:17
  You know, as seemingly in a way that's common enough that it's, you know, it's come to be their name.
• 31:23
  And it is, you know, generally thought that they feed on blood during really extreme environmental conditions where there's not other resources available.
• 31:31
  Okay.
• 31:31
  And it does seem to be something that's been seen in other finches on other islands.
• 31:34
  But these really isolated populations out on Darwin and Wolf seem to do it much more frequently.
• 31:39
  And so their bill is, you know, their bill is super pointed.
• 31:43
  And, you know, that facilitates this feeding on blood from the seabirds.
• 31:47
  It's really amazing.
• 31:49
  Amazing adaptation to these extreme conditions, you know, far beyond these minuscule changes in beak size that I talk about for changes in seed composition and so forth.
• 31:58
  Yeah, it is crazy.
• 31:59
  Blood is not a super nutritionally dense thing and it's hard to digest.
• 32:04
  I think because we see vampires and then they have all these superpowers, we think, ooh, blood must be a good resource, but it's not really.
• 32:11
  So it makes sense that they're only in very extreme conditions trying to eke out a living by biting open and drinking the blood of like a Nazca boobie, which is...
• 32:23
  It's a pretty crazy picture because these boobies are stark white.
• 32:26
  And then if they've been attacked by a vampire finch, they're also stained in their own blood, which is just pretty gnarly.
• 32:34
  But yeah.
• 32:35
  It's pretty gnarly.
• 32:37
  One thing you mentioned is that it's an extreme dietary adaptation to feed on blood.
• 32:41
  I mean, it's extremely unusual in general in vertebrates.
• 32:44
  And that's why, of course, there's these vampire bats that are famous.
• 32:47
  But the other aspect of the finches that's amazing is that not only do you have that blood feeding, you've also got these cactus finches that are feeding on pollen and vegetarian finches that actually eat leaves, which is also extremely unusual.
• 33:00
  And there's even one species of finch, the Woodpecker finch, that uses tools to exudate food from trees.
• 33:05
  And so overall, all these adaptations, these very...
• 33:11
  to varying different types of diet that are extreme and unusual globally in birds is remarkable within 18 species on this tiny little archipelago.
• 33:18
  Yeah, that's a really good point.
• 33:20
  Even across the other species, there's these incredible dietary specializations.
• 33:24
  And I got to see...
• 33:27
  one of the woodpecker finches feeding once up on the top of Santa Cruz.
• 33:32
  And it was pretty mind blowing to see tool use in a bird that just looks like a local finch.
• 33:39
  But yeah, super impressive and really interesting dietary specializations.
• 33:43
  We've reached the part of the show we call That's BS or That's Bird Stuff, where we give our guests an opportunity to debunk a myth that ruffles their feathers.
• 33:50
  So Eric, what do you want to call BS on?
• 33:52
  What I'd like to talk about is Darwin's finches and how they're not actually finches, of course.
• 33:58
  And the amazing thing is that it turns out they're related to tanagers.
• 34:04
  this much more colorful group of birds from the mainland.
• 34:08
  And the reason they were originally called finches is that the phenotype looks much more like we see in other finch species in the world, and yet they have this name.
• 34:16
  So this very famous group of birds is commonly surprising to people who actually see them and read about them.
• 34:22
  They're not, in fact, finches.
• 34:23
  They're actually part of this tanager group.
• 34:26
  And one of the things that I think is sort of a useful takeaway from that is that species or names of species, kind of like the species themselves, like we've been talking about this whole episode, these names are not immutable, right?
• 34:40
  They're not things that always carry meaning.
• 34:43
  You've got American robins in the US that are completely different than robins in the UK that are different completely from robins in Australasia.
• 34:50
  And so these names tend to be social constructs that often do represent something about how birds look, but they aren't necessarily something that's completely objective.
• 35:00
  Totally, totally.
• 35:01
  Awesome.
• 35:01
  Well, it's been really great to chat with you all about Darwin's finches.
• 35:04
  I'm excited to see where the research goes next as these amazing birds keep telling us cool things about evolution and climate change and all the rest.
• 35:12
  So thanks so much for taking the time to join us on the podcast.
• 35:15
  Well, thanks so much, Scott.
• 35:16
  This has been a lot of fun.
• 35:17
  And thanks again for inviting me to be a part of the show.
• 35:19
  Of course.
• 35:20
  Thank you.
• 35:24
  Birds are dinosaurs, and around here we like our snacks.
• 35:26
  So we end each episode with a dinosaur nugget.
• 35:29
  Today's nugget is, 18 species of little brown birds spread across volcanic islands have taught us more about how evolution actually works than almost anything else on Earth.
• 35:37
  Not because Darwin figured them out.
• 35:39
  He didn't.
• 35:40
  Because a handful of scientists went back, banded every bird on one island, and watched it happen.
• 35:44
  A drought hits, the small seeds vanish, and within a couple of years, the surviving beaks are bigger.
• 35:48
  So, let's get started.
• 35:49
  Species trade genes, phenotypes shift.
• 35:52
  That's evolution in real time on an island that's only 0.12 square miles.
• 35:57
  That's a wrap on this week's episode.
• 35:58
  Okay, but what's the deal with Darwin's finches?
• 36:00
  If you enjoyed this little field trip, leave us a rating or review.
• 36:03
  We're a small but mighty operation and those few seconds matter.
• 36:06
  All right, everybody back on the boat.
• 36:08
  Watch your step.
• 36:09
  There's a sea lion.
• 36:10
  Bye.
• 36:15
  Okay.
• 36:16
  Okay, But Birds is hosted by Scott Taylor with production and creative by Zach Carl.