Fall2025-week02-Natural Selection, Homology, and Reading the Tree of Life

Eric Marquette

Hey everyone, welcome back to the BIO 110 Recap podcast! I'm Eric Marquette, and as always, I’m here with Avery Lin. Avery, you got your coffee?

Avery Lin

Always, Eric. I’ve got a full cup and I’m ready to talk natural selection—a topic that always blows students' minds, honestly. So, shall we jump into the Florida mice story?

Eric Marquette

Absolutely. So this experiment is one of my favorites because it’s just so clever. They were looking at these two populations of mice down in Florida—one group lives on the sandy beach, and the other in the nearby forest. Same species. Only difference? Their coats. Beach mice are light, forest mice are brown, which right away kinda suggests this “matching the background” adaptation thing.

Avery Lin

And the real kicker here is how the scientists tested whether the coat color is actually helping them survive. But, instead of risking any real mice—you know, ethics and all that—they used clay models that looked just like real mice. Some were light, some were brown. They scattered them on the sand and in the forest, let the predators do their thing, and then checked for bite marks.

Eric Marquette

I just love how straightforward it is. If the color didn’t matter, you’d see attacks kinda evenly distributed. But nope—the brown models on the beach and the white models in the forest got picked off way more often. So, natural selection in action, right? Survival of the most camouflaged.

Avery Lin

Yeah, and it’s just such a neat workaround for the whole “don’t harm the animals” dilemma. And actually, this concept—how the environment shapes which traits succeed—comes back again and again. Next example: the soapberry bugs. Now, I’m warning you, I get a little obsessed with these guys. It’s the “ramen chopsticks of the insect world” analogy that cracks me up every time.

Eric Marquette

Go for it. The soapberry bug thing is wild. So, originally, these bugs had a long beak—think… 9mm or so—which let them get to the seeds of their native balloon vine fruit. But then, this new plant, the golden rain tree, invades Florida. It’s got fruit with seeds way closer to the surface, so suddenly having a long beak becomes—well, a bit of a disaster if you’re trying to eat.

Avery Lin

Exactly! It’s like, if you had two-foot-long chopsticks to eat a tiny bowl of ramen—you’d probably starve before you got a bite. But here’s the coolest part: when scientists went back to measure these bugs just 35 years after the golden rain tree showed up, the bugs had already evolved shorter beaks. I mean, you’re talking real evolutionary change in a handful of decades, not millions of years!

Eric Marquette

And they even checked old museum specimens to show this wasn’t just some fluke—those older bugs all had longer beaks, across the board. Plus, this shift has happened in other places and with other plants. That’s just, I dunno, textbook evolution in real time.

Avery Lin

It is! And there’s one more example—super relevant to everyone’s lives, honestly—evolution driving antibiotic resistance. I might be wrong, but I think a lot of people forget that bacteria evolve so fast, and it’s, uh, terrifying. Methicillin-resistant Staphylococcus aureus, or MRSA, started out as just a few random resistant bacteria. But the more we threw antibiotics at infections, the more we killed off all but the resistant ones. They reproduced, and now resistance is super common in hospitals.

Eric Marquette

Right, by the ’90s, cases of methicillin resistance in Staph jumped from, like, less than 5% to something like 40% in teaching hospitals. And that trend is not unique to one drug—that’s a whole healthcare challenge, right there. It’s a huge lesson in what happens when selection pressures are this strong and rapid.

Avery Lin

So, natural selection isn’t just an abstract principle; it’s happening all around us, whether it’s mice dodging birds, bugs retooling their eating strategies, or hospitals battling evolving bacteria. I love how these examples make evolution something you can actually observe. All right, Eric, now that everyone’s picturing mice and bugs and germs, wanna pivot us to the bigger evidence for how life’s all connected?

Eric Marquette

Let’s do it. So, let’s move from individual cases to those, uh, deeper connections—homology and the fossil record. I think we touched on this a little in past episodes, but it’s one of those things that’s key to understanding the whole “descent with modification” idea.

Avery Lin

Yeah, so, in class, Dr. Rosario went back to the Galápagos finches. Remember, homology is all about inherited similarity—structures, or even molecules, that are similar because they come from a common ancestor. Like how every finch beak is a beak, even though they’ve all been modified to fit different food sources.

Eric Marquette

Exactly, and it goes even deeper than bones or shapes. There’s this example of myoglobin—a protein that grabs oxygen in muscles. If you look at the myoglobin from a human, a horse, a whale, or even a tuna, there’s this little chunk, this heme group, that’s basically identical across all those species. It’s so critical, it just doesn’t change, and that sameness is literally genetic inheritance at work. The surrounding bits vary more, especially for, like, distantly related species.

Avery Lin

I always kind of nerd out over the ear bones too. So, in humans, we’ve got these three teeny-tiny bones in our middle ear that help amplify vibrations—we need them to hear, basically. But if you look in reptiles, those same bones are way bigger and are actually part of their jaw. Developmental biology literally tracks those bones moving up to your ear as the embryo develops—so, a jawbone in a lizard becomes an ear bone in us. How’s that for “descent with modification”?

Eric Marquette

It’s amazing! So those are anatomical homologies. And it’s not just bones and proteins—if you zoom out to the fossil record, things get super interesting. I always mix this up—was Pakicetus the “walking land whale”? I think that’s right…

Avery Lin

That’s totally it. Scientists found fossils of Pakicetus in Pakistan—hence the name—and it’s considered a super early ancestor of modern whales. What’s wild is, if you look at the ankle bones from Pakicetus, they have this “double bump” design you only find in even-toed ungulates—like deer, pigs, cows, hippos. Whales have it too, so that’s one thing linking whales and, weirdly, hippos together.

Eric Marquette

And then you track fossils up through Rhodocetus, Dorudon, all the way to modern whales. Each one’s a snapshot along the road from land mammal to the fully aquatic whales we know today. You don’t get every single step—all kinds of fossils are missing—but you can see enough of those changes in bone structure to make that evolutionary connection really strong.

Avery Lin

Yeah, and we should hedge a bit—science is always about hypothesis-building, not proof, but when you combine molecular, anatomical, and fossil evidence, you get this really compelling case for evolution. It’s not any one line; it’s the whole story they tell together. Honestly, it’s evidence that stays robust even as we find new fossils or sequence new DNA.

Eric Marquette

And it really ties back to previous stuff we’ve covered, like the Tree of Life and evolutionary relationships. Avery, want to walk us through how to read those evolutionary trees, and maybe some of the classic ways students get tripped up?

Avery Lin

Oh, for sure. So, we talked about the Tree of Life before, but let’s get into some of the nitty-gritty of reading these phylogenetic trees. First, a phylogeny is really just a hypothesis about how species are related through evolution—it’s about common ancestry, not just how things look or behave.

Eric Marquette

Right, and there’s a big difference between taxonomy and phylogeny, even though those words sound interchangeable. Taxonomy is basically about organization—grouping things by traits. But phylogeny adds that evolutionary hypothesis—it says, this group is together because they shared a more recent common ancestor.

Avery Lin

Totally. There’s this classic classroom story I love: the Eastern Ribbon Snake versus the Eastern Glass Lizard. They both look a lot alike—slithery, legless, you know—but once you dive in, you see that they’re not close relatives at all. The glass lizard is actually a lizard that’s lost its limbs, not a snake—so they ended up with that body shape separately. It’s this thing called convergent evolution, which is basically nature reinventing the same solutions in different branches of the Tree of Life.

Eric Marquette

And when you look at a phylogenetic tree, there’s a few terms that always show up. “Sister taxa” are like two branches that come from the same split point—they’re each other’s closest relatives. So, humans and chimps? Sister taxa. Their most recent common ancestor is unique to them.

Avery Lin

Basal taxon is another classic term. It’s basically the very first group that branches off from the common ancestor of the entire tree. Think of it as the “not-quite-root” but close. And just to trip everyone up, sometimes people call this the “outgroup.” Usually, if you’re looking at a tree with fishes, amphibians, reptiles, and mammals, “fishes” would be the basal taxon because they split off earliest from the main group.

Eric Marquette

And one of the super common questions you get in class or on exams is, “Which species are more closely related?” The trick is, it’s not about which ones are drawn next to each other on the tree. It’s about finding their most recent common ancestor on the internal nodes. Like, are lizards more closely related to frogs or to humans? If you trace the branches, lizards are actually closer to humans than to frogs, because that’s where their common ancestor is further up the tree.

Avery Lin

And you can rotate or “flip” bits of the tree around and it doesn’t change the relationships at all. So don’t get tripped up by the order of tips at the edge. Focus on the branching points, not the lineup.

Eric Marquette

Also, sometimes questions will ask about “sister groups” to a branch with multiple species. The way to handle it is to bundle all those species and their shared ancestor, treat it like a single grouping, and find what’s branching off next to that—don’t just look for single species!

Avery Lin

Definitely! And quick tip for tackling those exam questions: always figure out who you’re comparing, then trace their branches back until you find their most recent common ancestor. If you’re comparing lizards and frogs, it’s a deeper split than lizards and humans, so lizards and humans are more closely related on the tree. And, seriously, the visual part is important—use your finger or a pen to help track, even if it feels silly.

Eric Marquette

And, as a last practical piece—the order that species are listed in, top to bottom, doesn’t matter. Rotate, flip, doesn’t change the actual evolutionary relationships. So don’t let that trip you up.

Avery Lin

Exactly, and remember, phylogenetic trees are hypotheses. We’re learning as we go, sometimes switching things up as new info comes in. But the way you read them—tracing common ancestors, understanding these relationships—those are skills that stay useful as the story of life’s diversity gets clearer.

Eric Marquette

All right, I think we’re just about out of time, Avery. This was a packed week—clay mice, soapberry bugs, whales with double-bump ankles, and all those tangled branches on the Tree of Life!

Avery Lin

Yeah, that was a whirlwind tour before the first exam. So, everyone, keep reviewing those Mastering assignments, double-check your browser for those pop-ups, and don’t forget, the more phylogeny practice you get, the less intimidating those trees will be. Eric, thanks for another fun recap!

Eric Marquette

Thanks, Avery. Always a pleasure. We’ll be back next week to untangle even more BIO 110 mysteries.

Avery Lin

Bye, everyone! Good luck with your quizzes and see you next episode!

Eric Marquette

Take care, folks!