| Natural selection. Survival of the Fittest. Darwinian Evolution! Easily the most famous of the 4 evolutionary forces we’ve covered. So famous, I struggled with what to focus this newsletter on.I quickly nixed an email on how the beak sizes of Darwin’s finches are linked to food availability on the Galapagos Islands. It would be review for many of you. I considered guppy predator-prey dynamics. Covering how these tiny fish have different reproductive strategies, based on the presence of predators.But then it hit me… Lets cover my favorite group of organisms! The Bolitoglossa salamanders and their incredible “suction-cup” feet. Easily my favorite story of adaptive evolution. Or is it? Bolitoglossa salamandersThe some 140+ salamander species belonging to the genus Bolitoglossa are incredible. Like other salamanders in the family Plethodontidae, they lack lungs, have prehensile tails, and can shoot their long tongue to capture prey. I could fill an entire book on their amazing behaviors, life history traits, and morphology!The first Bolitoglossa I ever found was during my first field experience to Panama 10 years ago this Spring. What I didn’t know then, was how much I would end up obssessing over these slimy organisms all these years later.The hallmark trait of the Bolitoglossa are actually their webbed feet! Here’s a photo of a Mexican Climbing Salamander (B. mexicana) from my 2018 surveys in Belize that shows them well.  What’s up with their feet? In my undergraduate herpetology course, I learned these webbed feet were an adaptation for climbing. Nearly all Bolitoglossa are arboreal and readily climb smooth leaves during periods of high humidity. Essentially, these webbed feet would function like suction cups! The climbing capabilities gained by webbed feet were used as an early explanation for an adaptive radiation of Bolitoglossa into Mesoamerica. Adaptive radiations are where lineages rapidly diverge into new forms. Often this presents as an ancestral lineage rapidly dispersing across a landscape and then subsequently specializing into different niches. The classic example would be African cichlids, which rapidly evolved different forms, diets, and reproductive strategies.  From Brawand et al. 2014 In Bolitoglossa salamanders, it was believed the webbed feet signaled a transition to an arboreal life history. Arboreality enabled ancestral Bolitoglossa to exit their allegiance to an aquatic stage. Indeed, all Bolitoglossa salamanders are terrestrial and lack the aquatic larval stage of their sister groups in Plethodontidae. These salamanders lay eggs in bromeliads or under leaf litter which ultimately hatch into tiny terrestrial salamanders. Thus, the detethering of the aquatic constraint enabled a rapid spread through Mesoamerica and their subsequent diversification into the 140+ species we see today. In fact, we can see that diversification by the microhabitats these salamanders exist in! Many are bromeliad specialists living inside these plants. Some species exist high up in the trees and are rarely seen by ground bound biologists. Other species have readapted for a terrestrial life history. Based on the available evidence, it seemed that this was another classic example of adaptive radiation! But it wasn’t so simple.  My photo from 2019 surveys I’ve shared this story of Bolitoglossa salamanders dozens of times. Naturally so! This story was a pivotal one for my biologist career and centered on my favorite group of organisms. But in 2019, I read the paper “Macroevolution of arboreality in salamanders” that shook up my entire understanding of these amphibians.They coded the microhabitats for 450 species in the salamander family Plethodontidae (which includes Bolitoglossa). They then used a phylogeny to infer when in their evolutionary history transitions between microhabitats occurred. They found evidence for arboreality evolving independently 5 times in Plethodontidae. However, they found 60 instances of terrestriality evolving independently. In fact, they quantified the rate of transition from arboreal to terrestrial microhabitats as 24 times higher than the vice versa. In fact again, most of these transitions to terrestriality were found in Bolitoglossa! I know that entire paragraph is bolded, but there’s more. The authors included one additional analysis. They compared these results to body shape and foot morphology. Remember that foot? That webbed extremity which enabled the spread of Bolitoglossa through Mesoamerica? Turns out, its not as adaptive as we once thought. If webbed feet provided some selective advantage for arboreality, we would expect arboreal taxa to have a distinct foot shape compared to terrestrial taxa. Divergence. However, the authors found no meaningful difference in foot morphology between arboreal and terrestrial taxa. What does this mean for our Bolitoglossa salamanders? We believed selection favored arboreality in these salamanders, yet it seems selection actually favors a terrestrial form. We also believed the adaptive radiation was aided by these webbed, suction cup feet. However, the prevalence of webbed feet was not tied to arboreal or terrestrial conditions. So why do these salamanders have such a distinctive foot morphology?Unfortunately, I’ll need to save that information for the next newsletter. We’ll cover how this information affects our understanding of their evolutionary history and I’ll be incorporating a few other papers to add some clarity. It’ll be a great discussion on spandrels of evolution and the surprising effects of climate on morphology. If you want to know early, read the Macroevolution paper we discussed here But if not, ill be in your inbox next week with all that you need to know. Thank you! -D |