Dr Burbink began his lecture by noting that most of science involves contrast, for example control groups vs experimental groups. As a biologist, he is concerned with how organisms evolve, and how they speciate. He is also concerned with the topic of evolutionary hotspots. He described the species as the biologists' 'atom', a basic unit. There are believed to be about ten million non-microbial species on the planet, but it's estimated that only about 20% of species have been delineated. It is difficult to find new species, but it is crucial to understanding biodiversity. It used to be that species were described by how different they look, but this can be misleading- to illustrate this, he displayed photographs of three conspecific rat snakes with different color morphs, then three distantly related but similar looking racer species. He noted that these racer species had diverged around seven million years ago, approximately the same age when chimpanzees and humans diverged.
Speciation generally involves a separation in time and space. When populations become isolated, there is reduced gene exchange. Genetic drift and adaptation to different ecological niches then occur to further divide populations. Computer programs that summarize gene sequences are instrumental in determining the relationships between organisms. DNA can be traced back to a common ancestor, and gene flow (and when it stops) can be modeled. Divergence is a matter of reduced gene flow and adaptation to a new environment. Certain species are wide-ranging and exhibit diverse morphologies- Dr Burbrink gave the example of the milk snake (Lampropeltis triangulum). In areas in which milk snakes overlap with venomous coral snakes, they tend to be more colorful, a case of Batesian mimicry. Dr Burbrink joked that coral snake recognition poems can kill you, morphologies being variable.
Dr Burbrink displayed an image of a snake cladogram similar to this one:
The nodes are divergences. DNA sequencing can be likened to a 'time machine', the timing of divergence can be reconstructed. In the case of the milk snake, the species diverged from its nearest relatives in the Miocene.
In researching the process of speciation, certain topics are of particular interest. How does the rate of speciation change over time? What causes speciation? Factors such as climate change, the opening of ecological niches, and the effects of disease play a role. In the case of the ray-finned fishes, which comprise about thirty percent of vertebrate species, the K-T extinction event which killed off about seventy-six percent of the Earth's species, the effects were relatively insignificant. The morphological changes among these fishes occurred after the asteroid hit. One major controversy among biologists is the question over whether ecological niches can be filled to capacity, so speciation is limited. The current extinction event poses a problem- if species are lost, information useful to reconstructing the 'tree of life' is lost. Deforestation and new pathogens are particularly dangerous- recently observed fungal infections are ravaging amphibian, bat, and snake populations. In a poignant moment, Dr Burbrink displayed a photo contrasting a gorgeous healthy fox snake with one infected by a fungus, then noted that the first snake he caught as a boy in Illinois was a fox snake.
The spread of this fungal infection was modeled using an AI, in an effort to determine how many species could be infected. It is now believed that all snakes, or at least all Nearctic snakes, are at risk. Dr Burbrink noted that we stand to lose a lot if we lose snakes, which control rodent populations and help to suppress Lyme disease. The origin of the fungal infection is unknown, but it is currently limited to Europe and the Nearctic.
The subject of the talk then shifted to Madagascar, which he described as an 'upside down world' and a hotbed of discoveries of snake species. Dr Burbrink delivered an entertaining Madagascar travelogue. He ignores the lemurs, joking that all mammals bite, but only twenty percent of snakes do. He displayed pictures of the Avenue of Baobabs, of rival churches in towns. He displayed a picture of a fossa and a recounted a cautionary tale from a guide, "If it smells blood, it will destroy you!" He described a land with no bridges, river crossings being done with ferries. Camp is then set up, and a lab. The specimens are split between the institution and the authorities on the island. The beer is bad. While the suspension bridges are dodgy, the trails down the sharp karst formations are even more harrowing. He displayed beautiful photos of the plentiful gecko species, numerous chameleons (among them a tiny critter), snakes galore, and a rare Old World iguanid. One of the best sources of genetic material comes from recently road-killed snakes. Dr Burbrink joked about grad students eating jerky while collecting dead snakes. Outreach, while crucial, can be frought... many people believe in black magic, so an illness coinciding with a visit from the snake people can do a lot of harm. One particular arboreal snake, possessing a red tail, is believed to drop from trees, impaling hapless persons or cattle passing beneath. In one field season, the DNA of 730 individual snakes was sequenced and forty new species were found.
It is believed that the snakes of Madagascar arrived at the island from Mozambique twenty million years ago, and are descended from arboreal forebears. The speciation rate is fairly even, with 109 species expected to diverge over twenty million years. Dr Burbrink displayed a graph depicting the speciation rate of snakes on the island and noted that there was a 'dip' vis-a-vis expected numbers. The is speculation regarding the dip, does it represent species which remain to be found? Was there a 'saturation' of available niches? One of his colleagues, who discovered the 'ghost snake' has a knack for coining evocative species names. One particularly interesting snake species has unusual probosces and exhibits sexual dimorphism- males having spear-shaped noses and females having ones shaped like glaives but not glaive-glaive-glaive-guisarme-glaives. Despite the difficulties, a lot of headway is being made in the search for new species.
Dr Burbrink then offered up a quick autobiography, noting that as a youth he was inspired by two guys named Chuck D. He advised the audience to listen to a wide variety of music, to watch transgressive films, to be stable geniuses, and to have cool families-he showed a picture of his wife thrusting her hand into a rat snake hybernaculum in upstate New York (noting that torpid snakes can become active very quickly), and pictures of his older son catching a snake and his younger son holding a giant marine toad. Actually, his advice was pretty good for all endeavors.
The lecture was followed up by a Q&A session. The first questioner totally ninja'ed the Bastard by asking about the evolution of venom production in snakes. Venom pops up all over the snake family tree, with the viperids and elapids being the best-known groups. It is possible that all snakes are venomous to some extent, but that most snake species produce venom that is not harmful to humans (a controversial theory). Another question regarded the snake-infecting fungus- it is possible that it was historically prevalent, but has only recently become dangerous due to environmental factors. The bastard asked about snake cladistics, and snake evolution- all snakes are, biologically speaking, lizards. There are thirty extant lizard groups that have lost their limbs, notably the glass lizards and amphisbaenids. All snakes have a common ancestor and basal snakes had limbs. Among lizard groups, the iguanids, agamids, and chameleons are more closely related to snakes than to other lizard groups such as geckos. Some classification schemes propose lumping snakes and some lizards into one group. The explosion in snake diversity occurred about twenty million years ago, in the Miocene Epoch, and snakes became one of the most successful tetrapod (though they lack legs) groups. The fossil record shows numerous speciation events, but most species go extinct. The fossil record in North America is good, but Madagascar has a poor fossil record- the best known extinct species from Madagascar are the elephant birds, which survived into historic times, pygmy hippos, and an alarmingly sized frog dubbed Beelzebufo. Another question concerned the evolution of limblessness- getting rid of limbs is useful in three means of locomotion- swimming, burrowing, and 'grass swimming'. Another question involved the defining of species- as Dr Burbrink put it 'the million dollar question'. The biological concept of a species is a population that breeds true, with viable offspring that can successfully reproduce. Certain closely related species cannot reproduce because there are portions of the genome, known as genomic islands, which inhibit gene flow. Other closely related species may be prevented from interbreeding due to morphological or ecological differences.
Another fellow asked the derivation of the common name for the milk snake, and Dr Burbrink recounted a common folktale- ignorant lummoxes, seeing the snakes in barns, surmised that the snakes were sucking milk from the teats of cows, rather than hunting for the tasty rats that can infest barns. Dr Burbrink pronounced this folktale udderly ridiculous. He then joked about the color variations among milk snakes, with southern populations being brighter than northern ones... it would suck to mimic a species which isn't there.
A question about hibernation elicited a digression about snakes' ability to reduce their gut size between feedings. They can shrink their guts to a thin cell layer, then rebuild them after ingesting prey. The snakes come out of hibernation active, Dr Burbrink quipped that this would be a good model for astronauts' cryogenic sleep.
Another question concerned the origin of limblessness in snakes, the probable 'locomotive' reason for the group losing its legs. Certain features of snakes suggest an origin as burrowers- a lot of burrowing animals have degenerate eyes. Snake eyes are weird- they focus their eyes by moving the lens back and forth rather than by bending it, and their eyes contain oil drops. It is possible that they were burrowers who regained more functional eyes.
Being a lover of snakes, this lecture was right up my alley, hitting that 'Secret Science Sweet Spot'- that combination of hard science, beautiful imagery, adventure narrative, and personal journey. Dr Burbrink lectured with passion and humor, making a great case for the importance of snakes, the importance of studying snakes and working to protect them. Kudos to the good doctor, the staff of the beautiful Bell House, and Margaret and Dorian. High fives all around! Here's a video of Dr Burbrink giving a lecture on the snakes of Madagascar:
Pour yourself a nice beverage and soak in that science.
No comments:
Post a Comment