A false boa, Pseudoboa nigra, eating a lava lizard, Tropidurus hispidus.
Image credit: Ivan Prates, University of Michigan.
100 million years ago the ancestral snake was just another lizard but then suddenly (on the evolutionary timescale) something happened that enables snakes to diversify into thousands of different species while the other lizards plodded along slowly, diversifying at a much slower rate than the snakes.
But what was it that allowed this sudden radiation into so many species and why did it give the snakes the edge over their cousins the lizards? It was an event that an international team led by University of Michigan biologists have called an evolutionary singularity, in an analogy with whatever was at the start of the Big Bang.
The team has estimated that snakes have evolved up to three times faster than lizard, an ability that was facilitated by three things - an elongated body and loss of legs; enhanced sensory detection enabling them to find and track prey, and a flexible skull that enabled them to swallow large prey.
In an attempt to understand this, the team assembled a large database of lizard and snake diets from examining the stomach contents of tens of thousands of museum specimens. They also sequenced the partial genomes of almost 1,000 species from which they were able to construct the evolutionary trees of lizards and snakes.
Their findings are the subject of a paper in Science and of a news release from the University of Michigan:
Sadly, the team's paper in Science is copyright protected, but the abstract is available:Those crucial changes allowed snakes, as a group, to pursue a much broader array of prey types, while simultaneously enabling individual species to evolve extreme dietary specialization.More than 100 million years ago, the ancestors of the first snakes were small lizards that lived alongside other small, nondescript lizards in the shadow of the dinosaurs.
Then, in a burst of innovation in form and function, the ancestors of snakes evolved legless bodies that could slither across the ground, highly sophisticated chemical detection systems to find and track prey, and flexible skulls that enabled them to swallow large animals.
Those changes set the stage for the spectacular diversification of snakes over the past 66 million years, allowing them to quickly exploit new opportunities that emerged after an asteroid impact wiped out roughly three-quarters of the planet’s plant and animal species.
But what triggered the evolutionary explosion of snake diversity—a phenomenon known as adaptive radiation—that led to nearly 4,000 living species and made snakes one of evolution’s biggest success stories?
A large new genetic and dietary study of snakes, from an international team led by University of Michigan biologists, suggests that speed is the answer. Snakes evolved up to three times faster than lizards, with massive shifts in traits associated with feeding, locomotion and sensory processing, according to the study scheduled for online publication Feb. 22 in the journal Science.
For the study, researchers generated the largest, most comprehensive evolutionary tree of snakes and lizards by sequencing partial genomes for nearly 1,000 species. In addition, they compiled a huge dataset on lizard and snake diets, examining records of stomach contents from tens of thousands of preserved museum specimens.Fundamentally, this study is about what makes an evolutionary winner. We found that snakes have been evolving faster than lizards in some important ways, and this speed of evolution has let them take advantage of new opportunities that other lizards could not. Snakes evolved faster and—dare we say it—better than some other groups. They are versatile and flexible and able to specialize on prey that other groups cannot use.
Professor Daniel L. Rabosky, senior author
Museum of Zoology and Department of Ecology and Evolutionary Biology
University of Michigan, Ann Arbor, MI, USA.
They fed this mountain of data into sophisticated mathematical and statistical models, backed by massive amounts of computer power, to analyze the history of snake and lizard evolution through geological time and to study how various traits, such as limblessness, evolved.
This multipronged approach revealed that while other reptiles have evolved many snakelike traits—25 different groups of lizards also lost their limbs, for instance—only snakes experienced this level of explosive diversification.
Take Australia’s legless gecko, for example.
Like snakes, this lizard lost its legs and evolved a flexible skull. Yet the creature has barely diversified over millions of years. No evolutionary explosion—just a couple of species scraping out a living in the Australian outback.
So, it seems there is something special about snakes that enabled them to hit the evolutionary jackpot. Maybe something in their genes that allowed them to be evolutionarily flexible while other groups of organisms are much more constrained.
The ultimate causes, or triggers, of adaptive radiations is one of the big mysteries in biology. In the case of snakes, it’s likely there were multiple contributing factors, and it may never be possible to tease them apart.A standout aspect of snakes is how ecologically diverse they are: burrowing underground, living in freshwater, the ocean and almost every conceivable habitat on land. While some lizards do some of these things—and there are many more lizards than snakes—there are many more snakes in most of these habitats in most places.
Associate Professor R. Alexander Pyron, co-author
Department of Biological Sciences
The George Washington University, Washington, DC, USA.
The authors of the Science study refer to this once-in-evolutionary-history event as a macroevolutionary singularity with “unknown and perhaps unknowable” causes.
A macroevolutionary singularity can be viewed as a sudden shift into a higher evolutionary gear, and biologists suspect these outbursts have happened repeatedly throughout the history of life on Earth. The sudden emergence and subsequent dominance of flowering plants is another example.
In the case of snakes, the singularity started with the nearly simultaneous (from an evolutionary perspective) acquisition of elongated legless bodies, advanced chemical detection systems and flexible skulls.
Visualization of diets for 1,314 species of lizards and snakes. Each point is an individual species: lizards = blue, snakes = red. Points that are closer together indicate greater similarity in diets. The size of each point is proportional to the diversity of food types that each species will eat: Small circles indicate highly specialized diets, for example. Lizards and snakes show very little overlap: Lizards feed largely on insects, spiders and other arthropods. Snakes generally eat frogs, fishes, mammals, birds, and other vertebrates.From Title et al. in Science, February 2024.
A western blue-tongued skink (Tiliqua occipitalis), a large lizard from Australia. This species feeds heavily on plant matter, including fruits, stems, flowers and leaves. But it will also consume insects and other arthropods.Image credit: Dan Rabosky, University of Michigan.
A knob-tailed gecko (Nephrurus levis) from Shark Bay, Australia. This large gecko primarily feeds on insects but will occasionally consume other lizards.Image credit: Dan Rabosky, University of Michigan.
CT scan of a cat-eyed snake (Leptodeira septentrionalis) reveals a frog (blue skeleton) in its digestive tract. Snake specimen from U-M’s Museum of Zoology.Image credit: Ramon Nagesan,
University of Michigan Museum of Zoology.
A blunt-headed tree snake (Imantodes inornatus) eating its way through a batch of tree frog eggs.Image credit: John David Curlis, University of Michigan.
A copperhead (Agkistrodon contortrix). This venomous species occurs in the southern United States and feeds on a variety of vertebrate prey.Image credit: Alison Davis Rabosky, University of Michigan.
A false boa (Pseudoboa nigra) eating a lava lizard (Tropidurus hispidus).Image credit: Ivan Prates, University of Michigan.
Today, there are cobras that strike with lethal venom, giant pythons that constrict their prey, shovel-snouted burrowers that hunt desert scorpions, slender tree snakes called “goo-eaters” that prey on snails and frog eggs high above the ground, paddle-tailed sea snakes that probe reef crevices for fish eggs and eels, and many more.
For the study, the researchers got an inside look at snake dietary preferences by reviewing field observations and stomach-content records for more than 60,000 snake and lizard specimens, mostly from natural history museums. The contributing museums included the University of Michigan Museum of Zoology, home to the world’s largest research collection of snake specimens.One of our key results is that snakes underwent a profound shift in feeding ecology that completely separates them from other reptiles. If there is an animal that can be eaten, it’s likely that some snake, somewhere, has evolved the ability to eat it.
Professor Daniel L. Rabosky.
The study’s 20 authors are from universities and museums in the United States, the United Kingdom, Australia, Brazil and Finland.Museum specimens give us this incredible window into how organisms make a living in nature. For secretive animals like snakes, it’s almost impossible to get this kind of data any other way because it’s hard to observe a lot of their behavior directly.
Dr. Pascal O. Title, co-lead author
Department of Ecology and Evolution
Stony Brook University, Stony Brook, NY, USA.
What I love about this study is how it integrates hard-earned field and museum data with new genomic and analytical methods to show a basic biological truth: Snakes are exceptional and frankly quite cool.
Dr. Sonal Singhal, co-lead author.
Museum of Zoology and Department of Ecology and Evolutionary Biology
University of Michigan, Ann Arbor, MI, USA.
AbstractOf particular note for creationists to ignore, is the complete dependence of the team on the Theory of Evolution to explain these findings, with never so much as a hit that they regard it as inadequate or lacking what creationist superstitions have to offer in terms of fitness to explain the facts without resorting to unprovable childish assumptions such as the existence of evidence-free invisible entities making chemistry and physics do things they couldn't do on their own.
Snakes and lizards (Squamata) represent a third of terrestrial vertebrates and exhibit spectacular innovations in locomotion, feeding, and sensory processing. However, the evolutionary drivers of this radiation remain poorly known. We infer potential causes and ultimate consequences of squamate macroevolution by combining individual-based natural history observations (>60,000 animals) with a comprehensive time-calibrated phylogeny that we anchored with genomic data (5400 loci) from 1018 species. Due to shifts in the dynamics of speciation and phenotypic evolution, snakes have transformed the trophic structure of animal communities through the recurrent origin and diversification of specialized predatory strategies. Squamate biodiversity reflects a legacy of singular events that occurred during the early history of snakes and reveals the impact of historical contingency on vertebrate biodiversity.
From a phylogenetic perspective, snakes are indisputably nested within lizards (1, 2) and are simply one example of a particularly species-rich and cosmopolitan group of “scaled reptiles” (Squamata). Yet, unlike lizards, snakes engage with human emotions in a visceral manner unmatched by almost any other group of organisms and for this reason have played important cultural roles in human societies (3). Most readily known for their lack of limbs and unique prey-capture strategies, snakes exhibit an incredible degree of ecomorphological diversity and specialization. The ~4000 extant snake species include shovel-snouted burrowers that hunt desert scorpions, slender arboreal predators that prey on tree snails, and paddle-tailed marine forms that probe reef crevices for fish eggs and eels. However, more than 25 clades of lizards have independently evolved limblessness (4), and other lineages also evolved dietary specialization, venom, highly mobile skulls, and/or advanced chemoreception (5–7)—all attributes typically associated with snakes. This convergence raises fundamental questions about how and why particular traits have influenced squamate diversification more generally.
Viewed across multiple traits, snakes nonetheless appear distinct from lizards with respect to ecology, morphology, and biogeography (8–10). These observations suggest that evolutionary dynamics in snakes are qualitatively different from those in lizards. If so, the origin of snakes is potentially consistent with a Simpsonian view of macroevolution (11, 12), whereby major biodiversity expansions occur through qualitative phase shifts into new adaptive zones. These phase shifts can be conceptualized as macroevolutionary “singularities”: patterns of rapid change across multiple organismic and ecological axes that, when viewed retrospectively through the prism of geological time, are sufficiently clustered together so as to seem virtually instantaneous. The term “singular” also refers to the fact that these transformations typically appear unpredictable from prior character states and phylogenetic position alone; such transitions have been documented within birds (13, 14), mammals (15), and other taxa (16). Here, we characterize the tempo and mode of ecological and morphological innovation across squamate reptiles to address the role of macroevolutionary singularities in generating large-scale patterns of lizard and snake biodiversity. In particular, we test the extent to which phenotypic shifts have predictable consequences for evolutionary diversification across squamates.
Title, Pascal O.; Singhal, Sonal; Grundler, Michael C.; Costa, Gabriel C. et al.
The macroevolutionary singularity of snakes
Science 383 6685, 918-923 (2024). DOI:10.1126/science.adh2449
© 2024 The Authors, published by American Association for the Advancement of Science.
Reprinted with kind permission under license: #5735571435014
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