A Day Late and a Dollar Short

Please forgive the absence. June was spent running analyses and putting together my talk for the Joint Meeting of Ichthyologists and Herpetologists (nerds working on fishes and reptiles/amphibians, respectively) in Providence. Two glad tidings: I have returned to wax scientific on subjects, and I was honored to receive the ASIH Stoye award in General Ichthyology at the conference. Before the year is out, I will write an article here on the same: what the interrelationships of batoid fishes, my study group (skates, rays and allies), can tell us about widespread convergent evolution and the effects of the end-Cretaceous extinction event on their current patterns of diversification.

Speaking of mass extinction...

This one has been a while coming. Toward the end of last year, several science news outlets picked up a striking article in the journal Biological Conservation. In short, it suggested that current guidelines for setting minimum population sizes for protected species, like the black rhinoceros, are at least an order of magnitude (10x) too low to adequately protect them from extinction in this century. The current guidelines adhere to a "50/500" rule, in which a minimum of 50 adults are required to avoid the negative effects of inbreeding, and a minimum of 500 to be able to adapt to long-term environmental changes or rebound from a catastrophic event.

Ten times too low. Put a big, flashing exclamation point at the end of that one. Horrifying corollaries are immediately evident: many species are already too far gone to be preserved in the long term even if we dropped everything to triage them now, and species we think we've done a bang-up job of protecting to date may just need one calamity to drop them below that point of no return. In light of the political difficulties - an understatement by any measure - of maintaining even today's meager/modest conservation measures, a tenfold increase will never happen. These species, humble to majestic, mountain gorilla (~400) to desert pupfish (42!), are going away sooner than we'd like.

It is not an encouraging picture.


Black rhinoceros. Photo credit John and Karen Hollingsworth, USFWS, from Wikimedia Commons.

This is not a political blog, so we'll leave the issue behind and look at what these numbers mean. Why do we need X number of animals to avoid catastrophe? The first barrier, inbreeding, is intuitively obvious: offspring are produced by close genetic relatives. Put another way, the two copies that an individual carries of each her genes have a high probability of being identical by descent, or having come from the same ancestor (grandma) independently through each parent. Gross, yeah. But why is this necessarily a bad thing? Every human has two identical copies - alleles - of at least some genes, and we're not all running around with horrible genetic abnormalities. Things go south (no pun intended; I am from Texas after all) when you deal with genes that have a healthy version and a defective one.

At many gene locations in your DNA, you have one healthy allele and one defective one. In most cases, the normal copy steps up and does a stalwart job compensating for the freeloader. You'll never notice that one copy is bad, and so we call that copy a recessive, or hidden allele. That's the good news. The bad news is that they're ticking time bombs on a generational timescale. When you reproduce, one of those alleles is "drawn" at random to end up in your sperm or eggs. If you pass on the healthy version, great, your child will be A-OK for that gene. If not, you'd better hope that your mate contributes a healthy copy to pull the weight for your freeloader. If two recessive alleles end up in the offspring, she won't be able to make that gene's normal protein product. The effects may be mild, and even desirable to some: blond hair or blue eyes. Or instead, they may cause a serious genetic malady like cystic fibrosis or sickle cell anemia. If you have one healthy and one hidden, defective allele for a disease-associated gene, you are a carrier. Everyone on the planet is a carrier for an unknown, but probably not inconsiderable, number of genetic disorders. This is the basis behind genetic testing, which is becoming cheaper and more widely available every year.

The rest is easy to follow. If you're a cheetah and there are only a handful of your species left (the cheetah-reality is not that dire), the odds that you are closely related to any other cheetah you see are much higher than they are for a species with larger population sizes, like industrialized humans. If you mate with that cheetah, your odds of having the same set of alleles from a common ancestor are high, and an increasing proportion of offspring will either have two healthy or two defective copies of a gene - allele fixation. You can see how the situation degrades quickly once disease traits become universal in a population. Lost genetic diversity takes a long, long time to be recovered.

The second, higher requirement for a minimum viable population size (the 500 of the 50/500 rule) is trickier to understand. It builds on the concept of inbreeding, but the higher number of organisms keeps a larger gene pool - the total genetic variation in a population - available into the future. High genetic variation gives a population options, so to speak, in the face of environmental change. There may be warmer-adapted cheetah alleles that would do better under increased temperatures, or slightly faster cheetahs that can better keep up with Thomson's gazelles, which are under similar pressures. Perhaps most importantly, it provides options for [pathogenic] disease resistance.

Viruses, bacteria, and other parasites are engaged in a constant arms race with their hosts at a molecular level. Genetic variability goes a long way to ensure that at least some cheetahs will survive a potent disease, while if they are genetically homogeneous, a single bacterial strain that has "figured them out" can wipe out the whole population in one fell swoop. As a great recent example, humans of European descent have a much higher incidence of a genetic mutation called CCR5-delta32 that provides resistance to HIV. This mutation also appears to confer resistance to the great historical European plagues. That is, some Europeans had this mutant allele and became resistant to plague, while other didn't and were selected against. If Europe's population had been much smaller, this mutation may have never arisen and the entire continent could have been taken out in one of those dread epidemics.

A grim picture, to be sure. The next steps are political, but having been introduced to the science, hopefully you are in a better position to weigh the costs and benefits of conservation measures for yourself.

Here's to an educated democracy.

This reView of Life(?): Viruses, Part 2

The first decade of this brave new century was characterized by dramatic upheavals, often violent, in politics, business... and film. Perhaps through widespread disillusionment in the wake of crises in security and finance, perhaps merely the pendulum swinging back from the fantastic (read: "divorced from reality") flavors of storytelling in the 1980s - early 90s, the public has rushed to embrace the gritty and believable. Joel Schumacher's candy-colored Batman films were usurped by Christopher Nolan's dark masterpieces, achieving almost ridiculous commercial success. Peter Jackson breathed life into a vision of The Lord of the Rings that eschewed most of the magic and occasional levity of the book, and also featured a rather mundane, if attractive, aesthetic. Additional existing franchises and concepts from other media leapt to the big screen or were rebooted, and surprisingly often attempt to feature some "scientific" hook to enhance the believability of the tale. One of the rising stars of this movement is our enigmatic acquaintance, the virus.

Viruses are now almost universally implicated as the nefarious causal agent in modern zombie tales or close cousins, such as the animalistic, deranged "infected" in 28 Days Later. Alas, the point of this article is not to tackle the questionable physiology of a zombie (actually, that sounds like a good one for another day), but rather to touch on Hollywood's mixed success in portraying the pathology of viruses.


The virus in 28 Days Later is particularly terrifying in that, 30 seconds after exposure, a victim is reduced to a hemorrhaging, raging volcano of virus-laden fluids. One infected person in a crowd can effect some sick parody of The End Times in mere minutes. Viruses in many other films spread with similar urgency, in some cases reanimating a corpse within two minutes of exposure (Dawn of the Dead reboot). What does the real world have to say about this?
©2002, 20th Century Fox.

Viruses are constrained by the same physical laws that the rest of us are, regardless of your definition of "life." A generic viral life cycle, and there are all sorts of bizarre variants, goes something like this:
 1) find a host cell;
 2) enter the host cell, or inject genetic material;
 3) use own or hijacked machinery to replicate genes and coat proteins;
 4) stay forever, or have all your copies erupt out of the host cell;
 5) repeat.

Each of these steps requires time, particularly step (3). This is even more of a problem for related scifi concepts involving rapid gains of animal/plant/fungal tissue, or what I call "The Werewolf Problem." I'll leave that for another day.

The window of time between initial exposure and the point at which symptoms first emerge is called the incubation period. During incubation, viruses are reproducing and spreading through host cells but are at insufficient numbers to make much of an impact. Yet. So how long is this timetable? Clearly 30 seconds for complete system takeover is unrealistic.

The fastest acting human virus, as far as I have been able to discover, is the stringy little fiend below, ebola. The incubation period of ebola is 2 - 21 days, but usually 5 - 18 [source: Emerging infectious diseases 9(11):1430-7; via Wikipedia]. The fastest.


Ebola virus. Photo Credit Centers for Disease Control, from Wikimedia Commons.

A common pattern of viral strategies is a correlation between their agressiveness and their ability to be transmitted. Ebola and similar viruses have evolved a strategy that is very much living (yes, I said it) on the edge. They emerge, spread and kill so quickly that they run the risk of not being able to find a new host and suffering local extinction. Compare this to the less common strain of the virus that causes AIDS, HIV-2. HIV-2 often degrades the immune system more slowly than does HIV-1, leading to higher long-term survival rates. It also is less successful at being transmitted between people. For an analogy in honor of the ongoing NBA finals, it may only shoot 60% from the field compared to HIV-1's 80%, but gets an extra five minutes of playing time. The natural world is full of these trade-offs, in which organisms fine-tune their life strategies over time.

The ones that fail? Well. We don't see those guys anymore.

On that note, next time I will likely bring up some recent, distressing speculations about the fates of endangered species.

This reView of Life(?): Viruses, Part 1

There are no absolutes in science. The closest you'll get is in physics, but most of the so-called Laws remain either fundamentally unexplained and/or not quite what we thought they were. If asked to name one "obvious truth" about the natural world, I suspect most folks would immediately come up with something about gravity. "If I let go of this cute kitten, it will fall." But would you believe that we have almost no idea why this is so? We have nothing but competing theoretical frameworks for why things fall, and to get more technical, why gravity should exactly equal an object's inertia. No [prevailing] idea whatsoever. To make matters worse, gravity appears to "misbehave" at astronomical distances. The concepts of dark matter and dark energy are stabs at explaining why our observations of big objects' movement through the universe do not at all conform to expectations under gravitational theory, and our most distant manmade satellites are typically not where we expected them to be. The explanations could be mundane... or not.

Biology is far messier, if much better understood. In fact, as a biologist it is difficult to even define our field! Think on this for a moment: how would you define life? Which set of characteristics makes something living, from people to peas to protists? Let us consider two different candidates.

The first is from NASA, whose stalwart crew of scientists are engaged in the very business of finding weird, unexpected kinds of life out there in the big empty.

"Life is a self-sustained chemical system capable of undergoing Darwinian evolution."


For a different perspective, let's go to the textbook I'm using to teach non-majors intro bio this summer, Essential Biology by eds. Campbell, Reece & Simon.
"The set of common characteristics that distinguish living organisms, including such properties and processes as order, regulation, growth and development, metabolism, response to the environment, reproduction, and the capacity to evolve over time."

Clearly there's something going on behind the scenes to have two such dramatically different definitions. NASA's definition is more inclusive, which is to be expected when anticipating exotic forms of "life," but what exactly are the textbook makers ruling out?

Why, it is your friend and mine, the pesky little bugger that's had me coughing and cursing for the last week: the virus.

Viruses and bacteria are together responsible for most human infectious diseases, but are fundamentally very different things. A bacterium is an organism by any definition, composed of a single discrete unit called a cell. Its arrangement into wholly or semi-autonomous cells puts it in the same league as all other life (by the textbook definition) on Earth: it grows, it divides, it responds to stimuli, and as an imperfect reproducer its populations are capable of change over time.

The virus is not a cell. Writers for pop media often get this dead wrong. For example, consider an episode of the current Fox television series Fringe. The show features both surprisingly excellent characters and some of the mostly howlingly terrible "science" I've ever seen. A sluglike parasite used in assassinations was revealed to be a genetically engineered supersized cold virus "cell." Sigh.

Viruses are actually a diverse grab-bag of maybe-organisms consisting, at a minimum, of genetic information and a protective coat. The info can be stored either as DNA, as in all cellular life on Earth, or as its chemical cousin RNA. The coat is made of protein and sometimes fats. A virus is basically the nasty hacker of the biosphere: it comes into contact with some cellular organism, injects its genetic material, and hijacks that cell's machinery to make more copies of the virus. In some cases, the cell basically explodes in order to release a swarm of new viruses. Other, more sinister viruses actually incorporate themselves into the host's DNA and "hitch a ride." Potentially forever. A good deal of the so-called "junk DNA" in your genome is actually viral information: some dormant, some not so much!

How do you fight something like that? We have antibiotics against cellular organisms, like bacteria: chemical compounds that selectively interfere with the pest's way of life. Not so with viruses. Antibiotics have zero effect on them, and are actually counterproductive in that they give rise to resistant strains of bacteria that can then feel free to rise up and take on humanity. Vaccination is currently the best defense against viruses, since the limited supply of drugs that can actually hamper the viral life(?) cycle often do a good deal of damage to the patient's cells too.

So. What do viruses lack that makes some consider them non-living? Do they possess order/structure? Of course; take a look at this micrograph of swine flu. So does ice, great. Reproduction? Check, but so does fire. Evolution? Yes oh yes. Metabolism? That one's a maybe, since it hijacks a host cell's metabolic machinery. The "problem" lies in virus' inability to directly grow (although they self-assemble) and, more importantly, maintain an internal environment different from what's outside their coat.
Swine flu (H1N1). Photo Credit C. S. Goldsmith and A. Balish, CDC, from Wikimedia Commons.

Okay, I could buy that, except some bacteria run into the same problem when they face certain stresses. In these circumstances they form spores, which are dormant, hardcore, last-chance structures used to ride out what would otherwise be certain death. Some important causes of human disease form spores, like the agents behind anthrax and botulism. Spores do not have metabolism, do not reproduce, do not grow, do not maintain that internal environment... they are basically virus-like, waiting for favorable circumstances before developing back into bacteria. While a bacterium may form a spore while it waits for nutrients to come along, viruses wait for new host cells.

Personally, I side with NASA and find it hard to call spores and viruses nonliving. There is a tremendous gulf between viruses and uncontroversially nonliving particles, far greater than that between viruses and some bacterial stages. At worst, viruses are renegade bits of life. Hypotheses of the origins of viruses, events which have probably occurred innumerable times through the history of life on Earth, usually stipulate these bits of genetic information "going rogue" and escaping the cell with the bare minimum of machinery for propagating themselves. Once they're out in the biosphere, evolution takes over and all hell breaks loose.

And that's what life is all about.

Next time: touching again on the misrepresentation of viruses in pop culture, particularly movies.

Brief Hiatus for A Related Project

Sorry for the extended absence. I'm currently writing my unit of this summer's Intro Bio for Non-Majors, which poses many interesting problems. Moreso given that I drew the "Cell and Genetics" unit, which is the driest by far of the lot. My task is thus: condense and sugar-coat a colossal and diverse set of fields into eight lectures for people who really, really don't want to be there.

So far, my syllabus includes includes beer, Ridley Scott's Alien, Maury, Jurassic Park, and mass extinction among others. I think I'm going to focus on three things: the molecular nature of life (and the nebulous definition of the latter), diseases/disorders, and how to recognize bad biology in movies, tv and other pop culture.

I suspect that I may adapt one or two of those topics for this blog.

Back soon.
-NA

Sharks, Stasis and Fake ID

You've heard it before: "Sharks are among Earth’s oldest life-forms," [Discovery Channel] having patrolled the seas, "essentially unchanged, for 400 million years" [National Geographic]. Among the most prevalent of sharky soundbites, gems like these are sensationalistic, misleading, and born of a strange sort of biased observation. Let's see if we can't challenge their grim persistence.

What of the first claim, that sharks are Very, Very Old? This one is repeated so often as to be clichéd and conceals an interesting and quite different reality. All living sharks (and rays, to which we'll return in a moment) are descended from a common ancestor dating to the Triassic Period, probably somewhere between 240 - 220 million years ago. This makes the set of all living sharks and rays about the same age as dinosaurs and, surprisingly, just a hair older than mammals! So how do they get away with the perception that they're almost twice as old as that: the paleontological equivalent of an eleven-year old bellying up to the bar and scoring a beer?

It turns out that living sharks and rays are the only survivors of a much older and previously much larger set of fishes. Collectively known as chondrichthyans, or fishes-with-cartilage-skeletons, this crew is at least 400 million years old and was both amazingly diverse and often bizarre in appearance. Calling a more recently derived subset (living sharks and rays) of this ancient group 400 million years old is like calling me 200 years old because that's how far back I can trace my lineage. It doesn't work.


Cladoselache, an early chondrichthyan. From Wikimedia Commons.

So when people speak of sharks as being twice as old as dinosaurs, they're hopefully not referring to the age of the living group (although I fear this is often the case), but rather to the broader idea of "the shark." This is a different concept, that of the body plan or general architecture of the beast. Torpedo-shaped, toothy, with the typically sharky complement of fins slicing through the water. In doing so, they are placing "the shark" in the same bin as "the shrimp": an artificial group of organisms that pretty much look the same. Critically, this ignores 400 million years of weird experiments in the shark body plan. Just a few examples of extinct chondrichthyans include forms that resemble lumpy rocks, or eels with a long spine behind their head, or strange little undersea birds, or one with a spiral of teeth that no one quite knows where to place on the fish. For some beautiful reconstructions of these oddities, check out this article in Dive Magazine.

And let's not forget about those living exceptions that happen to comprise the group I study: the rays. Rays, skates and their relatives (collectively called batoids) are the majority of living shark and ray species, and precious few of them look anything like sharks. One colleague has gone so far as to call her skates "charismatic slimy pancakes of wonder." The group also includes what is considered to be the most derived, non-sharklike chondrichthyan of all, the manta ray.


Manta ray. Photo credit Richard Harvey, from Wikimedia Commons.

With this background, we can finally address the second claim. Has the shark body plan remained "essentially unchanged" over deep time? [Some creationist websites claim zero change, which is ludicrious to anyone with eyes and Google - try it yourself.] The answer to this question is more complex than that of the first.

At first blush, it seems simple. Having defined "the shark" as a body plan rather than as a natural set of organisms, the comparison between ancient and modern sharks is revealed to be circular. We've designated starting and ending points based on their similarity and then raise our eyebrows when they turn out to be similar. Wow. Consider an analogy from finance. If a trader tracks a thousand stocks over the course of a year, most will finish either up or down from their starting price. A few will finish at, or close enough to, their initial value. Is it then meaningful for the trader to ignore all but those few static stocks, and in his bias marvel at how little they've changed over time?

But fortunately the natural world isn't the DJIA, and perhaps there is some kernel of wisdom that we can salvage from this debacle. Just how similar are the body plans of shark-like chondrichthyans today and those of 400 million years ago? There have been important modifications to jaw suspension, the internal girdles supporting the paired fins, and so on, but much beyond that we're going out of bounds of our artificial body plan playing field. Sharks' hydrodynamic, torpedo-shaped architecture is what some biologists cheekily call A Good Trick: a trait that a group retains or multiple groups independently stumble upon because it's highly adaptive for their environment and way of life.

In one last attempt to address the question, let's turn for perspective to sharks' sister group, the osteichthyans or fishes-with-bony-skeletons. There are two subsets of these: the ray-finned fishes, which are nearly all "typical" fishes (see last week's post); and the lobe-finned fishes, which include coelacanths, lungfishes, and limbed vertebrates like... you! Both ray- and lobe-finned fishes date to a common ancestor that lived about 420 million years ago, which had that torpedo-shaped, vaguely shark-like body plan (the Good Trick). Since they've been evolving for as long as have sharks, what have they done with that body plan since?

Like their chondrichthyan cousins, the ray-finned fishes have diversified into many different amazing forms, from millstone-like sunfish to gulper eels to sleek barracuda. And also like chondrichthyans, the basic shark-like body plan persists as a common "starting material" for most of these daring experiments in anatomy. Some ray-finned fishes alive today resemble the earliest members of this group, at least to the extent that some modern sharks resemble their ancestors.

And the lobe-finned fishes? Living coelacanths and lungfishes number only enough species to count on both hands, but like living sharks and rays they represent formerly much larger groups that also explored weird body plan variations. Still being aquatic, they've retained the same basic architecture of their ancestors. But what about the third member of this group, the tetrapods ("four-footed" beasts)? Some surviving members have changed relatively little over time despite their own extinct and experimental offshoots, and again these are typically aquatic or amphibious forms like salamanders. Where we see dramatic deviations from the ancestral body plan are in groups that have escaped the water and stumbled upon a new Good Trick. Among many examples are: flight, which evolved three separate times in tetrapods; new, faster ways of running in mammals and dinosaurs; and treetop leaping and swinging in many primates. Others came full circle and returned to the water, re-evolving shark-like body plans from very different starting material. The best example of these are not whales but rather the ichthyosaurs ("fish-lizards").


Ophthalmosaurus, a Jurassic ichthyosaur. Ichthyosaurs re-derived a shark-like body plan from a lizard-like ancestor. Image credit Nobu Tamura, from Wikimedia Commons.

Perhaps, someday in the far future, humble mice will be the only surviving members of what was once the most body-plan-diverse group of vertebrates, the mammals. Will some alien television narrator compare these mice to their tiny ancestors that once crept around Jurassic jungles? Might she remark, "My! How little they have changed...?"
-NA

Nested Sets of Sushi

This colorful opinion piece in Time by Josh Ozersky followed the 175-nation Convention on International Trade in Endangered Species (CITES); the outcome of which was, to be charitable, an unmitigated disaster for science-based fisheries management. At issue were US- and EU-backed proposals to ban trade in a number of marine species including sharks and bluefin tuna, which are being overfished vastly beyond the ability of these large, slow-growing fishes to replenish their populations.

It is difficult to deliberately fish a widespread, open-ocean species into extinction. The commercial inviability of continuing to target vanishing fishes will in most cases spare them from true extinction. Where these beasts can get in trouble is if they're hit by one or more extra liabilities: for example, going through a bottleneck in their life history where it's easy to find and fish them all up, or if they command exorbitant prices sufficient to keep fishing pressure intense to the point of complete stock exhaustion. The Atlantic bluefin tuna Thunnus thynnus bears both of these albatrosses. These voracious, silver leviathans can reach ten feet and 1400 pounds, taking decades to reach maturity. Populations are harvested wholesale when they gather in the Mediterranean, which is one of their exactly two spawning grounds. And to top it off, they are tasty. Very tasty.


Tuna being shaped at Tsukiji fish market, Tokyo.

The Japanese in particular are crazy about these things. A single, epic fish sold for $175,000 in Tokyo last year. It was no surprise that Japan spearheaded the effort to torpedo the proposed CITES ban on bluefin trade, along with a number of fishing nation allies including Canada, Indonesia, Venezuela, UAE and friends. The proposed ban was destroyed by a vote of 68 to 20, with 30 abstentions. That is a go-limping-home kind of whooping, crowned by a strident Libyan denunciation of sound fisheries science as "lies." Nice.

But this post isn't about conservation. It's about a curious twist in Mr Ozersky's mostly rational and impassioned call to boycott his once-beloved o-toro, bluefin sushi, in response to the shenanigans of Japan et al. Ozersky writes:

"It's been around for more than 400 million years, which means it is older than the trees, older than the Himalayas, older than the Atlantic Ocean itself. ... But either way, the loss of a creature that has been living here since before the continents formed won't be on my hands."

Wait. Huh?

Four hundred million years is old. Even in evolutionary time, this goes back to approaching the rise of large animal life on the planet. Paired fins and jaws had just made the scene. At this time near the beginning of the Devonian period, jawed fishes were finally getting their act together and beginning to diversify while their jawless cousins began a slow decline into obscurity. Most of these fishes were armored oddities called placoderms that have no living descendents. The first dinosaurs, lithe little things, wouldn't trod the earth for another 170 million years. So... is the Atlantic bluefin nearly twice as old as dinosaurs??

To answer this strange puzzle, let's consider the nature of life as a series of nested sets. This is the pattern of evolution by cladogenesis, in which an ancestral species gives rise over time to two distinct daughter species. This also has the nice consequence of producing a hierarchical arrangement of organisms. For example, all ferrets belong to a larger group called mustelids, along with weasels and sea otters and wolverines. All mustelids are placental mammals. All placental mammals are vertebrates, and all vertebrates are animals. Therefore, you don't predict to find weasels before mammals evolved, or mammals before the advent of animals.

Now how about tunas? Tunas are in a family of fishes called scombrids along with mackerels, bonitos, and other tasty things. They are nested well within the "set" of fishes called teleosts, which comprise over 99% of all living bony fishes, and the next larger group we'll worry about are the ray-finned fishes. So we can't have tunas without teleosts, we can't have teleosts without ray-finned fishes. Here's the problem.


Divergence times for groups containing bluefin tuna.

Teleosts, the crown group of bony fishes from marlins to mollies, date to the early Triassic period at the dawn of the dinosaurs, sometime around 240 million years ago (with some uncertainty). Right off the bat, we can see that there is no way that the bluefin can be older than that - you can't be your own grandpa, so to speak. What about ray-finned fishes? They crop up around 420 million years ago in the period right before the Devonian. Could this be what Ozersky meant? Even given that benefit of the doubt, does it matter? His heart is in the right place, but conservation may be legitimately justified by appealing to the maintenance of viable commercial markets, or ecosystems, or even an organism's aesthetic value (which I submit the bluefin has). Lemurs are worth protecting because they represent the entirety of a major offshoot of the primate family tree, not because they are vertebrates or animals or organisms with a discrete nucleus.

Species longevity is an interesting concept that gets at the heart of the pattern of "punctuated equilibrium" characterizing the fossil record of many groups. Punctuation is itself intriguing: why did this plant/animal/fungus' morphology change rapidly in a short period of time? But the equilibrium often gets lost in punctuation's spotlight: why do so many species not visibly change over long periods of time?

Across vertebrate taxa, species longevity tends to be on the order of only a few million years; perhaps 2-5. Even without knowing more about the timing of the scombrid fishes' radiation, we would not predict that the Atlantic bluefin is much older than that. The number of "famous" animals that buck the trend and appear to have changed very little over time can be counted on one hand, but are usually overblown. We'll return to the coelacanth, sharks, and similar cases in the next post, with a revelation or two that may surprise you.
-NA

Welcome to "This reView of Life"

The line with which Darwin closed On The Origin of Species captures much of why I am, why I always knew I would be, a biologist.

"There is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved."

And as Darwin ended his transformative work, so begins this humble experiment in running commentary on science. I am inexhaustibly fascinated with and often appalled by the portrayal or synopses of science in the media, and hope to use this outlet to highlight interesting topics, redress wrongdoings, and expound upon loose ends in articles that come my way. I hope you will join me.