Tuesday, September 8, 2015

Letter to the editor

I've gotten old.

Once upon a time, I relished a good fight. It helped that I was a student of evolutionary biology at a university of around forty thousand who were decidedly neither Down With It nor afraid to show it. I'll never know who produced them, but maroon "Darwin Lied - Genesis 1:1" shirts were as commonplace as "Truth" fish, devouring Darwin sigils, on vehicles. I wanted a piece. Nineteen-year old business majors at the coffee shop, professing a profound mastery of biochemistry and physics that "disproved evolution"? Baited hooks. Mostly harmless crackpot Tom Short spouting nonsense on a sunny day in the quad, rocking Hawaiian shirts and six-day creationism? Blood in the water.

I was the kid who was first up on the mike in the Q&A sessions following engineers/executives/whatever who were brought in to attempt to discredit science. I could never understand why most of my professors, and in particular a short, irascible Yankee, would never get involved. Save one bizarre and ill-advised [more on this in a moment] debate between our department chair and Mike Behe, poster boy of Intelligent Design (think Creationism with a spoiler) and some weird beers out afterward, most of the silverbacks would just shake their heads and listen to me vent.

I loved the fight. Upon meeting Genie Scott, former director of the National Center for Science Education, while I was in grad school, I probably would have quit on the spot to accept a job offer.

I've gotten old.

...but I haven't given up the fight. It took me a long time to realize that my advisors weren't choosing to abstain, to abdicate some responsibility to the maintenance of sanity and reason in society. They were just doing it smarter, and they didn't have the teenage Gibraltar-on-one's-shoulder that I bore. They knew that Some Men You Just Can't Reach. Indeed, that's why I would go spar with Tom Short; not to win anyone over, but to correct any gross falsehoods (and boy, were there some howlers!) that he or his scions would sling to see what stuck.

These guys just coolly breathed science and reason into everything they did in class. Arguably the best positioned to do so was not the Evo prof, but rather the mild-mannered and brilliant guy teaching Embryology. There is perhaps no better class for demonstrating key principles of both evolution and development, and without the Day One stigma to the recalcitrant of a class called "Evolution."

This is the best that we can do through science. Demonstrate how evolution is the only remotely viable lens through which to view the objective reality of life on Earth; in other words, Daniel Dennett's "universal acid." This has so far been my joy of the semester - teaching a small but bright group of students how "Life found a way" across the scope of biodiversity.

So what do we do for everyone else? Not for the pre-physical therapist or budding researcher, but for the lawyers and librarians and contractors of the world, who are every bit as bright but are neither a captive nor particularly interested audience (on average)?

The one thing we should not do is consent to the entirely sisyphean, futile prospect of public debate. I've seen it in our Chair vs. Behe, in Bill Nye vs. Ken Ham. Civil debates work, and only work, if the participants present data that are generally recognized to be credible and can bolster one perspective or position or the other. That is most definitely not how "debates" over evolution vs. creationism work. Please indulge a dramatic reenactment:

Participant A: "Here is something that is the case."
Participant B: "That is false."
Audience: "Well, crap. I guess we have to go look everything up when we go home."

And, without fail, people will cleave to their narrowcast sources of choice and will reinforce their previous opinions irrespective of anything that actually transpired on stage. It certainly doesn't help that much of the subject matter is esoteric as all get-out; see coverage of the Dover v. Kitzmiller trial if you want to visualize a judge's eyes rolling back in his head from days of testimony over the majestic bacterial flagellum.

I have been invited to participate in such a debate, and I will no more fall into this snare than engineers on the Apollo project will debate moon landing hoaxers. The optimal outcome is that I would do a passable job of defending the notion that we can use our minds to learn more about objective reality, with the inevitable side effect of lending credence to mysticism by elevating it to a position of false equivalency and providing the opposition with exposure and a fund-raising opportunity.

It worked out pretty well for Ken Ham, and we was trashed by The Science Guy.

So. Uh. Why are we here today? Because this letter to the editor was published in our local paper last week, and I saw in it an opportunity to respond from a less conventional perspective.

Probably the thing I love most about my school, which is a church-affiliated institution, is the opportunity it's given me to befriend and collaborate with bright, thoughtful theologians. Last spring, I gave a joint lecture with my buddy on the Biology and Theology of Monsters ("T-Rex vs. Leviathan" - more on this in upcoming posts), and our friendship has broadened my perspectives on the historical interplay between religion and science / natural philosophy.

The truth is that young-earth creationism is a bankrupt philosophy that in its current, fundamentalist form is surprisingly young and born of a rejection of societal change. Think of it as a weed. Refuting (again, ultimately up to the audience to fact-check claims with reputable sources, a grueling exercise) scientific claims one-by-one is like snipping the terminal leaves of the weed. It won't kill or keep it in check.

You have to go for its deep roots, exposing them as deeply and philosophically rotten: science is not inherently evil nor anti-religious, and indeed science and religion have been and continue to be two complementary Tools of Knowing for most of humanity.

When I chose to respond to the creation "science" letter, I knew it was fruitless to lance sciencebolts from on high. I took the other path. Here is the link to the QC Times letter, which is edited and for which I am trying hard not to read the comments, although I was pleasantly surprised by the retorts to the creationists' post. Below, I reprint the original, uncut letter.

I hope my nineteen-year old self would approve. My biologist and theologian friends have.


To address Mr Brouard’s comments in his recent letter:

The claim that science must “hide behind judges” is one of the most astonishing bits of misdirection I have seen outside of the caucuses. It is in fact creationists who have fled to litigation as well as legislation after being soundly defeated in court. From 2004 to 2011, over forty “academic freedom” bills promoting creationism in public school science classes were filed in 13 states. Science and rational thought are on defense for once.

Mr Brouard lists several prominent scientists who believed in a Creator. This is irrelevant. All of these men were also white males. Are only theistic, white males capable of good science? The tremendous technological achievements of recent decades suggest otherwise.

The statement about “falsehoods” is a mishmash of, ironically, abject falsehoods and straw men. Trotting out old Haeckel’s drawings to undermine modern biology is akin to denying modern astronomy because early sketches of Mars were inaccurate. Just as we now have rovers and orbiters exploring the red planet, biology has advanced to a degree that Mr Brouard would likely find incredible.

Finally, the idea that science leads inevitably to atheism belies a poor understanding of both theology and history. This is known as the “wedge strategy” of those who recognize that they can’t win with science. Historically, scientists used natural philosophy as a way to interpret creation and God’s design. Metaphors in the Bible were understood to be just that: symbols. Biblical literalism is a surprisingly recent contrivance of Western society.

Creationism is head-in-the-sand denialism that is neither science nor sound philosophy. It obscures two of the most beautiful truths of all: the objective history of life on Earth, and our place in God’s creation. Science and religion can be in harmony if we use evidence and our gift of reason to shine light on the How of creation, and our hearts to understand the Why.

Neil C. Aschliman, Ph.D.


After publication, I received a voicemail invitation to debate, as well as a letter from the NCSE that mentioned cribbing my "head-in-the-sand denialists" line and made me geek out pretty hard.

It was a good day.

Tuesday, July 23, 2013

The Devil Ray's in the Details

So mantas aren't a thing anymore... - paraphrase, David Shiffman's liveblog of my talk at the recent meeting of the American Elasmobranch Society.

Wow, is that going to require some explanation! Did I steal these guys away under cover of night? Did they pull a "so long, and thanks for all the fish" on us? No, this is a story about the power of naming, and one that may have serious implications for the conservation of these amazing animals and their close relatives.

Human beings love to name things. We do it vigorously and redundantly: a single fish species may be christened with a dozen or more common names by people in different geographic areas, times, or even marketing departments! It helps to have an international-standard system of naming animals to give this enterprise some consistency. It doesn't matter if you know it as Chilean Sea Bass or the Patagonian Toothfish, people worldwide will be happy to confirm for you that Dissostichus eleginoides is indeed one ugly customer.

This system of precise identification in which scientists apply a formal code to name organisms is called binominal nomenclature ("two-part name"). This is often incorrectly called "binomial" nomenclature ("two number," a mathematical expression), even by professionals! The first part of the name is the genus, which can apply to between one and many species that are closely related and resemble each other. The second part of the name is the specific epithet, which applies to one and only one species. These names are usually derived from Greek and Latin. For example, Batrachognathus volans translates as "flying frog-jawed" one, an apt appellation for an odd pterosaur from the Late Jurassic.

A genus (plural genera) should describe some small set of similar organisms. This group should be defined by a common ancestor, all of its descendants, and nothing else (Figure 1A). We call these natural groups, or clades.

This is the ideal case. What happens if you end up with something like Figure 1B? Here, you can see that one genus is "nested" inside the other one: Mobula is not a natural group because it inappropriately excludes Manta. It's easy to analogize this to how, in some cultures, family names are passed down the paternal line. If you, your father, his father, and all male relatives in between are named "Lannister" but one of your brothers is named "Baratheon," something's gone wrong in the process of naming!

Under the current classification scheme, there are two genera of "devil rays": two species in the genus Manta and nine species in the genus Mobula. Besides some slight differences in body proportions, there are a few characteristics that distinguish Manta from Mobula. Most notably, Manta lack teeth on the upper jaw and have a mouth at the end of the head, while the mouth of Mobula is underslung as in most sharks. In recent years it has usually been held that the classification of devil rays is a valid arrangement, following Figure 1A.


Evidence from both DNA sequences and comparative anatomy suggests that current devil ray classification actually fits Figure 1B, making it invalid.

DNA sequence analyses from my doctoral dissertation work [1], followed by a large-scale study performed by my graduate advisor and colleagues [2], suggested that Manta is nested within Mobula. While not all species were sequenced, Manta birostris was indicated to be more closely related to Mobula japanica than it is to other sequenced Mobula. This is based mostly on evidence from one or two genes, with a couple of others rather uninformative. It was compelling enough to get me to delve back through the scientific literature on the anatomy of the group, and what I found was surprising.

It turns out that biologists who have taken a close look at devil ray anatomy have been quietly suggesting for nearly 15 years that Mobula is not a natural group! [3,4] For example:

 • Most devil rays have comb-like teeth, while Manta, Mobula japanica, and Mobula mobular have distinctive peg-like teeth. [3,4]

 • Mobula japanica (>3m wide) and Mobula mobular (>5m?) are among the largest devil rays, comparable in size to the recently described Manta alfredi. [5,6]

 • Among devil rays, only Manta, Mobula japanica, and Mobula mobular have caudal spines (the "sting" in "stingrays"). [5,6]

So is this enough to overturn the current system of classification for devil rays? Not yet. If any changes are to be proposed, someone first needs to perform a formal "taxonomic revision" of both Manta and Mobula. This means that they need to present a sound, peer-reviewed case for the reclassification of these species, which will require physically re-examining them and ideally presenting corroborating DNA evidence. This must include the "type species" for each genus: Manta birostris and Mobula mobular. A type species is the one to which the name of the genus is permanently attached; in this case, it defines "what a Mobula should be." There are to date no available DNA sequence data for Mobula mobular, although we can predict that they will group most closely with Manta and Mobula japanica.

Assuming the evidence continues to build that Manta is nested within Mobula, what happens next? The most likely case is that the genus Manta will cease to be valid, subsumed under Mobula as a "junior synonym." Mobula has precedence under the rules since it was described in 1810, versus 1829 for Manta. There are mechanisms by which a name can be suppressed: the proposed renaming of the fruit fly species that's the cornerstone of much of modern biology would be a nightmare beyond measure (see here), but Manta is unlikely to qualify. Their common names will remain "Manta Rays" and the world will go on turning. Given the scarcity of materials from some species and their often colossal size, I don't envy the gal/guy to undertake the formal review of the devil rays... but it needs to be done. Five bucks says that Manta is going away in the next ten years.*

So why does this matter? Legal issues and conservation.

Savvy folks sitting in the conference room or reading David's liveblog quickly seized upon this question. With Manta recently receiving protection under the Convention on International Trade in Endangered Species (CITES), would lumping these guys in with Mobula expose them (1) under a legal technicality and/or (2) as less unique and worthy of conservation than when they had their own genus? Fortunately, shark conservation experts including Sonja Fordham (President at Shark Advocates International / Deputy Chair at IUCN Shark Specialist Group / conservation rockstar) were on hand to address these concerns.

Fears of CITES loopholes appear to be unfounded. Current protection schemes are thought to be engineered to continue to protect the two named (plus a third suspected but undescribed) species of Manta. Prying at loopholes is sadly unnecessary: countries that would otherwise seek to do so can apparently just opt out of CITES protections (I am aiming an unimpressed glare squarely at you, Canada and Guyana).

In short, this is less a threat to mantas than it may be an opportunity to extend protections to the other devil rays. Most of these species are heavily affected by both targeted fisheries and as bycatch, but they exhibit human-like life histories that make them extremely vulnerable to such pressure. Despite this shared and imminent danger, only Manta - with its tremendous charisma - seems to have found many champions in conservation, evidenced by the fact that only Manta species were proposed for CITES listing. Changing its name won't make these folks give up the cause, but folding all of the devil rays into a single genus can underscore the similarities in vulnerability across species in this group, strengthening the case for offering them equal safeguards in the future.

"Mantas aren't a thing anymore"? Sounds good to me.

*Offer valid for first claimant as of ten years from the date of this article's publication. If I win, I'll pick one of you at random and demand my fiver.

1 Aschliman NC (2011) The batoid tree of life: recovering the patterns and timing of the evolution of skates, rays and allies (Chondrichthyes: Batoidea). Dissertation, Florida State University
2 Naylor GJP, Caira JN, Jensen K, Rosana KAM, Straube N, Lakner C (2012) Elasmobranch phylogeny: a mitochondrial estimate based on 595 species. In: Carrier JC, Musick JA, Heithaus MR (eds) Biology of sharks and their relatives, 2nd edn. CRC Press, Boca Raton, Florida, pp 31-56
3 Herman J, Hovestadt-Euler M, Hovestadt DC, Stehmann M (2000) Contributions to the study of the comparative morphology of teeth and other relevant ichthyodorulites in living supra-specific taxa of Chondrichthyan fishes. Part B: Batomorphii 4c: Order: Rajiformes - Suborder Myliobatoidei - Superfamily Dasyatoidea - Family Dasyatidae - Subfamily Dasyatinae - Genus: Urobatis, Subfamily Potamotrygoninae - Genus: Potamotrygon, Superfamily Plesiobatoidea - Family Plesiobatidae - Genus: Plesiobatis, Superfamily Myliobatoidea - Family Myliobatidae - Subfamily Myliobatinae - Genera: Aetobatus, Aetomylaeus, Myliobatis and Pteromylaeus, Subfamily Rhinopterinae - Genus: Rhinoptera and Subfamily Mobulinae - Genera: Manta and Mobula. Addendum 1 to 4a: erratum to Genus Pteroplatytrygon. Bull Inst R Sci Nat Belg Biol 70:5-67
4 Adnet S, Cappetta H, Guinot G, Notarbartolo di Sciara G (2012) Evolutionary history of the devilrays (Chondrichthyes: Myliobatiformes) from fossil and morphological inference. Zool J Linn Soc 166:132-159
5 Notarbartolo di Sciara G (1987) A revisionary study of the genus Mobula Rafinesque, 1810 (Chondrichthyes: Mobulidae), with the description of a new species. Zool J Linn Soc 91:1-91
6 Marshall AD, Compagno LJV, Bennett MB (2009) Redescription of the genus Manta with resurrection of Manta alfredi (Krefft, 1868) (Chondrichthyes; Myliobatoidei; Mobulidae). Zootaxa 2301:1-28

Photos from Wikimedia Commons
Manta: LINK
Mobula: LINK

Wednesday, April 25, 2012

Shattering the Vegetarian Myth: Meat Consumption Was Intrinsic to Human Evolution

Apologies for the absence. It takes time to finish one life and start another.

At least this dragged me back into the scene: Kathy Freston's "Shattering the Meat Myth: Humans Are Natural Vegetarians." The piece is, and I hesitate to write this in an age where one's Google News feed bears a striking resemblance to The Onion, a breathtakingly poor scrap of doggerel. Even for ill-researched propaganda, this beast is remarkable in its reckless and wanton distortion of not only science, but of history as well.

This rebuttal in no way touches upon the relative morality or nutrition of diets that include or exclude animal products. In fact, the article came to my attention via Facebook by a respected friend and colleague who takes a valid moral stance against meat consumption. However, with 20,000 other Facebook Likes at the moment, I cannot let the grotesque inaccuracy of the arguments from biology stand. To crib from Samuel L. Jackson's character in Pulp Fiction, "Well, allow me to retort!"

Freston argues as follows (and by all means, read the original to evaluate my summary).
1. The inclusion of meat in the human diet is a product of agricultural civilization (circa 10,000 years ago) and is incompatible with a plant-based biochemistry that dates back "at least tens of millions of years." Prior to the rise of herding, "we may have needed a bit of meat… in times of scarcity."
2. "Humans are herbivores" because we lack physical adaptations that make it easy to tear flesh and hide, such as overdeveloped canine teeth and claws. We resemble the other great apes in that we process food with our hands and must have similarly had "a largely plant-based diet."
3. Humans have "never adapted" to a meat-inclusive diet because meat-eaters have a higher incidence of heart disease, cancer, and diabetes.

Freston's deeply flawed arguments can be attacked from historical, evolutionary, and logical angles. Let's get the observed history out of the way so we can get on to the evo.

A History of Meat Consumption
Let us assume, for the purposes of clarity (since Freston surely provided little), that the "natural" state of humanity refers to traits expressed prior to civilization and its concomitant horrors. Freston has asserted that, before the advent of agriculture-dependent herding 10,000 years ago, humans consumed only small quantities of meat in times of hardship.

Um. But.

This ignores the physical record of at least 70,000 years of hunting. Hunting big things. From some of the earliest human art on cave walls depicting the hunt, to no shortage of archaeological sites riddled with thousands of mammoth bones (one example), remains of other large and small mammals, deep middens of fish bones and mollusk shells, and so on. By about 10,000 years ago, we were in part responsible for the disappearance of nearly every large mammal in the Western Hemisphere by eating them toward extinction.

South African cave art. Photo Credit San Felszeichnung, from Wikimedia Commons.

Of course, then civilization struck and, well… a grand total of zero of the remotely successful cultures on the planet, now or in recorded history, have categorically excluded meat from their diet. Jared Diamond has a little more to say on the subject.

But let's throw Freston a bone (hah) and examine her argument in the time frame of millions of years. What does biology have to say?

Pretty Much The Same Thing
Freston views humanity through the lens of our closest living relatives, the other great apes, so let's follow. Our sister group, the two chimpanzee species, eat lots of lovely fruits and vegetable bits. Er, and also insects, birds, and mammals including other primates and small relatives of cows and pigs(!). Meat only comprises about 5% of their caloric intake, but for a species relying on its hands, primitive tools and wits, it's seriously impressive that they can muster as much.

Is Freston correct that the human gut tract is like the intestines "of other herbivores" in being very long? No, we're rather intermediate on the spectrum between hypercarnivores and herbivores. Incidentally, carnivore gut tracts are not particularly short "so they can quickly get rid of all that rotting flesh they eat" - another of the bountiful instances in which Freston's sensationalism throws science on the fire. Carnivores have a short and efficient intestinal tract in that they don't need to process large quantities of generally indigestible plant fibers. Our barrel-shaped rib cages, rather than the conical arrangement exhibited by primates that consume more plant matter, in part reflects this reduction in gut length.

Freston laments the absence of wicked claws and fangs (actually, in primates the size of the canine teeth is strongly correlated with social structure rather than with diet) and such. If only humans had a way of compensating for that!

Oh. Right. The whole running and tools thing.

A proper treatment is well beyond the scope of this post, but the evolution of the human body plan from that of a four-footed ancestor appears to have been driven in large part by selective pressure on the ability to run long distances efficiently - and later, while holding tools. Our legs and feet are superbly adapted to long distance running. The NYT has a decent, if brief summary of a few of these characters, and there's a skeletal outline of the endurance running hypothesis over at Wiki. This ability may be nearly 2 million years old, dating back to Homo erectus.

There are a couple reasons for us to run long distances: to poach kills (in fact, that's how we got tapeworms from large cats and hyenas), and to run prey into the ground. Turns out we're pretty good at that (awesome Attenborough video!). We run, we track, and unlike many of our large prey, we can sweat to cool off in the chase. Persistence pays off.

Might meat consumption have put evolutionary pressure on not only our body plans, but on our developmental timing as well? Humans wean their offspring at a very early age relative to other great apes (2 years and change in humans / 5 in chimps / 7 in orangutans, which consume very little animal protein). This is a pattern common to carnivorous mammals (PLoS ONE original). Carnivores wean their offspring faster than do herbivores due to improved milk quality and/or the ability of the offspring to eat high-energy meat. After weaning, females again become receptive to mates; the upshot is that a shorter weaning period increases reproductive capacity.

The preponderance of evidence suggests that Freston's thesis could not be more wrong. The increased means to acquire and utilize meat was likely one of the major driving forces behind the evolution of our bipedal body plan optimized for endurance running, our ability to make and manipulate tools, and our developmental timing. There are other human relatives that were more clearly geared toward eating tough plant matter. They didn't make it.

But It'll Still Kill You
Meat is an extremely efficient source of nutrients, including proteins, fats, vitamins, and minerals. When combined with leafy greens and other nutritious foods and an active life style, it helps build healthy bodies. In the short term. In the long term, the consumption of, in particular, red and processed meats is linked to increased risk of cardiovascular disease, certain cancers, and diabetes.

Apparently needing a screamingly ludicrous sound bite from an otherwise distinguished individual, Freston concludes her sad crusade with this gem from Dr. William Roberts, editor of the American Journal of Cardiology. In its entirety: "Although we think we are, and we act as if we are, human beings are not natural carnivores. When we kill animals to eat them, they end up killing us, because their flesh, which contains cholesterol and saturated fat, was never intended for human beings, who are natural herbivores." Coupled with another MD suggesting that "our bodies have never adapted to [eating meat]," it begs the obvious question: why did our ancestors eat meat - and as we have seen above, they certainly did - if it predisposed them to disease? The answer, again, rests in timing.

Except in cases of gross excess and sedentary life styles, these pathologies tend to strike late in life. Specifically, they largely affect post-reproductive (or nearly so) individuals. If you die at 55 from a heart attack but raise ten vigorous, meat-fed offspring, you still have dramatically higher fitness than a vegetarian who lives to 90 but raised five offspring before menopause. Additionally, during the course of human history, "post-reproductive" is a luxury that an incredibly small portion of the population would ever live to see; those disorders would be invisible to natural selection. The MDs' argument is completely vacuous. While they may mean well, I won't stand by to see them misinform the public in order to do so.

It's good to be back.

[Note: Desmond Morris tackled the evolution of humans as a balancing act between carni- and herbivory in his classic The Naked Ape.]

Thursday, July 15, 2010

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.