E. O. Wilson mistakenly touts group selection (again) as a key factor in human evolution

February 26, 2013 • 9:43 am

As most of you know, Edward O. Wilson is one of the world’s most famous and accomplished biologists.  He was the founder of evolutionary psychology (known as “sociobiology” back then), author of two Pulitzer-Prize-winning books, one of the world’s great experts on ants, an ardent advocate for biological conservation, and a great natural historian. His legacy in the field is secure.

So it’s sad to see him, at the end of his career, repeatedly flogging a discredited theory (“group selection”: evolution via the differential propagation and extinction of groups rather than genes or individuals) as the most important process of evolutionary change in humans and other social species. Let me back up: group selection is not “discredited,” exactly; rather, it’s not thought to be an important force in evolution.  There’s very little evidence that any trait (in fact, I can’t think of one, including cooperation) has evolved via the differential proliferation of groups.

In contrast, there is a ton of evidence for an alternative explanation for cooperation: kin selection, the selection of genes based on how they affect not just the fitness of the individual, but the fitness of relatives that share its genes.  Features like parental behavior, parent-offspring conflict, sibling rivalry, and preferential dispensing of favor to relatives, as well as features like sex ratios in insects—all of these are all easily explained by kin selection.  And many aspects of cooperation can easily be explained by individual selection: individuals that live in small groups, especially those in which one can recognize group members, can evolve cooperation as an individual good based on reciprocity: the “I scratch your back, you scratch mine” hypothesis.  And, as I’ve discussed before, the cooperative and “altruistic” behavior seen in our own species shows many features suggesting that it evolved via individual or kin selection and not group selection.

I’ve covered this issue many times (e.g., here, here, here, here, and here), so I won’t go over the arguments again. Wilson’s “theory” that group selection is more important than kin selection in the evolution of social behavior (published in Nature with Martin Nowak and Corina Tarnita) was criticized strongly by 156 scientists—including virtually every luminary in social evolution—in five letters to the editor, and sentiment about the importance of group selection has, if anything, decreased since Wilson’s been pushing it.

But Wilson persists, to the detriment of his reputation. In a new piece at the New York Times “Opinionator” site, “The riddle of the human species,” Wilson continues to make the same argument that group (or “multilevel”) selection was a key force in making humans (and social insects) the socially complicated species they are.  Since his arguments are virtually identical to those published in NYT Opinionator piece last June, and in his book The Social Conquest of Earth (see part of my review here), I won’t dissect them in detail. I just want to highlight three points that I think make Wilson’s argument for group selection—and against kin selection—deeply misleading. I wouldn’t spend my time writing time-consuming critiques like this were Wilson not famous, influential, and given a big public forum in the New York Times. Someone has to address his arguments!

Here are Wilson’s errors (quotes indented), and my responses:

1. Wilson: Humans are a “eusocial species”:

. . the known eusocial species arose very late in the history of life. It appears to have occurred not at all during the great Paleozoic diversification of insects, 350 to 250 million years before the present, during which the variety of insects approached that of today. Nor is there as yet any evidence of eusocial species during the Mesozoic Era until the appearance of the earliest termites and ants between 200 and 150 million years ago. Humans at the Homo level appeared only very recently, following tens of millions of years of evolution among the primates.

My response:  “Eusociality” as defined by Wilson and every other evolutionist is the condition in which a species has a reproductive and social division of labor: eusocial species have “castes” that do different tasks, with a special reproductive caste (“queens”) that do all the progeny producing, and “worker castes” that are genetically sterile and do the tending of the colony. Such species include Hymenoptera (ants, wasps and bees, though not all species are eusocial), termites, naked mole rats, and some other insects.

But humans don’t have reproductive castes, nor genetically determined worker castes.  Wilson is going against biological terminology, lumping humans with ants as “eusocial,” so he can apply his own theories of “altruism” in social insects (i.e., workers “unselfishly” help their mothers produce offspring while refraining themselves from reproducing), to humans. But human cooperation and altruism are very different from the behavior of ants, most notably in our absence of genetic castes and genetically-based sterility associated with helping others reproduce. Human females aren’t sterile, and don’t usually refrain from reproduction just to help other women have babies.  My guess is that Wilson lumps humans with insects as “eusocial” because he wants to subsume them both under a Grand Theory of Social Evolution.

2. Wilson: Kin selection doesn’t work, ergo it certainly couldn’t have played a role in the evolution of eusociality and human cooperation.

Still, to recognize the rare coming together of cooperating primates is not enough to account for the full potential of modern humans that brain capacity provides. Evolutionary biologists have searched for the grandmaster of advanced social evolution, the combination of forces and environmental circumstances that bestowed greater longevity and more successful reproduction on the possession of high social intelligence. At present there are two competing theories of the principal force. The first is kin selection: individuals favor collateral kin (relatives other than offspring) making it easier for altruism to evolve among members of the same group. Altruism in turn engenders complex social organization, and, in the one case that involves big mammals, human-level intelligence.

The second, more recently argued theory (full disclosure: I am one of the modern version’s authors), the grandmaster is multilevel selection. This formulation recognizes two levels at which natural selection operates: individual selection based on competition and cooperation among members of the same group, and group selection, which arises from competition and cooperation between groups. Multilevel selection is gaining in favor among evolutionary biologists because of a recent mathematical proof that kin selection can arise only under special conditions that demonstrably do not exist, and the better fit of multilevel selection to all of the two dozen known animal cases of eusocial evolution.

My response:  There is so much fail here I don’t know where to start.  The first paragraph is basically correct except that Wilson omits “individual selection” along with “kin selection” as an accepted evolutionary process that can promote the evolution of cooperation. As I mentioned, selection on individuals in small groups can allow the evolution of cooperation without any need to invoke the unparsimonious process of differential group survival based on genes.

Wilson’s claim that the “special conditions of kin selection” demonstrably do not exist is an egregious and (I think) willful misstatement.  Kin selection can cause evolution whenever the genes in an individual benefit relatives that share copies of that individual’s genes, and can do so whenever the benefit of that behavior to the recipients, devalued by their degree of relatedness to the donor (a figure usually ranging between 0 and 1, but which can be related if an individual helps another less related to it than the average member of the population) is greater than the reproductive cost to the donor.  (“Hamilton’s rule”: rb > c.) That is known to obtain in many cases, and explains things like parental care, parent-offspring conflict, sex ratios in insects, and many other features (see the five letters in Nature mentioned above, which list some features of social behavior that clearly evolved by kin rather than group selection).

The mathematical “proof” given by Nowak et al. does not show that group selection is a better explanation than kin selection for social behavior in insects, for their “proof” does not vary the level of kinship, as it must if it could allow that conclusion.

The second egregious and false claim in this paragraph (a paragraph that’s the highlight of the piece) is that “multilevel selection is gaining in favor among evolutionary biologists” because of the Nowak et al. paper. That’s simply not true.  The form of multilevel selection adumbrated in that paper is, to my knowledge, embraced by exactly four people: the three authors of the paper and David Sloan Wilson. There is, and has been, no increase in acceptance of group or multilevel selection in the past ten years. The Nowak et al. paper has sunk without a stone, except to incite criticism by other biologists and excitement by an uncomprehending press.

3. Wilson: Eusociality in insects arose not via kin selection, but via the initial construction of a defended nest site.

The history of eusociality raises a question: given the enormous advantage it confers, why was this advanced form of social behavior so rare and long delayed? The answer appears to be the special sequence of preliminary evolutionary changes that must occur before the final step to eusociality can be taken. In all of the eusocial species analyzed to date, the final step before eusociality is the construction of a protected nest, from which foraging trips begin and within which the young are raised to maturity. The original nest builders can be a lone female, a mated pair, or a small and weakly organized group. When this final preliminary step is attained, all that is needed to create a eusocial colony is for the parents and offspring to stay at the nest and cooperate in raising additional generations of young. Such primitive assemblages then divide easily into risk-prone foragers and risk-averse parents and nurses.

My response:  Phylogenetic studies show that eusociality in Hymenoptera always originated in species whose females mated only once: this is a statistically significant result.  And that alone militates for kin selection as an important factor in eusociality: if a female founds a colony consisting only of full siblings (as is the case when she mated only once), they are more related to each other than if she had mated multiply. In the later case, colonies would consist of half-sisters or even more distant relatives, making kin selection less efficient.

Further, relatedness is high in virtually every species of eusocial insect with the exception of a few highly derived species of ants that have many queens.  The connection between relatedness and eusociality is exactly what we expect if kin selection is important in social evolution, and is not expected if Wilson’s nest-based group selection was important. The model of Nowak et al., which starts with the construction of such nests by single females who stay in the nests with their offspring, produces precisely the condition in which relatedness can promote the evolution of sterility and cooperation.  They argue that this relatedness is a consequence of their model and not a cause of eusocial evolution, but that’s unconvincing, for they do not vary the level of initial relatedness in their model.

*****

Wilson’s claim, the theme of his newest book, is that humans are both angels and devils: we are both selfish and cooperative species, and this combination of good and bad is what makes our species unique. (That’s not true, of course, because many species show that mixture of behavior. Lions, for instance, cooperate when hunting, but when males take over a pride they immediately kill all the female’s cubs, which are unrelated to them. And that, by the way, is due to kin selection, because those cub-killing males replace the cubs with new cubs containing their own genes, including the genes for killing cubs. Cub-killing could have evolved only by individual selection and not group selection, for while killing another male’s cubs is good for an individual, it’s bad for the group, forcing females to waste reproductive energy.)

Yes, we have both selfish and cooperative behaviors, though most of our “cooperative” behaviors that didn’t arise through culture arose through forms of selection that involve maximizing our reproductive output—individual and kin selection.  There is not a scintilla of evidence, in humans or any other species, that group selection has been responsible for the evolution of any adaptation.  In contrast, individual and kin selection have productively explained the evolution of “problematic” traits like altruism and cooperation. They have been tested and work.

Why does Wilson keep writing article and article, and book after book, promoting group selection? I’m not a psychologist, so I don’t know the answer. What I do know, though, is that his seeming monomaniacal concentration on a weakly-supported form of evolution can serve only to erode his reputation.  His theories have not gained traction in the scientific community. That doesn’t mean that they’re wrong, for, in the end, scientific truth is decided by experiment and observation, not by the numbers of people initially on each side of an issue. But the facts of science already show that Wilson is unlikely to be correct. What is sad is that, as a great natural historian, he doesn’t recognize this.

Wilson’s reputation is secure. It’s sad to see it tarnished by ill-founded arguments for an unsubstantiated evolutionary process.

h/t: Phil Ward, Laurence Hurst

Ducky orchids and insects

February 21, 2013 • 9:41 am

When I first saw these pictures I was startled, for the resemblance of this Australian orchid (Caleana major) to a flying duck is amazing.

Picture 1

In fact its common names are the “flying duck orchid” and the “big duck orchid”.

From Friends of the Cove National Park, Inc: http://www.friendsoflanecovenationalpark.org.au/Flowering/Flowers/Caleana_major.htm
From Friends of the Cove National Park, Inc: http://www.friendsoflanecovenationalpark.org.au/Flowering/Flowers/Caleana_major.htm

Kuriositas has the botanical details:

The duck orchid is a perennial but blooms in late spring or early summer.  At up to 45 centimeters in height you might think it would stand out in its natural habitat.  However, because of the reddy-brown colors of both the stem and flowers it moulds in to its Australian environs so expertly that it becomes almost invisible – unless you are deliberately seeking out its company.

Image Credit Flickr User Davidfntau: http://www.flickr.com/photos/96936558@N00/4208765488/
Image Credit Flickr User Davidfntau: http://www.flickr.com/photos/96936558@N00/4208765488/

I was tempted to write that this orchid is pollinated by male ducks, who try to copulate with the flowers and thereby affix pollen to their heads (this is in fact true for insects pollinating the wasp and bee orchids), but I knew at least one reader would be taken in. But the facts are just as striking:

The ‘upside-down’ flower is reddish-brown, 15-20 mm long. The labellum or tongue, at the top, is a deep red and attached to the rest of the flower by a sensitive strap. Pollination is via male sawflies. When the insect touches the sensitive labellum it snaps shut, trapping the insect in the sticky body of the column. It deposits pollen it may be carrying and picks up more. It is then released to fly to the next orchid.

I’d love to grow one of these (I have several wild orchids in my lab), but, alas, that won’t be. As Kuriositas notes:

 If you have suddenly been gripped by the desire to own your very own duck orchid then you will be disappointed.  Despite numerous attempts, this orchid stubbornly refuses to be propagated, and is only found in the wild. This is because the roots of caleana have a symbiotic relationship with the vegetative part of a fungus which only thrives in the part of Australia in which it originates. The fungus helps the plant to stave off infections and without its help the duck orchid never lasts long.

And the Aussies, God bless them, have put the orchid on a stamp:

Caleana-Orchid-04-210x300

Finally, in a bizarre coincidence, I found this—a duck-faced lacewing fly! (It’s actually a “spoon-winged lacewing” in the genus Nemia, family Nemopteridae.) Spoon-winged lacewings are also called “thread-winged antlions”, for their larvae are predators on ants and other insects.

It’s described on Piotr Naskrecki’s website, The Smaller Majority. Here’s the bill:

The head and mouthparts of spoon-winged lacewings is elongated and well-adapted for fitting into long corollas of flowers [Canon 1Ds MkII, Canon 100mm macro, 2 x Canon 580EX]; photo by Piotr Naskrecki
The head and mouthparts of spoon-winged lacewings is elongated and well-adapted for fitting into long corollas of flowers [Canon 1Ds MkII, Canon 100mm macro, 2 x Canon 580EX]; photo by Piotr Naskrecki
But it’s not just the face that’s weird—check out its hindwings!:

Spoon-winged lacewings (?Nemia sp.) from Richtersveld National Park, South Africa [Canon 1Ds MkII, Canon 100mm macro, 2 x Canon 580EX]; photo by Piotr Naskrecki
Spoon-winged lacewings (?Nemia sp.) from Richtersveld National Park, South Africa [Canon 1Ds MkII, Canon 100mm macro, 2 x Canon 580EX]; photo by Piotr Naskrecki
As Naskrecki explains, the “duckface” is adapted to dip into flowers to eat nectar and pollen, but we don’t know why those hindwings are so large:

These lacewings are easily recognizable thanks to their unique, extremely elongated or enlarged hind wings, reminiscent of the long plumes seen in some birds-of-paradise. The function of this unusual morphology is still not entirely known. In species with particularly enlarged hind wings their function appears to be to deter some predators by giving a false impression of the insect as much larger—and thus potentially stronger—than it really is. In species with long, thread-like wings their function may be related to the aerodynamics of the flight, and in members of the subfamily Crocinae the hind wings play a sensory function in cavernicolous habitats that these insects occupy.

I would have thought sexual selection is involved, making these beasts the insect equivalent of long-tailed widowbirds, but that would lead to sexual dimorphism, with males having much longer wings than females. And that’s apparently not the case.

To see other species in this bizarre group, go here.

h/t: GN

The assassin bug: aggressive mimicry of prey

February 15, 2013 • 1:35 pm

I’m shamelessly stealing this story from Alex Wild’s great Scientific American website, Compound Eye. His latest post describes a paper from the Proceedings of the Royal Society B (link below) by Wignali and Taylor, who show that assassin bugs from Australia (Stenolemus bituberus; these are true bugs in the order Hemiptera) kill spiders by entering their webs and producing vibrations that lure the spider by mimicking either the vibrations made by normal prey trapped in the web.  This shows that the evolution of mimicry need not involve any change in appearance but simply a change in behavior: in this case natural selection has favored those assassin bugs who are able to vibrate spider webs with the proper frequency.

Assassin bugs are usually called “thread-legged bugs” for obvious reasons:

Reduviidae: Emesinae (Belize); not the species used in the study but a related one. Photo used with permission.
Reduviidae: Emesinae (Belize); not the species used in the study but a related one. Photo used with permission.

They’re cryptic, too; as Alex notes: “In the field the insect looked like so little I thought it merely debris in a disorganized spider’s web. I didn’t see the faint outline of a young assassin bug until the debris shuddered, ever so slightly.” (Remember that some spiders, and of course bug-hunting birds, have keen vision.)

I hope my readers are now biology-savvy enough to understand the paper’s abstract:

Assassin bugs (Stenolemus bituberus) hunt web-building spiders by invading the web and plucking the silk to generate vibrations that lure the resident spider into striking range. To test whether vibrations generated by bugs aggressively mimic the vibrations generated by insect prey, we compared the responses of spiders to bugs with how they responded to prey, courting male spiders and leaves falling into the web. We also analysed the associated vibrations. Similar spider orientation and approach behaviours were observed in response to vibrations from bugs and prey, whereas different behaviours were observed in response to vibrations from male spiders and leaves. Peak frequency and duration of vibrations generated by bugs were similar to those generated by prey and courting males. Further, vibrations from bugs had a temporal structure and amplitude that were similar to vibrations generated by leg and body movements of prey and distinctly different to vibrations from courting males or leaves, or prey beating their wings. To be an effective predator, bugs do not need to mimic the full range of prey vibrations. Instead bugs are general mimics of a subset of prey vibrations that fall within the range of vibrations classified by spiders as ‘prey’.

Here’s another photo showing the bug entering a spider’s web for nefarious purposes:

Picture 2

Finally, a video (taken from the original paper via Alex) showing an assassin bug luring a spider to its death:

Some assassin bugs also kill spiders not by mimicking prey vibrations, but by sneaking up on them and stabbing them with their mouthparts (ergo their name). An earlier BBC report notes that, when using this latter tactic, assassin bugs are most likely to move toward their spider prey when the wind is blowing, masking any vibrations produced by their movement. They’re like ninja cats! This was demonstrated in clever experiments using fans to mimic the vibrations of spider webs produced by wind.

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Wignali, A. E. and P. W. Taylor. 2013. Assassin bug uses aggressive mimicry to lure spider prey. Proc. R. Soc. B 7 May 2011 vol. 278 no. 1710 1427-143, Published online October 27, 2010 doi: 10.1098/rspb.2010.2060

See also: Wignall, A.E. . & Taylor, P.W. (2008). Biology and life history of the araneophagic assassin bug Stenolemus bituberus including a morphometric analysis of the instars (Heteroptera, Reduviidae).. Journal of Natural History 42: 59-76. (pdf here)

How the pebble toad rolls

January 12, 2013 • 12:40 pm

The best part of being an evolutionary biologist is learning about the endless ways that animals adapt to their existence and environment.  (The classic aphorism is “Natural selection is cleverer than you are.”)

And here’s a behavior completely new to me: the escape behavior of the pebble toad, Oreophrynella nigra, from Bolivia and Guyana. The inimitable Attenborough tells the tale:

An article at BBC EarthNews notes:

The toad is so small and light that the forces of impact are too tiny to cause it any harm.

However, as well as being less than impressive jumpers, the toads do not swim well.

So while most that land in puddles survive, there are reports of toads drowning after tumbling into deeper pools of water.

h/t: Christopher

The amazing mimicry of frogfish

November 29, 2012 • 4:54 am

I have a penchant for cases of mimicry, not only because they served as some of the earliest evidence for natural selection in Darwin’s time, but also because they show how far natural selection can achieve “perfection”—that is, how far do developmental and physical constraints prevent the evolution of an “optimum phenotype.” The answer is that constraints don’t matter much.

There are few cases in nature where one can judge how “optimum” an adaptation is, and mimicry is one of them. (Sex ratio is another.) What it shows, as this post demonstrates, is that it can be remarkably precise; that is, natural selection is pretty good at molding animals (and some plants) to hide their true nature by evolving to resemble either another organism or their environment. The resemblance can be astonishingly precise.

Finally, many examples of mimicry are simply unexpected, cool, and stunning. The first one below was sent to me by Matthew Cobb who got it from a tweet by M.J. Walker from the Blue Planet Society The photo is by Andrew Taylor.

One of these animals is a frogfish; the other is a sponge (yes, sponges are animals).  If you look closely you can see which is which, but it may not be so easy for a predator or a prey item. (Frogfish are almost all predators.)

Frogfish, sometimes known as “anglerfish” are in the order Lophiformes and the family Antennariidae; 47 species are recognized. 

Wikipedia has a good section on frogfish mimicry. I’ve reproduced it in the indented parts below, and inserted some pictures of different species of frogfish:

The unusual appearance of the frogfish is designed to conceal it from predators and sometimes to mimic a potential meal to its prey. In ethology, the study of animal behavior, this is known as aggressive mimicry. Their unusual shape, color, and skin textures disguise frogfish. Some resemble stones or coral while others imitate sponges, or sea squirts with dark splotches instead of holes. In 2005, a species was discovered, the striated frogfish, that mimics a sea urchin while the sargassumfish is colored to blend in with the surrounding sargassum.

Here’s the one, Antennarius striatus, that’s supposed to mimic a sea urchin:

Some frogfish are covered withalgae or hydrozoa. Their camouflage can be so perfect, that sea slugs have been known to crawl over the fish without recognizing them.

Here’s one that looks like an algae-covered rock:

Here’s another that looks like a sponge, hiding in a sponge:

For the scaleless and unprotected frogfish, the camouflage is an important defense against predators. Some frogfish can also inflate themselves, like pufferfish, by sucking in water in a threat display. In aquariums and in nature, frogfish have been observed, when flushed from their hiding spots and clearly visible, to be attacked by clownfish,damselfish, and wrasse, and in aquariums, to be killed.

Many frogfish can change their color. The light colors are generally yellows or yellow-browns while the darker are green, black, or dark red. They usually appear with the lighter color, but the change can last anywhere from a few days to several weeks. It is unknown what triggers the change.

If your appetite for mimetic frogfish isn’t sated, there’s a whole site devoted to them, http://www.frogfish.ch, which has a great page showing many mimetic animals.

To show how the mimicry works, here’s a sponge-mimicking frogfish nomming a cardinal fish. It’s fast!

Finally, a short clip of a frogfish feeding. It was filmed at 1000 frames per second and played at 10 frames per second, so this whole 17-second video represents 0.17 seconds in real time. Note how opening the mouth creates a suction that draws the prey in:

A free journal issue on experimental evolution

October 31, 2012 • 10:41 am

Biology Letters is offering free access to its latest issue on “experimental evolution,” an issue edited by Thomas Batailon, Paul Joyce, and my friend Paul Sniegowski. You can see the table of contents at the link above, and here are the free articles:

Feature Articles

Introduction – As it happens: current directions in experimental evolution
by Thomas Bataillon, Paul Joyce and Paul Sniegowski

Temperature, stress and spontaneous mutation in Caenorhabditis briggsae and Caenorhabditis elegans
by Chikako Matsuba, Dejerianne G. Ostrow, Matthew P. Salomon, Amit Tolani and Charles F. Baer

Mutational effects depend on ploidy level: all else is not equal
by Aleeza Gerstein

Genetic background affects epistatic interactions between two beneficial mutations
by Yinhua Wang, Carolina Díaz Arenas, Daniel M. Stoebel and Tim F. Cooper

Epistasis between mutations is host-dependent for an RNA virus
by Jasna Lalic and Santiago F. Elena

The role of ‘soaking’ in spiteful toxin production in Pseudomonas aeruginosa
by R. Fredrik Inglis, Alex R. Hall and Angus Buckling

Experimental evolution of multicellularity using microbial pseudo-organisms
by David C. Queller and Joan E. Strassmann

Model and test in a fungus of the probability that beneficial mutations survive drift
by Danna R. Gifford, J. Arjan G. M. de Visser and Lindi M. Wahl

Evolution of clonal populations approaching a fitness peak
by Isabel Gordo and Paulo R. A. Campos

Evolutionary rescue of a green alga kept in the dark
by Graham Bell

Competition and the origins of novelty: experimental evolution of niche-width expansion in a virus
by Lisa M. Bono, Catharine L. Gensel, David W. Pfennig and Christina L. Burch

Related Content

Discussion Meeting issue ‘Genetics and the causes of evolution: 150 years of progress since Darwin’ organized and edited by Michael Bonsall and Brian Charlesworth

‘Genomics of Adaptation’ Guest Edited by Professor Jacek Radwan and Dr Wiesław Babik

Evolution articles
Special Feature articles

Free articles on the genomics of adaptation

October 30, 2012 • 11:00 am

The Proceedings of the Royal Society (B) has a special issue on the genomics of adaptation that it’s making available for free to everyone. You can see the contents here; they include these articles, which can be accessed directly from my links.

Introduction: The genomics of adaptation
by Jacek Radwan and Wieslaw Babik

Research article: Genomic consequences of multiple speciation processes in a stick insect
by Patrik Nosil, Zach Gompert, Timothy E. Farkas, Aaron A. Comeault, Jeffrey L. Feder, C. Alex Buerkle and Thomas L. Parchman

Review: How does adaptation sweep through the genome? Insights from long-term selection experiments
by Molly K. Burke

Review: Gene duplication as a mechanism of genomic adaptation to a changing environment
by Fyodor A. Kondrashov

Review: The probability of genetic parallelism and convergence in natural populations
by Gina L. Conte, Matthew E. Arnegard, Catherine L. Peichel and Dolph Schluter

Giant arthropods, then and now

October 25, 2012 • 7:43 am

by Greg Mayer

The Naturmuseum Senckenberg in Frankfurt am Main, one of Europe’s great natural history museums, has announced the discovery in Laos of one of the world’s largest known daddy longlegs by Senckenberg researcher Peter Jager. The apparently new species is now being studied by Jager and his Senckenberg colleague, Ana Lucia Tourinho. Daddy longlegs are also called harvestmen, although I grew up with lots of daddy longlegs, and never knew them to be called harvestmen except in books.

(The current incarnation of the Senckenbergischen Naturforschenden Gesellschaft incorporates the museum in Frankfurt and several other German natural history museums and research institutes.)

Many people mistake daddy longlegs for spiders. While both spiders and daddy longlegs are eight-legged arachnids, daddy longlegs have a more compact body with the abdomen and cephalothorax not separated by a constriction as in spiders, and their legs are invariably thread-like. The new giant form from Laos, which has not yet been formally described as a new species, has a leg span of over 33 cm. The record is 34 cm for a South American species.

Although science fiction films abound in giant arthropods or arthropod-like aliens (my favorite: Attack of the Crab Monsters), it is in fact hard for an

Attack of the Crab Monsters (1957), original poster from Wikipedia.

arthropod to get very large. The size limitation of arthropods is due to constraints on their chitinous exoskeletons and their ability to breathe. The University of California, Berkeley, website Understanding Evolution has great animations explaining these size constraints. Both kinds of constraints arise from the disproportion of size and shape as animals get bigger. Basically, many of the functions of animals (including their weight bearing skeleton and their respiratory surfaces) increase as the square of their increase in size, whereas their need for the functions increases as the cube of their size. So, if you double the size of an animal, without changing its shape, you will quadruple its surface area, but its volume will increase by a factor of eight. Thus if you have a physiologically important surface area (say the lining of your respiratory system), you are going to be lacking by a factor of two, as your oxygen needs are set by your volume. The “square/cube problem” in biology has long been known, and J.B.S. Haldane, one of the founders of modern evolutionary theory, wrote an influential popular article on the subject, “On being the right size“, in 1926. (Haldane, by the way, a geneticist-physiologist-soldier-pacifist-communist-Hindu-atheist-patriot-expatriate, was the original most interesting man in the world, as we’ve noted before here at WEIT.)

The largest insects today are not very big. They grew larger in the distant past, and this is thought to be related to a higher concentration of oxygen in the atmosphere at that time (35% during the Carboniferous period, compared to 21 % today). The largest known arthropod ever, also an arachnid, but an aquatic one, a sea scorpion or eurypterid from the Devonian, was discovered by Simon Braddy and colleagues a few years ago (pdf).

Giant arthropods from the fossil record compared with the average height of a (British) human male; (a) the eurypterid Jaekelopterus rhenaniae, Early Devonian, Germany; (b) the trilobite Isotelus rex, Late Ordovician, Manitoba, Canada; (c) the dragonfly Meganeura monyi, Late Carboniferous, France; (d) the millipede Arthropleura armata, Late Carboniferous, Europe. Scale bar (a–d), 50 cm. (e) Chelicera of the giant eurypterid J. rhenaniae from the Early Devonian of Willwerath, Germany, PWL 2007/1-LS. Photograph, the disarticulated fixed (above) and (rotated) free ramus (below). Scale bar, 10 cm.

They were 2.5 m long, and longer if you stretched out their claws (and imagine the size of the turds produced by d!). This is, as shown in the figure above somewhat larger that the typical British male (who, I must say, appears admirably buff and well-muscled in this outline drawing; perhaps it’s all that wrist-wrestling, or is it elbow-bending, down at the pub).

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Braddy, S.J., M. Poschmann, and O.E. Tetlie. 2008. Giant claw reveals the largest ever arthropod. Biology Letters 4:106-109  (pdf)

Haldane, J.B.S. 1926. On being the right size. Harper’s Magazine (March) 424-427. (retyped pdf)

h/t Andrew Sullivan