Category: Articles

Nabokov was right all along

January 27, 2011 • 7:31 am

This is a really cool result: a paper published 66 years ago, speculating about the evolutionary history of a group of butterflies, has just been vindicated by a combination of new molecular and ecological work.

But what is even cooler is that the author of the earlier paper was Vladimir Nabokov.

You know Nabokov (1899-1977) as a famous writer, author of, among other books, Lolita, Pnin, Pale Fire, and Speak, Memory.  He also taught literature at Cornell from 1948 to 1959; his classroom lectures have been published and they are absolutely superb.

But what you probably didn’t know was that he was also a world-class lepidopterist, specializing in the “alpha taxonomy” (description and publication of new species) of butterflies.  From 1945-1948 he was Curator of Lepidoptera at Harvard’s Museum of Comparative Zoology (MCZ), the institution where I got my Ph.D.  If you read his novels, you’ll often find a mention of butterflies.

Here’s Nabokov at the MCZ in 1945, from the article on his butterfly work at the New York Public Library Site:

Nabokov’s speciality was the butterfly subfamily Polyommatinae (called “blues” because of their color) in the family Lycaenidae.

Lycaeides melissa, subspecies samuelis. A “blue” from the northeast U.S.  The species was first described by Nabokov.

In 1945 Nabokov published a formal description and revision of some lycaenid butterfies in Psyche, the journal of the Cambridge Entomological Club (I’m a member!).  Here’s the title of his paper:

The paper is long (62 pages), and largely devoted to describing species based on their genitalia, the character that seems to evolve most rapidly among insects—and many other groups, probably because of sexual selection (see William Eberhard’s excellent Sexual Selection and Animal Genitalia).  At the end of the paper he speculates, based on the morphology of the various species, that the “blues” arrived in the New World from Asia over the Bering Strait.  He further speculated that there were actually five successive invasions from Asia, each giving rise to a different New World group.  And the first invasion produced species that made it to Central and South America, with the North American representatives eventually becoming extinct.

A group of investigators from a bunch of places, led by my friend Naomi Pierce at Harvard, just tested Nabokov’s theory with the modern tools of molecular biology and systematics.  Their results, published in a paper in the Proceedings of the Royal Society (free access!) and described by Carl Zimmer in yesterday’s New York Times, show that Nabokov was right on all counts.

Here’s what the authors found:

  • Using six genes, they made a molecular phylogeny of 73 species of blues from all five “sections” described by Nabokov.  This analysis showed that, indeed, the five separate invasions posited by Nabokov each produced a monophyletic group (i.e., a group descended from a single ancestral species). The butterflies also descended, as Nabokov posited, from Asian ancestors, and certainly came to the New World by migration over the Bering Strait.
  • The oldest group was, as Nabokov suggested, the butterflies in the neotropics: Central and South America.  The other groups were younger, and in the precise age sequence that Nabokov posited had arrived from Asia (and then radiated in North America).
  • The branches of the phylogeny were dated using a “molecular clock.” They were found to have begun about 11 million years ago, and continued until about 1 million years ago.  During the earliest part of that period the Bering Strait was not continuous between Asia and North America, and so some of the ancestral blues must have crossed an expanse of ocean.
  • The authors then determined the temperature-tolerance ranges of each group based on the climate where its members are now living.  From that, they reconstructed the temperature tolerance of each group’s ancestors. (You can do this for any measurable trait, morphological or physiological, using a method called “ancestral character state reconstruction”.)  And they found that the temperatures tolerated by each successive invader declined over time.  The earliest invader tolerated higher temperatures than the next, and so on for all five invasions.
  • The spiffy result: the temperature tolerances posited for each ancestor matched very nicely the temperatures thought to have prevailed in “Beringia” (the region on either side of the Bering Strait) at the time of the invasions.  Here’s part of the authors’ Figure 1, showing the temperature tolerances of ancestors during each of the five invasions (vertical bars), and the temperatures posited to have existed in Beringia, all over the 11 million years from the first to the last invasion:


Note the good match between the actual temperatures in Beringia and the temperatures tolerated by the ancestral invader of each of the five groups. Clearly, the butterflies now in the neotropics tolerated high but not low temperatures, and so were likely driven to the south as the climate became colder.  This also implies that each group has pretty much retained the ancestral temperature tolerance of its ancestor, though the ancestral five groups did diverge from one another in tolerance.

What a lovely piece of work, and how nice that the modern work, using all the highfalutin tools of molecular biology, systematics, ancestral state reconstruction, and so on, managed to confirm the speculations of one itinerant zoologist/author equipped only with a microscope and a bunch of butterfly genitals!


h/t: Carl Zimmer for providing pdfs.

____________

R. Vila, C. D. Bell, R. Macniven, B. Goldman-Huertas, R. H. Ree, C. R. Marshall, Z. Bálint, K. Johnson, D. Benyamini, and N. E. Pierce.  2011.  Phylogeny and palaeoecology of Polyommatus blue butterflies show Beringia was a climate-regulated gateway to the New World.  Proc. R. Soc. B published online before print January 26, 2011, doi:10.1098/rspb.2010.2213

Dogs are smarter than you think

January 11, 2011 • 9:09 am

. . . well, at least one dog: Chaser, a female border collie born in 2004.  Because of their marked inferiority to felids (the King of Pets), I don’t usually feature goggies on this website.  But this bit of research, published in Behavioural Processes, was too good to pass up.

When I lived in Scotland, the one television show I never missed was the BBC’s “One Man and His Dog,” in which border collies and their owners would vie for a prize in sheepherding. Despite my indifference to dogs, I was fascinated at the skill with which these dogs herded errant packs of sheep using commands from their owners.  I was sad to hear that the show was canceled, though Wikipedia says it’s still alive. (UK readers: is it?)

Border collies are clearly alert and intelligent beasts, and this new paper demonstrates it, showing that they have a stunning ability to learn and (supposedly) to combine nouns and commands, an ability to recognize that objects have names, and a talent for distinguishing different commands about how to deal with those objects.

The paper, bearing the turgid title of “Border collie comprehends object names as verbal referents” (free online, and you can see a summary/press release here), is by John Pilley and Alliston Reid, two psychologists at Wofford College in Spartanburg, South Carolina.  I don’t want to produce a long summary, for the paper is well written and easily comprehensible to laypeople. Further, you can read the press release for a shorter take.  I’ll just discuss the salient points here.  Do consult the paper if you’re worried about controls, etc., since the experiments did seem well controlled.

Also, the link to the paper will take you to four videos (on the right) that you can play to see Chaser’s talents for yourself.

Fig. 1.  Chaser, the erudite border collie

Over a period of three years, Pilley and Reid trained Chaser to recognize various objects: toys, stuffed animals, plastic items, etc., by telling the dog to “go to” that item and fetch it. (They eliminated the “clever Hans” effect by having the owner give orders when out of sight of the dog.)  Once Chaser had learned to fetch a number of these items, they did further experiments. There were four in total.

  • First, the learning of names.  Chaser’s ability here was astounding: at the end of the training period, she had learned the names of 1,022 objects, and was able to reproduce them faithfully, almost without error.   In one series of tests, for example, a group of 20 of the 1022 objects would be dispersed randomly on the floor.  Chaser was then asked to select one item out of the 20.   Then he would be asked to select another without replacement (order random, of course), and then another, until all 20 were gone.  This was done in more than 50 successive trials, until all 1000-odd objects had been used.  This meta-test often took many hours.

Amazingly, in no test—and there were many of them—did Chaser make more than two mistakes (in other words, she always got at least 18 objects correct). And she retained this ability to remember names for at least two years after training, as shown by retesting when she was five.

Here are some of the objects Chaser learned, with their names on the left (click to enlarge):

  • Second, understanding new combinations of different words. What seems to be the Big Result of the paper, but one that doesn’t completely convince me, is the authors’ contention that Chaser “understood the separate meanings of proper-noun names and commands.”  What they did was first train Chaser to perform three actions, apparently using objects that were not part of her previously-learned repertoire. These commands were “take” (i.e., fetch), “paw,” and “nose.”  Then, once the commands were learned, Chaser was given combination commands using only three of the 1,022 objects combined with a requested action.  For example, “take lamb,” or “nose lips” (this was an object resembling human lips), or “paw ABC” (a cloth cube with those letters on it).  Note that the dog had learned the commands and the objects separately, and had never been given a directive that combined them.  This was her first exposure to the two-word commands.

There were 14 trials (see Table 1 of the paper), and Chaser did the right thing all 14 times.  The cumulative probability that this would happen by chance alone is 0.000000000000044.  It would have been nice to do this with all 1,022 objects to get a better judgment on Chaser’s “combinatorial” abilities, but this is still telling. Chaser was obviously able to combine an action command with a noun command, demonstrating (to the authors) that she has “combinatorial understanding.”  As the authors say, “She responded as though the commands and the proper-noun names were independent entities or morphemes Thus, in effect, Chaser treated phrases like ‘fetch sock’ as though the ‘sock was a sock and not a ‘fetch sock’—indicting [sic?] that her nouns referred to objects.”

This does demonstrate combinatorial abilities, which is pretty remarkable.  My only quibble is whether Chaser understood the term “sock” as a noun by itself, or rather as a combination noun, “go to [i.e., “fetch”] sock”, which is the way she learned “sock”; and that this combination-noun was overriden by a third term, like “nose,” that preceded the object. In other words, while I’m convinced of Chaser’s combinatorial abilities, I’m not convinced that she learned that the objects really were nouns that weren’t attached to verb commands.

  • Third, learning different noun categories.  Chaser was trained to recognize not just the 1,022 names of objects, but also their classification under the rubric “toys.”  She was also taught to distinguish these from a large number of other objects that she had not been allowed to play with.  After training, 8 toys and 8 non-toys were strewn about and, with the investigator hidden, Chaser was asked to “fetch a toy.”   She was then asked again, and the objects were not replaced after being fetched.  She performed perfectly.

Beyond this broad category, Chaser was also trained to recognize those 116 objects that were balls under the name of “ball.”  She was likewise trained to recognize 26 of her disk-like toys under the name of “frisbee.”  She was again tested with 8 balls and 8 “non-balls”, and also with 8 frisbees and 8 “non-frisbees” (i.e., “fetch a frisbee”).   In both cases she performed perfectly, even though she knew each of those objects not only as “ball” of “frisbee”, but also by their unique name and the general name of “toy.”  Here is a sample trial showing 8 frisbees and 8 non-frisbees:

  • Fourth, learning words by exclusion.  In the last experiment, Chaser was asked to retrieve novel objects with novel names.  There were 64 of these novelties, which differed from the 1022 objects whose names she had already learned.  One of the novelties was placed with seven familiar objects.  In the first two commands, Chaser was asked to fetch a familiar object. In the third, she was asked to bring a novel object with a novel name, one she hadn’t heard before.  This forced her to discriminate by exclusion.  She was successful eight times out of eight.  Remarkable! However, this ability to remember novelty decayed quicky: in other tests, conducted immediately after the successful novelty trial, then ten minutes later, then 24 hours later, she was often unable to pick the novel named item out of a group of four novel and four familiar items.  After 24 hours, her memory for the novel ones had decayed completely.

I think even caninophiles would be surprised by Chaser’s talents, which, of course, could probably be seen in other border collies—though not necessarily other dog breeds.  Her abilities far exceeded anything necessary or useful in ancestral canids—dogs, after all, don’t have to remember names in the wild, though they certainly do have to recognize different conspecific individuals (but not 1022 of them!).  Part of her performance is probably due to a co-option of brainpower used for other things (just like humans can learn to read music using neurons evolved for other reasons), and part to the fact that border collies are trained to recognize different commands.  I’m not an expert on dogs, and I bet there are border-collie owners among the readers, so by all means recount your experiences or theories about the dogs.

Of course, any random cat could do exactly what Chaser did—and much more.  It just wouldn’t want to!

_______

h/t: Matthew Cobb

Pilley, J. W., and A. K. Reid. 201o.  Border collie comprehends object names as verbal referents.  Behavioural Processes, in press. doi:10.1016/j.beproc.2010.11.007

“Psychic” paper provokes backlash

January 7, 2011 • 5:02 am

Okay, not really on psychic powers, but on precognition.  Last October I posted about Daryl Bem’s paper in the Journal of Personality and Social Psychology, “Feeling the future: Experimental evidence for anomalous retroactive influences on cognition and affect,” (download a preprint on his webpage).  I summarized its results as follows:

The paper purports to show that a choice that you make in a computer test can be influenced by stimuli you receive after you’ve already made the choice.  This implies you have some way, consciously or unconsciously, of detecting things that haven’t yet happened.

I also mentioned some criticisms of these results by others; and many of the hundred-odd comments were also critical.

Yesterday’ss New York Times reports a strong backlash by scientists about the paper.

“It’s craziness, pure craziness. I can’t believe a major journal is allowing this work in,” Ray Hyman, an emeritus professor of psychology at the University Oregon and longtime critic of ESP research, said. “I think it’s just an embarrassment for the entire field.”

The editor of the journal, Charles Judd, a psychologist at the University of Colorado, said the paper went through the journal’s regular review process. “Four reviewers made comments on the manuscript,” he said, “and these are very trusted people.”

All four decided that the paper met the journal’s editorial standards, Dr. Judd added, even though “there was no mechanism by which we could understand the results.”

But many experts say that is precisely the problem. Claims that defy almost every law of science are by definition extraordinary and thus require extraordinary evidence. Neglecting to take this into account — as conventional social science analyses do — makes many findings look far more significant than they really are, these experts say.

“Several top journals publish results only when these appear to support a hypothesis that is counterintuitive or attention-grabbing,” Eric-Jan Wagenmakers, a psychologist at the University of Amsterdam, wrote by e-mail. “But such a hypothesis probably constitutes an extraordinary claim, and it should undergo more scrutiny before it is allowed to enter the field.”

Dr. Wagenmakers is co-author of a rebuttal to the ESP paper that is scheduled to appear in the same issue of the journal.

Wagenmakers commented on my post and gave a reference to his own paper rebutting that of Bems.

The report goes on:

Peer review is usually an anonymous process, with authors and reviewers unknown to one another. But all four reviewers of this paper were social psychologists, and all would have known whose work they were checking and would have been responsive to the way it was reasoned.

Perhaps more important, none were topflight statisticians. “The problem was that this paper was treated like any other,” said an editor at the journal, Laura King, a psychologist at the University of Missouri. “And it wasn’t.” . .

. . . So far, at least three efforts to replicate the experiments have failed. But more are in the works, Dr. Bem said, adding, “I have received hundreds of requests for the materials” to conduct studies.

I’ll bet big bucks that the effect vanishes, simply on the grounds that a mechanism for precognition seems unlikely. Perhaps the paper did deserve more thorough reviewing given its controversial nature, but the truth will out.  And in the the advocates of precognition will be on even weaker ground.

The dubious arsenic bacterium

December 8, 2010 • 9:26 am

I didn’t post on the bacterium that supposedly evolved to incorporate arsenic into its DNA because I was ill, late to the party, and, frankly, not really equipped to judge that paper, which was published in Science.  In yesterday’s Slate, however, Carl Zimmer wades into the fray, talking to a number of scientists who attacked the study, including Rosemary Redfield of the University of British Columbia, who published a withering takedown of the original paper. Zimmer’s piece is straightforwardly called “This paper should not have been published.”

Zimmer’s interviewees are pretty unanimous in claiming that the evidence that arsenic was really incorporated into the DNA—the paper’s major finding—is unconvincing.  It might well have been a contaminant. Critics blame the paper’s appearance on shoddy science, credulous authors, and poor reviewers.

Zimmer asked two of the paper’s authors to respond to these criticisms, and they refused, claiming that they weren’t going to debate their results in the media.  As Zimmer reports, others see this as evasion:

While Redfield considers Wolfe-Simon’s research “flim-flam,” she think it’s fine for the NASA scientists to hold off responding to their critics. She is working on a formal letter to Science detailing her objections. But Jonathan Eisen of UC-Davis doesn’t let the scientists off so easily. “If they say they will not address the responses except in journals, that is absurd,” he said. “They carried out science by press release and press conference. Whether they were right or not in their claims, they are now hypocritical if they say that the only response should be in the scientific literature.”

Eisen’s right.  It’s incumbent on the authors, who flogged their paper via press conference, to at least give some sort of public response, if only to say that they’re looking into it and repeating their experiments.  This is a new era of science, in which reaction to a paper by fellow scientists can be virtually instantaneous, and not always pretty.  I, for one, welcome it.  I’d rather know now rather than later if there are problems with this “new life form.”  Further, these critiques and counter-critiques aren’t always easy to find when they’re in the scientific literature: many journals bury them somewhere in the online version.

How, and how fast, did the human brain evolve?

November 24, 2010 • 2:29 pm

by Greg Mayer

While in Colombia last week, Jerry directed my attention to a paper by Roy Britten (abstract only free) in that week’s issue of the Proceedings of the National Academy of Sciences. Britten is a venerable figure in evolutionary molecular genetics, one of the pioneers of DNA-DNA hybridization who helped elucidate the structure of the genome long before sequencing was possible. The paper was indeed interesting. This post is a bit longer and more data-and-analysis-laden than usual, but I think the paper merits discussion.

Britten summarizes his latest paper’s conclusions succinctly:

The aim of this paper is an explanation for the high speed of evolution of the human lineage, which has been exceptional compared with other animals. The high speed of evolution of human lineage brain size is recognized by comparison of fossil brain sizes (1, 2). Evolution of the lineage leading to humans during the last several million years was striking. … A major source of variation [for brain evolution] has been the insertion of transposable elements (TEs).

He goes on to note that besides rapid brain evolution, humans have many TE insertions. For him

This is an extraordinary correlation. Human evolution has been rapid, particularly brain evolution in the last several million years. It is the only species known to make such rapid evolutionary progress. Now it is shown that human is the only species studied to have so many TE insertions. Recognition of this correlation leads to the concept that Alu insertions underlie rapid human evolution.

Importantly, he states up front that:

We believe the brain evolution was due to natural selection and genomic variation.

He is thus not seconding Colin Blakemore’s unwarranted claim that brain size is a neutral character, conferring neither selective advantage nor disadvantage, that must therefore have evolved via genetic drift. Britten is definitely not saying this, and is thus not open to the criticisms of Blakemore made by Jerry (here and here) and John Hawks. Britten thinks a big brain is advantageous.

So, how well supported are the claims he does make? I’d first note that one data point does not a correlation make, especially not an extraordinary one. TE’s, by increasing mutation rates, can certainly increase evolutionary rates, but all sorts of other singular characteristics apply to the Homo lineage. In addition to having many TE’s, they were savannah-dwelling, nearly hairless, bipedal, etc. Which of these correlates with rapid brain growth is the important one? I don’t know, but a more forceful argument than simple occurrence in the same lineage would be needed to establish which is the most likely causal factor.

And what about the factual points? Much of the paper is devoted to establishing the prevalence of TE’s in the human lineage, and I, as at least a first approximation, would yield to Britten’s expertise on this point. What about the rate of brain size evolution? (To be fair, Britten takes his cue here from the literature; the high rate is a premise, not a conclusion.) G.G. Simpson, one of the founders of modern evolutionary biology, spent a major part of his career documenting the variability of evolutionary rates. He showed that there is great variability of evolutionary rates between lineages, among characters within lineages, and within lineages at different times. The following figure is based on an original in Simpson’s 1953 Major Features of Evolution. It shows that the rate of evolution in lungfish was high about 300 million years ago, but not so much at other times (i.e. variation within lineages at different times).

Rate of lungfish evolution, from Mark Ridley's Evolution, 2004.

Historically, claims of human exceptionality have tended to become less exceptional when examined more closely. Huxley’s debate with Owen over the brain is perhaps the classic example: contra Owen, Huxley “showed that the brains of apes and humans were fundamentally similar in every anatomical detail.” Knowing this, I decided to check on this important premise of Britten’s paper. Is the speed of evolution of the human brain “striking”, and “exceptional compared with other animals”? In a word, no: over the last several million years, human brain size has evolved at rates which are typical of paleontologically measured evolutionary rates.

Here’s a graph by John Hawks (using the same or similar data as Lee and Wolpoff, 2003) showing the pattern of change in cranial capacity (on the vertical axis, in cubic centimeters) over the last 2 million years or so.

There are a number of ways of measuring evolutionary rates of morphological features such as cranial capacity. One useful measure is the haldane, developed by Phil Gingerich (1993) of the University of Michigan Museum of Paleontology, based on a suggestion made in 1949 by the original most interesting man in the world, the great geneticist-physiologist-soldier-pacifist-communist-Hindu-atheist-patriot-expatriate J.B.S. Haldane, one of the founders of modern evolutionary theory. The haldane is the change, on a logarithmic scale, of the feature in question in units of the standard deviation (a measure of how variable the feature is), per generation.

Using data from three papers on modern human cranial capacity, I found the average to be 1345 cubic centimeters (cc), calculated as the unweighted average of males and females from the measured populations from Korea, Turkey, and Nigeria (a total sample of 1151). 1.8 million years ago, the cranial capacity of the Homo lineage was 702 cc, calculated as the average of skulls of that age from Perning, Kenya (one each, given by Lee and Wolpoff, 2003), and Dmanisi (three skulls, given by Gabunia et al., 2000, for two of them, and the median of two estimates by Lee, 2005, for the third).

The logarithmic standard deviation is well approximated by the coefficient of variation (CV: the untransformed standard deviation divided by the mean; Lewontin, 1966). The unweighted average CV for the modern humans was .0955, and for the five early Homo it was .1149; averaging, we get .1052.

So, the amount by which cranial capacity has changed on the log scale is ln(1345)-ln(702)=.6502; dividing this by the estimated logarithmic standard deviation, .1052, gives 6.181. In the last 1.8 million years, our cranial capacity has increased about 6 standard deviations. We don’t know generation time for early Homo, but we do know it for modern humans (about 25-30 years) and chimps (19-24 years; Matsumura and Forster, 2008). Using 25 years as an estimate for the whole lineage, we get 72000 generations in the last 1.8 million years, giving an evolutionary rate for brain size of .00008584 (or 10^-4.066) haldanes.

Is this a high rate or a low rate? Neither– it’s absolutely typical for evolutionary rates measured over this generational time scale. Gingerich (2001) compiled a large data set on rates of evolution, measured in haldanes, over a wide variety of time scales. He states:

Macroevolutionary studies yield rates on the order of 10^-2–10^-6 haldanes calculated over intervals of geological time ranging from 10^2–10^6 generations.

If we look more precisely, at about 72000 (10^4.86) generations, we find measured rates of about 10^-3.5 to 10^-6.5. So the rate of human brain evolution is above the median, but nothing “exceptional”. Could this unexceptional result be due to the particular initial time (1.8 mya) selected? What if we looked further back in time? I redid the analysis using an average chimpanzee cranial capacity of 383 cc (McKee et al., 2005), and a divergence time of 6 million years. Using the same logarithmic standard deviation and generation time, we get  [ln(1345)-ln(383)]/.1052 = 11.940 standard deviations, nearly twice the 6 standard deviation change in the last 1.8 million years. Dividing by the 6 million years/25 years per generation = 240000 generations gives .00004975 (or 10^-4.3) haldanes. Again, above the median, but nothing exceptional.

So, there’s nothing much remarkable about the speed of human brain size evolution. If the TE’s (or hairlessness or bipedalism or whatever) of the Homo lineage had an effect on human evolution, it was not expressed as an unparalleled increase in the rate of evolution of cranial capacity.

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Acer, N., M. Usanmaz, U. Tugay, and T. Ertekin. 2007. Estimation of cranial capacity in 17-26 years old university students. Int. J. Morphol. 25:65-70. pdf

Britten, R.J. 2010. Transposable element insertions have strongly affected human evolution. Proceedings of the National Academy of Science 107:19945-19948.

Gabunia, L., et al. 2000. Earliest Pleistocene hominid cranial remains from Dmanisi, Republic of Georgia: taxonomy, geological setting, and age. Science 288:1019–1025.

Gingerich, P. D. 1993. Quantification and comparison of evolutionary rates.  American Journal of Science 293A: 453-478. pdf

Gingerich, P. D. 2001. Rates of evolution on the time scale of the evolutionary process. Genetica 112-113: 127-144. pdf

Haldane, J.B.S. 1949. Suggestions as to quantitative measurement of rates of evolution. Evolution 3:51-56.

Hwang, I.-L., et al. 1995. Study on the adult Korean cranial capacity. Journal of Korean Medical Science 10:239-242. pdf

Matsumura, S. and P. Forster. 2008. Generation time and effective population size in Polar Eskimos. Proc. R. Soc. B (2008) 275:1501–1508. pdf

McKee, J.K.,  F.E. Poirier, and W.S. McGraw. 2005. Understanding Human Evolution. Pearson, Upper Saddle River, New Jersey.

Lee, S.-H. 2005. Is variation in the cranial capacity of the Dmanisi sample too high to be from a single species? American Journal of Physical Anthropology 127:263–266. pdf

Lee, S.-H. and M.H. Wolpoff. 2003. The pattern of evolution in Pleistocene human brain size. Paleobiology 29:186-196. pdf

Lewontin, R.C. 1966. On the measurement of relative variability. Systematic Zoology 15:141-142.

Odokuma, E.I., P.S. Igbigbi, F.C. Akpuaka, and U. Esigbenu. 2010. Craniometric patterns of three Nigerian ethnic groups. African Journal of Biotechnology  9:1510-1513. pdf

Simpson, G.G. 1953. Major Features of Evolution. Columbia University Press, New York.

Why is sex good?

November 22, 2010 • 5:57 pm

by Greg Mayer

And by sex, I mean, of course, “… the union (SYNGAMY) of two genomes, usually carried by gametes, followed some time later by REDUCTION, ordinarily by the process of meiosis and gametogenesis” (Futuyma, 2009:388). Most of the organisms we know and love– oak trees, lobsters, goldfish, cats–  reproduce sexually. But a few of our favorite organisms– whiptail lizards prominent among them– reproduce asexually.

Cnemidophorus inornatus (sexual ancestor), C. neomexicanus (unisexual daughter species) , C. tigris (sexual ancestor). c Alistair J. Cullum. Used with permission.

At first glance, what the asexual whiptails are doing makes complete evolutionary sense: why bother producing unproductive males, when you can double your reproductive output by having nothing but daughters?  If we start a sexual population with one male and one female, and suppose that females on average have four surviving offspring, two of whom will be female, then the population increases from 2 to 4 to 8 to 16 to… etc. If we start with two asexual females, who also average four surviving offspring, all of whom are female, the population increases from 2 to 8 to 32 to 128 to… etc. You can see that asexuals reproduce a lot faster than sexuals. And it wouldn’t matter if the population wasn’t increasing– the asexuals would come to constitute a higher and higher proportion of the total population. This reproductive advantage of asexuality is called the cost of sex (google image that term for an interesting mix of scientific and non-scientific illustrations!).

So if sex has such a high reproductive cost, why are so many organisms sexual?  This is where the whiptails are revealing. Tod Reeder,  C.J. Cole, and Herb Dessauer, in their 2002 review of Cnemidophorus evolution, found that

the capability of instantly producing parthenogenetic clones through one generation of hybridization has existed for approximately 200 million years, yet the extant unisexual taxa are of very recent origins. Consequently, these lineages must be ephemeral compared to those of bisexual taxa.

Indeed, the asexual whiptails have evolved so recently that the ancestral sexual forms can in most cases be readily identified (see figure 6 in Reeder et. al). That asexual taxa are of recent origin appears to be true for animals in general (with some notable exceptions):  asexuality appears to be an evolutionary dead end. This implies that there is some long term advantage to sexuality, so that asexual species do not prosper and diversify, but rather are extinguished. The paucity of asexuals, despite their large reproductive advantage, argues for a short term advantage to sex as well. There have been a number of suggestions, most supposing that sex is advantageous in fluctuating or changing environments, so that sexual lineages would have higher fitness than asexual lineages within a population.

An essay by Matt Ridley posted at the PBS website for their Evolution series of a few years ago considers some of these issues, as does this page by someone at Brown University, and Nature has an open article collection on the subject. Two of the classic introductions to the subject are Sex and Evolution by George C. Williams, and The Evolution of Sex by John Maynard Smith.

Hybridization and parthenogenesis in whiptail lizards

November 19, 2010 • 12:26 am

by Greg Mayer

Not much in the way of culinary pleasures here. (Although Jerry’s piece on the Inquisition killed my appetite, anyway). Reader Pete Moulton asked for some references on hybridization and parthenogenesis in whiptail lizards (Cnemidophorus [or Aspidoscelis] and related teiid lizards), in particular C. (A.) uniparens.

Desert grassland whiptail, by Davepape from Wikipedia.

A. uniparens is a triploid unisexual. It resulted from a cross of two bisexual species (A. inornata [mother] and A. burti [father]), which produced a diploid unisexual, which then backcrossed to inornata to produce the triploid uniparens. The unisexuals reproduce clonally, i.e. offspring are exact genetic copies of their mothers, except for new mutations. Courtship and ‘pseudocopulation’ between parthenogenetic females promotes reproduction. The situation is summarized nicely by Cole et al. (2010):

The natural origin of diploid parthenogenesis in whiptail lizards has been through interspecific hybridization. Genomes of the parthenogens indicate that they originated in one generation, as the lizards clone the F1 hybrid state. In addition, hybridization between diploid parthenogens and males of bisexual species has resulted in triploid parthenogenetic clones in nature. Consequently, the genus Aspidoscelis contains numerous gonochoristic (= bisexual) species and numerous unisexual species whose closest relatives are bisexual, and from whom they originated through instantaneous sympatric speciation and an abrupt and dramatic switch in reproductive biology.

The selection of papers below includes both classics and recent papers, with a preference towards ones where online full text was available (see pdf links below). These papers are all about whiptails of the family Teiidae. Laurie Vitt and Jana Caldwell, in their fine text Herpetology (Academic Press 2009), record about 50 species of  parthenogenetic lizards (adding in a few they missed) in eight families (including the whiptails), and one species of parthenogenetic snake.

Wright, J.W. and C.H. Lowe. 1968. Weeds, polyploids, parthenogenesis, and the geographical and ecological distribution of all-female species of Cnemidophorus. Copeia 1968: 128-138. no pdf (A classic on unisexual ecology.)

Parker, E.D. and R.K. Selander. 1976. The organization of genetic diversity in the parthenogenetic lizard Cnemidophorus tesselatus. Genetics 84:791-805. pdf (A classic on unisexual genetics.)

Crews, D. and K.T. Fitzgerald. 1980. “Sexual” behavior in parthenogenetic lizards (Cnemidophorus). Proceedings of the National Academy of Science USA 77: 499-502. pdf (A classic on unisexual behavior.)

Reeder, T.W., H.C. Dessauer, and C.J. Cole. 2002. Phylogenetic relationships of whiptail lizards of the genus Cnemidophorus (Squamata, Teiidae) : a test of monophyly, reevaluation of karyotypic evolution, and review of hybrid origins. American Museum Novitates 3365:1-62. pdf (In this paper, the genus Aspidoscelis is resurrected for part of the genus Cnemidophorus; because there is such a huge literature under the name Cnemidophorus prior to 2002, both names must be used when searching the literature. The part on hybrid origin begins on page 25.)

Cole, C.J., L.M. Hardy, H.C. Dessauer, H.L. Taylor, and C.R. Townsend. 2010. Laboratory hybridization among North American whiptail lizards, including Aspidoscelis inornata arizonae × A. tigris marmorata (Squamata: Teiidae), ancestors of unisexual clones in nature. American Museum Novitates 3698:1-43. pdf

The American Museum of Natural History’s Digital Library has pdf’s of all the Museum’s publications, and it has been a center for studies of parthenogenetic lizards. More papers can be found by going to the Digital Library site and searching on ‘Cnemidophorus’, ‘Aspidoscelis’, and ‘parthenogenesis’.

UPDATE. The numbers of parthenogenetic species of lizards and snakes compiled by Vitt and Caldwell and given above refers only to obligately (or nearly so) parthenogenetic species, not facultatively parthenogenetic ones (like Komodo dragons, boa constrictors, and some other snakes; they have a separate discussion of the facultative species in their book).