Tag: lizards

Hangin’ on in the wind: Natural selection, hurricanes, and lizards

July 27, 2018 • 2:10 pm

by Greg Mayer

Colin Donihue at the Anolis Symposium, 17 March 2018.

At the Anolis Symposium at Fairchild Tropical Botanic Garden in March, one of the stars of the show was Colin Donihue of Harvard University, who gave a talk on the effect of last fall’s Hurricane Irma on Anolis scriptus, the endemic (and only native) anole of the Turks and Caicos. Colin and collaborators had chanced to visit and measure the morphology of the lizards just before the hurricane struck, and were able to return within weeks to see what had happened.

And something had happened. After Irma, the lizards had bigger toepads, longer arms, and shorter hind legs. The first two changes made sense—bigger toepads and longer arms are known to increase clinging ability in anoles– but the third seemed contrary to the first two. Longer legs would help them cling to the vegetation, and thus prevent them from being blown against the rocks or out to sea– so why did the ones with shorter legs survive better?

It was Colin’s exploration of this last question that made his talk one of the hits of the Symposium. In order to see the effect of Irma on the lizards, they used a garden leaf blower to simulate high winds, and recorded it all on video!

The video above is from Nature (not what Colin showed us in March), where the paper by Colin and colleagues will soon appear (already available online; there’s also a nice account of the field work by Colin at his website). What they have surmised, based on their leaf blower experiments, is that the hind legs of the lizards, once they’ve lost their grip on a perch, act as ‘sails’, catching the wind, and thus carrying the poor lizard away. “Yarr, ’tis an ill wind that blows a man out to sea.”

What surprised me was that the lizards held on to the last with their arms—I would have thought that they would grasp with all fours, and that the hind legs, having a greater toepad surface area, would give out last. Perhaps the wind caught their (larger) hind legs around the perch, and forced them off first, presaging the eventual cause of blowing away altogether. As expected during a round of directional selection, the variances of traits generally decreased. Also, the body condition of the lizards was good—they weren’t starving after the hurricane, supporting the idea that the differential mortality occurred at the time of the storm.

So, what we have here is a nice demonstration of natural selection, and a plausible, experimentally supported cause of the differential survival. But it is important to note that this is not a demonstration of evolution by natural selection, and the reason for that is interesting, and relates to the fact that evolutionary biologists use the term ‘natural selection’ in a number of contexts.

While natural selection is a major cause of evolution, as Fisher noted in the first sentence of his Genetical Theory of Natural Selection, “Natural Selection is not Evolution.” A short definition of natural selection, and one that I have used in classes and in print is that natural selection is “consistent differential survival and reproduction of heritable variants.” That this does not equate to evolution by natural selection can be readily seen in the case of heterozygote advantage, such as sickle cell hemoglobin in malarial environments. In such cases, the result of natural selection is that the genetic composition of the population doesn’t change—rather, it reaches an equilibrium, and stays there. There’s no evolution.

But there’s another sense in which natural selection does not imply evolution, and that is the sense used in quantitative genetics, and also very often in studies of changes in quantitative phenotypic traits (such as the study under discussion). Quantitative genetics derives from the work of plant and animal breeders (which was an important source of facts and inspiration for Darwin), and one of its key results has long been summarized  in the ‘breeder’s equation‘:

R=sh²; or

Response to selection is equal to the selection differential times the heritability (h²)

What this means is that the evolutionary change due to natural selection depends on both how much the selected organisms differ from the mean of the population (the selection differential), and what proportion of that difference is passed on the offspring (the heritability). The heritability is where genetics comes in—the variants that are hereditary have a (non-zero) heritability.

The structure of the breeder’s equation flows naturally from how breeders work. First, they pick an animal to breed from, based on its possession of desirable variation (e.g., having larger breast muscles than average for a turkey). Then, they breed it. Finally, they check to see how much of the desirable variation is present in the offspring. If the offspring are exactly like the parent in the selected trait (i.e. desirable), then heritability is 100% or 1.0. If the offspring have only half the desirable advantage of the parent (say, being 4 ozs. larger than average, as opposed to 8 ozs. larger in the selected parents), then the heritability is 50% or .5. So in these two cases, selection leads to evolution. So where’s the problem?

The problem, or rather conceptual subtlety, is that the heritability may be 0—the offspring of the selected parents may not differ at all from the general mean of the population. Thus we can have selection, but no response to selection, and thus no evolution. So, although natural selection is often defined as I did above (consistent differential survival and reproduction of heritable variants), it is often the case that we can measure the differential survival before we know whether or not the variation is hereditary. And that’s what the breeder’s equation captures—the two-step nature of differential first, inheritance second.

The same two-step sequence of observation often applies in nature as well as on the farm or in the lab, and thus, ‘natural selection’ is often used in the sense of the differential, with the heritability evaluated separately (as it usually must be, since the observation of a phenotypic difference does not generally imply anything, one way or the other, about heritability).

As regards the measurement of selection differentials, Colin’s study has the very nice feature that the measurements were taken within the same generation; i.e. no reproduction had occurred—the second set of measurements were taken on lizards that had lived through the hurricane. This allows them to exclude certain other possible explanations—e.g., phenotypic plasticity—for the change in average morphology. A similar advantage accrued to the classic studies of natural selection in Darwin’s finches by the Grants and their collaborators. The Grants had the additional advantage that their birds were individually marked, so that the individual identities of surviving birds were known; on the Turks and Caicos, the same generation of adult lizards was sampled before and after the hurricane, and some individuals might indeed have been measured both times, but as the lizards were unmarked, individuals cannot be followed over time.

The next step for Colin is to return to the Turks and Caicos, to see if the morphological shifts persist into the next generation, thus supporting that evolution by natural selection has occurred—i.e., that the offspring resemble the selected (=surviving) parents. This could be complicated by the fact that, with the selective environmental force (Irma) now gone, there may be directional natural selection back toward the previous trait means. Thus, measuring the persistence of the observed change may be confounded by further changes occurring. As in the Darwin’s finches studies, a multi-year approach is called for.

The lizard traits that were studied are likely to be at least moderately heritable, as morphological features such as these are usually found to be so. There have been few studies of heritability in anoles, and there have been conflicting results. Using common garden experiments, Shane Campbell-Staton has found that critical thermal maximum, a physiological trait, is heritable in Anolis carolinensis; but Mike Logan has recently reported that heritability was low for other thermally-related traits in Anolis sagrei. Studies of the heritability of morphological traits in anoles should be a fruitful area of inquiry. One advantage the Grants had is that, using the information on pedigrees provided by individual marking, they measured the heritabilities of a number of quantitative phenotypic traits in the populations of Darwin’s finches they have studied.

Campbell-Staton, S.C., S.V. Edwards, and J B. Losos. 2016.Climate-mediated adaptation after mainland colonization of an ancestrally subtropical island lizard, Anolis carolinensis. Journal of Evolutionary Biology 29:2168-2180. link  (links marked ‘link’ may not be to full text)

Donihue, C.M., A. Herrel, A.-C. Fabre, A. Kamath, A.J. Geneva, T.W. Schoener, J.J. Kolbe and J.B. Losos. 2018. Hurricane-induced selection on the morphology of an island lizard. Nature in press. link

Fisher, R.A. 1930. The Genetical Theory of Natural Selection. Oxford University Press, Oxford. full text

Grant, P.R. and B.R. Grant. 2014. 40 Years of Evolution: Darwin’s Finches on Daphne Major Island. Princeton University Press, Princeton, New Jersey.

Logan, M.L., J.D. Curlis, A.L. Gilbert, D.B. Miles, A.K. Chung, J.W. McGlothlin, and R.M. Cox. 2018. Thermal physiology and thermoregulatory behaviour exhibit low heritability despite genetic divergence between lizard populations. Proceedings of the Royal Society B 285 (1878): 20180697. link

Mayer, G.C. and C.L. Craig. 2013. Theory of evolution. pp. 392-400 in S.A. Levin, ed. Encyclopedia of Biodiversity, 2nd ed., volume 3, Academic Press, Waltham, Mass.

The horror, ctd.

November 12, 2014 • 2:15 pm

by Greg Mayer

We’ve had occasion previously to note some dastardly beings that eat lizards, and express dismay at their foul deeds. And now, thanks to Matthew, we have another opportunity to engage in a two minutes hate towards a transgressor.

Wasp spider (Argiope brunnichi) with common lizard (Lacerta vivipara) in Cheshire, UK, from Phil @Goldenorfephoto.
Wasp spider (Argiope bruennichi) with common lizard (Lacerta vivipara) in (probably) Cheshire, UK, from Phil @Goldenorfephoto.

That’s a wasp spider (Argiope bruennichi), a recent invader of Great Britain from the Continent. The picture was tweeted by Phil @goldenorfephoto. Phil is based in Cheshire, which is up where the NE corner of Wales touches England. So if he took the photo locally, then the spider is expanding its range rapidly (or, as Matthew put it to me when pointing this out, “pdq”), as can be seen from the map below, where it’s still quite a ways from Cheshire.

Distribution of Argiope bruennichi in the UK (from britishspiders.org.uk).
Distribution of Argiope bruennichi in the UK (from britishspiders.org.uk).

The tragic victim is a common lizard, Lacerta vivipara, one of the two three species of lizard native to Britain (the others being the much rarer sand lizard, Lacerta agilis, and the legless slow worm, Anguis fragilis; lizard count updated thanks to comment below by reader Dave). You can see some less traumatic photos of them here.

The picture reminded me very much of the large American orb weaving spider Nephila clavipes, which I have encountered in the West Indies and Central America, and which also eats lizards. The Nephila I’ve seen in the tropics are more yellow, rather than the orangey Florida ones pictured at the link above.  Nephila and Argiope are both orb weavers  (I didn’t realize they had big ones in England), and are sometimes placed in the same family.

h/t Matthew ~

Moar on lizards eating fruit

December 2, 2011 • 12:19 am

by Greg Mayer

I’ve previously noted a recent paper about fruit eating lizards that wind up as bird fodder. Fortunately, the cases I’m about to relate here don’t end tragically in an avian maw. The lizards that I study, anoles, are primarily insectivorous, but eat a modest amount of meat and fruit as well. I’ve seen the Jamaican Anolis opalinus eat runny banana (the banana had been sliced and left out to attract birds), Puerto Rican Anolis cristatellus pursue round, red fruits (pursue because the fruit kept rolling away as the lizard tried to grab it), and Virgin Island A. cristatellus defecate purplish feces with small black seeds (perhaps from Turk’s cap cactus, Melocactus).

My colleague Manuel Leal of Duke has posted at Anole Annals a video of Bahamian Anolis sagrei eating fruit. This is a tough-skinned fruit. It was opened by another type of lizard (a curly tail or lion lizard), and the sagrei is nomming the pulp left behind.

The video was taken by Dave Steinberg. For more on anole feeding, see Jon Losos’s book, Lizards in an Evolutionary Tree, noted here previously at WEIT.

The horror

November 28, 2011 • 7:56 am

by Greg Mayer

Lizards, as Grace Slick used to say, are the crown of creation. It thus is always a sadness to learn that horrible, predatory birds are eating them. And, what’s more, it turns out that seeds in the lizards’ stomachs wind up in the birds’ stomachs, which then eject the seeds in their pellets; this turns out to be an important form of dispersal for the plants.

Gallotia galloti, feeding on a plant. (c) Beneharo Rodriguez

These results are in a paper in press in the Journal of Ecology by David Padilla and colleagues in the Canary Islands. The lizards are members of a very interesting genus, Gallotia, which is endemic to the Canary Islands. They are in the family Lacertidae, which will be familiar to British and European readers of WEIT as wall lizards, sand lizards, and the like.

Shrike holding a now, sadly, ex-lizard. Seeds in the lizard's stomach my be dispersed by the bird. (c) Gustavo Pena

The BBC has covered the story, and has more superb pictures.

h/t Dominic

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Padilla, D.P., A. Gonzalez-Castro and M. Nogales. 2011. Significance and extent of secondary seed dispersal by predatory birds on oceanic islands: the case of the Canary archipelago. Journal of Ecology in press. pdf

Live-bearing lizards

October 13, 2011 • 5:44 pm

by Greg Mayer

One of the standard things we learn about animals are their modes of reproduction: budding, egg-laying, live-bearing, etc. And one of the standard things we “know” about modes of reproduction is that mammals are live-bearing, and reptiles lay eggs. Neither of these things we “know” is true, though– they are generalities, with exceptions. The platypus and its cousins the echidnas are fairly well known as egg-laying mammals, but that many lizards and snakes are live-bearers is not well known. Lizards and snakes are actually quite adept at evolving viviparity: over 100 instances of independent (i.e. convergent) evolution of live-bearing are known among lizards and snakes, versus only a single (or perhaps two) instances in mammals.

For many years, our foremost student of reptilian live-bearing has been Daniel Blackburn of Trinity College in Connecticut. In a paper in press in the Journal of Morphology, he and Alexander Flemming of Stellenbosch University report the most mammal-like placenta yet found in a reptile.

Detail from Fig. 8F, showing juxtaposition of fetal (vc) and maternal (uc) capillaries.

In most placental reptiles, exchange of nutrients, gases, and wastes occur through juxtaposition of fetal and maternal tissues, but not by direct contact with maternal capillaries. In the African skink Trachylepis ivensi, they have now found that this does occur, a condition previously thought  to occur only in mammals. Money quote:

Histological study shows that this species has evolved an extraordinary placental pattern long thought to be confined to mammals, in which fetal tissues invade the uterine lining to contact maternal blood vessels.

This species of skink is not very well known. Blackburn and Flemming did their histological studies on a small series of preserved specimens housed in the scientific collections of the Port Elizabeth Museum in South Africa.

h/t Dominic, Matthew Cobb

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Blackburn, D.G. 2006. Squamate reptiles as model organisms for the evolution of viviparity. Herpetological Monographs 20: 131-146. (abstract)

Blackburn, D.G. and A.F. Flemming. 2011. Invasive implantation and intimate placental associations in a placentotrophic african lizard, Trachylepis ivensi (Scincidae). Journal of Morphology in press. (abstract)

Blackburn, D.G., L.J. Vitt and C.A. Beuchat. 1984. Eutherian-like reproductive specializations in a viviparous reptile. Proceedings of the National Academy of Science (USA) 81:4860-4863. (pdf)

Burrowin’ lizards, Batman!

May 19, 2011 • 1:42 pm

by Greg Mayer       (Update below)

Lizards are far and away the most species-rich group of living reptiles, with over 7000 species. One of the first things you learn if you’re a little boy interested in such creatures is that snakes are lizards. One of the other things you learn is that snakes are not the only group of legless lizards. There are, in fact, many groups of lizards with reduced or missing legs, such as the European slow worm and American glass snakes (now preferably called glass lizards).  Snakes are just the most evolutionarily successful such group of lizards, comprising 3000 or so of the species of lizards. One of the most distinctive of the non-snake legless lizards are the worm lizards, or amphisbaenians, a group of about 150, mostly tropical, burrowing species. Perhaps our greatest student of the group, the late Carl Gans, thought them so distinctive that he championed a classification in which they were ranked equally with lizards and snakes within the Squamata (the taxon which includes lizards and all their derivatives, including snakes and amphisbaenians), although most other workers did not accept this ranking.

A worm lizard, Amphisbaena sp.

Gans wrote in his Biomechanics (he was a functional morphologist and physiologist as well as a systematist) that:

Unfortunately, we lack fossils intermediate between the Amphisbaenia and other groups, and can only speculate what their ancestors looked like.

A paper published in Nature today by Johannes Muller and colleagues (abstract only) goes a long ways towards constraining our speculations. In the paper, they describe a new species of lizard from the Eocene Messel shale of Germany (Messel is a famous lagerstatte: a deposit with extraordinary fossil preservation) as a transitional form from ‘normal’ lizards to the amphisbaenians.
Cryptolacerta hassiaca, holotype, from Nature 473:365.

Ever since Charles L. Camp’s 1923 classic, “Classification of the lizards”, amphisbaenians have bounced around a bit in terms of who their closest relatives are (this proposal being the most heterodox), but recent molecular work (summarized here and here by Blair Hedges and Nicolas Vidal) has connected them to the Lacertidae, a group of typical-looking Old World lizards (‘lacerta’ is Latin for ‘lizard’). In describing the new species, known from a single, well-preserved, and nearly complete specimen, Muller and colleagues write that the species shows “a mosaic of lacertid and amphisbaenian anatomical characters”. The skull, like that of amphisbaenians, is strongly constructed, and evidently adapted for a semi-fossorial life, while the limbs, though well developed proximally, are fairly short and have miniaturized digits. The body is not elongated. Morphometric comparison to modern lizards show that Cryptolacerta was likely a cryptic, leaf litter dwelling form.

Thus, the burrowing head evolved before the fully fossorial life style, while the body was as yet unenlongated, and the limbs still fairly well developed. We should not be surprised to find limbs in a transitional form from the well-limbed lacertids, but it is also the case that three extant species of worm lizards, the members of the Mexican genus Bipes, retain short front legs. Though very short, the limbs are well-developed for mole-like burrowing.

Amphisbaena sp. (left) and Bipes biporus

The New York Times has a story on this, which gets the gist of the story right, but the headline (“Fossil Sheds Light on the Lizard-Snake Divide”) and lede (“The origin of snakes is a perplexing matter”) are way off: the paper concerns the origin of amphisbaenians, not snakes.

h/t: Matthew Cobb

UPDATE: Burrowing lizards seem to be all the rage this week, as alert readers Dominic and James C. Trager have pointed out two other burrowing lizard events in the comments below. First, a new species of blind skink, Dibamus, has been described by Thy Neang and colleagues in the journal Zootaxa (BBC piece here). There are about ten species of dibamids, which lack forelimbs, but have flap-like hindlimbs. Like amphisbaenians, they have bounced around a bit in their classification; the latest work (see papers by Hedges and Vidal below) places them as the earliest branch within the lizards. I’m not sure why this new species merited news coverage, except insofar as all new species are newsworthy. One of the authors of the new species is Lee Grismer, whose alpha taxonomic exploits we’ve noted here at WEIT before.

The second item is a paper by Steve McAlpin and colleagues at Macquarie University in Plosone, describing heretofore unknown complexity in lizard social behavior (NY Times piece here). I’ll let the abstract speak for itself:

Here we provide the first example of a lizard that constructs a long-term home for family members, and a rare case of lizards behaving cooperatively. The great desert skink, Liopholis kintorei from Central Australia, constructs an elaborate multi-tunnelled burrow that can be continuously occupied for up to 7 years. Multiple generations participate in construction and maintenance of burrows. Parental assignments based on DNA analysis show that immature individuals within the same burrow were mostly full siblings, even when several age cohorts were present. Parents were always captured at burrows containing their offspring, and females were only detected breeding with the same male both within- and across seasons. Consequently, the individual investments made to construct or maintain a burrow system benefit their own offspring, or siblings, over several breeding seasons.

Complex social behavior is well known in crocodilians and, of course, birds (which are glorified reptiles), but this is a unique case for squamates (so far). They don’t seem to be eusocial though, which, in addition to overlapping generations, requires cooperative care of the young (there is at least some indirect parental care here), and a reproductive division of labor. The skinks involved are burrowing, but well-limbed.

A social skink, Liopholis kintorei, from Australia. Adam Stow,via NY Times.

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Camp, C.L. 1923. Classification of the lizards. Bulletin of the American Museum of Natural History 48:289-481. (pdf)

Hedges, S. B. and N. Vidal. 2009. Lizards, snakes, and amphisbaenians (Squamata). Pp. 383-389 in S. B. Hedges and S. Kumar, eds., The Timetree of Life,  Oxford University Press, New York. (pdf)

McAlpin, S., P. Duckett and A. Stow. 2011. Lizards cooperatively tunnel to construct a long-term home for family members. Plosone 6(5):e19041, 4pp. (pdf link)

Muller J., C.A. Hipsley, J.J. Head, N. Kardjilov, A. Hilger, M.Wuttke and R.R. Reisz. 2011 Eocene lizard from Germany reveals amphisbaenian origins. Nature 473:364-367. (abstract)

Neang, T., J. Holden, T. Eastoe, R. Seng, S. Ith, and L.L. Grismer. 2011. A new species of Dibamus (Squamata: Dibamidae) from Phnom Samkos Wildlife Sanctuary, southwestern Cardamom Mountains, Cambodia. Zootaxa 2828:58-68. (abstract)

Vidal, N. and S. B. Hedges. 2009. The molecular evolutionary tree of lizards, snakes, and amphisbaenians. Compte Rendus Biologies 332:129-139. (pdf)

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).