Italian Wall Lizards

"although usually present, the HG valves in P. siculus may be flexible in their morphology or even in their presence, and they are not fixed features in the Pod Mrčaru lizards." — Wehrle, Herrel, German and colleagues, 2020¹

Summary

In 1971 ten Italian wall lizards were moved from one Adriatic islet, Pod Kopište, to a smaller one nearby, Pod Mrčaru.  When biologists returned in 2004 to 2006, the lizards had turned to eating plants, grown bigger heads and a harder bite, and developed cecal valves, folds across the gut that slow food so microbes can ferment it.  The 2008 paper in PNAS was widely reported as a new organ evolving in 36 years.  This article reads that paper and every follow-up that could be found, and scores the case by the rules of Measuring Biological Information.

1. The observations are sound. Plants made up 34% to 61% of the Pod Mrčaru diet against 4% to 7% on Pod Kopište. Bite force was higher, density about five times higher, and territories gone. The native Dalmatian wall lizard had vanished from the islet.^{2 2}
2. The discoverers never claimed the change was genetic. Their paper says that plasticity and maternal effects "are needed" as further studies.²
3. The valves are not new to lizards. They occur in plant-eating iguanas, agamids and skinks, in other plant-eating lacertids, and in some mainland Balkan green lizards.^{3 4}
4. The valves behave like a response to diet. Twelve of 16 wild Pod Mrčaru lizards had them, and none of 20 kept on insects for 15 weeks did.⁵ A later study by the same group called them "not fixed features."¹
5. The head and bite differences did not persist in a common garden. Plasticity was among "the main drivers" at this stage.⁶ Whether valves were tested in that garden could not be read.
6. The genome shows a founder event and a handful of outlying loci, none tied to a trait.⁷
7. The popular accounts left out the open question, and the creationist replies mostly got it right. The press release and Dawkins's book did not mention it. Answers in Genesis predicted in 2008 that the valves would "dwindle" on an insect diet, and they did, although several creationist writers got the anatomy and genetics wrong.⁸
8. Comparable cases are mixed. Florida green anoles kept larger toepads in a common garden, and Bahamian brown anoles' legs proved partly plastic.^{9 10}

Verdict.  The case is not yet scoreable as gain, loss, modification or transfer, because no inherited change has been shown.  It cannot be cited as an observed new organ, and it cannot be dismissed, because a working valve that appears when needed is a capacity that itself needs explaining.  It fits the young-earth claim that animals were made able to adapt, and says nothing yet about inherited diversification.  One experiment would settle it:  hatch eggs from both islets and raise them on both diets.

Part 1: What was done and what was found

The 1971 move

In the summer of 1971 a team led by Eviatar Nevo was studying competition between two lizards on the small islands of the Adriatic.  The Italian wall lizard, Podarcis siculus, and the Dalmatian wall lizard, Podarcis melisellensis, rarely share a small island.  Nevo's paper opens with that pattern:  "Competetive exclusion between Lacerta sicula and L. melisellensis characterizes the small islands of the Adriatic Sea."¹¹  Earlier introductions had been followed up that summer.  On one island "the introduced species appears to be replacing the native form," and the paper closes by announcing "a reciprocal introduction involving the two species on the islands of Pod Kopiŝte and Pod Mrĉaru."¹¹

The experiment was about competition, not diet.  Five adult pairs of Italian wall lizards were moved from Pod Kopište to Pod Mrčaru, an islet nearby that was held by the Dalmatian species.²  Nothing about their guts was recorded at the time.

The return, 2004 to 2006

Anthony Herrel, Katleen Huyghe, Bieke Vanhooydonck, Thierry Backeljau, Karin Breugelmans, Irena Grbac, Raoul Van Damme and Duncan Irschick visited both islets in spring and summer of 2004, 2005 and 2006, and published in PNAS in March 2008.²  Their findings, taken from the paper itself:

1. The native species was gone. Twice-yearly visits "show that this species has become extinct on Pod Mrčaru."² The competition experiment had its answer.
2. The new lizards descend from the old ones, as far as the test could show. Two stretches of mitochondrial DNA from seven Pod Mrčaru lizards and eight Pod Kopište lizards were "genetically indistinguishable."² Two short mitochondrial genes cannot rule out a later arrival from elsewhere, and the paper's rate figures say they "assume no additional colonization of the island by P. sicula."²
3. The diet had changed. Stomach flushing of 330 lizards showed plants making up 34% of the food by volume in spring and 61% in summer on Pod Mrčaru, against 7% and 4% on Pod Kopište.² About half the plant matter on Pod Mrčaru was leaves and stems, which are hard to digest.²
4. Heads and bites were bigger. Pod Mrčaru lizards had longer, wider and taller heads, and not only because they were larger overall. Males bit with 11.25 newtons against 9.85, females with 6.70 against 4.99.²
5. The gut had valves. Dissection of preserved lizards found "cecal valves (Fig. 4) in all individuals, including a hatchling (26.4-mm snout-vent length, umbilical scar present) and a very young juvenile (33.11-mm snout-vent length) examined from Pod Mrčaru."² A cecal valve is a fold across the start of the large intestine. It slows food down and makes a chamber where microbes can ferment plant fibre.
6. The gut had worms. Nematodes "were common in the hindgut but absent from individuals from Pod Kopište."² The authors read this as a sign of a new partnership with gut microbes.
7. There were more lizards and fewer territories. Unbaited traps caught 0.05 lizards per trap per hour on Pod Mrčaru and 0.01 on Pod Kopište, and the Pod Mrčaru lizards "do no longer appear to defend territories."²

What the paper does not say

Three gaps matter for what follows.  The paper does not say how many lizards were dissected for valves.  "All individuals" could be a handful or dozens.  It does not say how many Pod Kopište lizards were opened and found without valves.  For the absence of valves in other P. siculus it cites an earlier paper by the same group rather than a count.²  And it gives no basis for its generation count.

Years and generations

The paper's headline is "in only 36 years (≈30 generations)."²  Thirty-six years runs from 1971 to 2007, when the paper was written.  The lizards were measured between 2004 and 2006, 33 to 35 years after the move.  Thirty generations in 36 years means a new generation every 1.2 years.  The paper does not say where that figure comes from.  A later genomic study repeats "~30 generations" and also gives no derivation.⁷  The count may be too high if these lizards breed first in their second year, but no study found gives the age at first breeding on these islets.  The arithmetic in Part 6 uses 20 to 30 generations to cover that doubt.

Two numbers that do not agree

Herrel's rates reach "up to 8,593 darwins or 0.049 haldanes."²  Neither figure is the largest in the paper's own Table 1, where female body mass reaches 9,441 darwins and female head width 0.051 haldanes, and no cell reads 8,593.²  The islet areas also disagree between papers.  Herrel gives Pod Kopište as 0.09 km² and Pod Mrčaru as 0.03 km².²  Maxime Taverne and colleagues, from the same research group eleven years later, list 7,915 m² and 2,931 m², about a tenth as large.¹²  The ratio of the two islets agrees.  Neither error changes the story, but both show why numbers from this case should be taken from the tables and not from summaries.

What later visits added

Later work by the same group and its partners filled in the picture.  Bart Vervust and colleagues, working in April 2008, found wider teeth, longer stomachs and small intestines, slower passage of food and better digestion in Pod Mrčaru lizards.⁵  Transect counts put the density at about 3,082 lizards per hectare on Pod Mrčaru against 1,045 on Pod Kopište.¹³  The gut bacteria differ:  plant-eating Pod Mrčaru lizards carry more kinds of microbe and more of the methane-making Methanobrevibacter.¹³  Three-dimensional models of the skull show that small changes in shape and jaw muscle together raise bite force.¹⁴  Beck Wehrle, Herrel, Donovan German and colleagues found that Pod Mrčaru lizards digested plant diets better in the laboratory, but found no cellulase activity "anywhere in their guts."^{1 1}

None of this is disputed.  A small population, founded by ten animals, changed its diet, its head, its gut, its microbes, its numbers and its social life inside four decades.  What is disputed is what kind of change it was.

Part 2: Are cecal valves new?

How rare they are

Herrel's paper stresses how unusual the structure is:  "The fact that <1% of all currently known species of squamates have cecal valves (13, 14) illustrates the unusual nature of these structures in this population."²  Squamates are lizards and snakes together.  The two works cited for that figure are the group's own 2004 survey of plant-eating lacertids and J. B. Iverson's 1980 study of iguanas.  Iverson's abstract gives no percentage, and the 2004 paper could not be read in full, so the figure rests on Herrel's authority.³

Iverson's work shows what the valves are.  Every iguana species he examined except the marine iguana has "from one to eleven transverse valves in the proximal colon," and they "apparently evolved as simple infoldings of the medical colic wall."³  "Medical" is the abstract's misprint for medial.  In 1980, "Herbivorous species of the Scincidae, Agamidae, and Iguanidae are the only lizards known to exhibit colic partitioning."³  So the valve is a fold of the gut wall, and it goes with plant eating across three families that are only distantly related.  That pattern alone says the capacity to make it is widespread in lizards.

They are known in the lizard's own family

Wall lizards belong to the Lacertidae.  In 1980 no lacertid was known to have valves.  By 2008 Herrel's group could write that the Pod Mrčaru valves "are similar in overall appearance and structure to those found in herbivorous lacertid, agamid, and iguanid lizards."²  The same paper calls the structure "rare in this family."²  Rare is not absent.

A second genus makes the point plainly.  Sagonas, Pafilis and Valakos dissected Balkan green lizards, Lacerta trilineata, from islands and from the mainland in 2015.  "The dissection of the hindgut revealed an unexpected finding, the presence of cecal valves that were more frequent in island lizards."⁴  More frequent on islands means present on the mainland too.  In that species the valve is a variable trait that some individuals have and others lack, and its frequency tracks where the lizards live.

The question list for this article named the Canary Island lizards, Gallotia, and the Balearic wall lizards, Podarcis lilfordi and P. pityusensis, as places to look for valves.  All of them eat a great deal of plant matter.¹⁵  No report of valves in them was found in the sources read here.  The lacertid valves that Herrel's 2004 survey describes may include some of them, but that paper was read only in abstract.¹⁶  This is a gap in this article, not evidence of absence.

Plant eating is common on small islands

Pod Mrčaru is not the only island where P. siculus turned to plants.  Taverne's survey of twenty Adriatic populations found diets "ranging from strictly insectivorous populations (e.g., Brusnik or Mala Palagruža) to almost completely herbivorous populations (e.g., Sušac or Pod Mrčaru)," and named Sušac, Pijavica and Pod Mrčaru as the plant-eating P. siculus islands.¹²  Valentin Pérez-Mellado and Claudia Corti wrote in 1993 that "Among lacertid lizards, plant consumption is fairly common in insular populations."¹⁵  A small island with few insects pushes lizards toward plants again and again.

What this means

The valve on Pod Mrčaru is new to that population and was not seen in its source.  It is not new to lizards, not new to lacertids, and not new as a kind of structure.  It is a fold of the gut wall of a type that appears wherever lizards eat leaves.  That is consistent with two readings.  Either the ability to make the fold is an old developmental capacity that a plant diet switches on, or a similar mutation has been selected many times.  The first reading needs no new genetic information on Pod Mrčaru.  The second would.  The experiments in Part 3 were designed to tell them apart.

Part 3: Genetic change or plasticity?

What the discoverers said

Herrel's team did not claim to have shown a genetic change.  Their paper says so in its discussion:

"Although the presence of cecal valves and large heads in hatchlings and juveniles suggests a genetic basis for these differences, further studies investigating the potential role of phenotypic plasticity and/or maternal effects in the divergence between populations are needed."²

Phenotypic plasticity is the ability of one set of genes to build different bodies in different conditions.  A maternal effect is a trait a mother passes on through the egg or her body rather than through her genes.  The hatchling with valves is the paper's best evidence for inheritance, and it is weak.  A hatchling on Pod Mrčaru has a Pod Mrčaru mother, eats Pod Mrčaru food from its first days, and picks up Pod Mrčaru gut microbes.  One animal cannot separate those causes.

Test 1, 2010: take the plants away

Bart Vervust, Panayiotis Pafilis, Efstratios Valakos and Raoul Van Damme ran the first test, partly by accident.  They caught lizards on both islets in April 2008.  In freshly caught animals, "In 12 of the 16 individuals (75%) examined from Pod Mrčaru, the hindgut contained cecal valves. In sharp contrast, we found no such valves in any of the 27 individuals from Pod Kopište."⁵  So even in the wild, one Pod Mrčaru lizard in four had no valve.

Twenty Pod Mrčaru lizards, eight males and twelve females, were taken to Antwerp and fed only arthropods.  After 15 weeks a power cut killed them:  "These 20 lizards had died of oxygen deficiency following a power cut in the animal house at Antwerp University."⁵  The team dissected them.  "In addition, we found no evidence of cecal valves in any of the twenty specimens that had been fed arthropods."⁵  At the wild rate of 75%, about fifteen of the twenty should have carried valves.  None did.  Their small intestines and hindguts were also lighter.⁵

The authors drew the conclusion carefully:

"Many comparative studies have tacitly assumed that the distinctive features of plant-eating lizards (large body size, skull dimensions, special dentition, gut morphology) are a product of genetic adaptation to the special demands of a plant-based diet (e.g., Van Damme 1999; Cooper and Vitt 2002; Espinoza et al. 2004; Herrel et al. 2008). Our results suggest that in P. sicula, at least some of the changes associated with a dietary shift toward a higher proportion of plant material may be plastic."⁵

Four limits apply.  The same animals were not examined before and after, so the valves are inferred to have gone, not watched going.  The text says 15 weeks, while the notes to two of the paper's tables say 6 months.⁵  Some differences survived the diet, because the laboratory Pod Mrčaru lizards still had longer guts than wild Pod Kopište lizards.⁵  And only one direction was tested.  Nobody has fed plants to Pod Kopište lizards to see whether valves grow.

Test 2, 2020: count again, across years

Beck Wehrle and colleagues opened more guts over several years.  They found valves in 28 Pod Mrčaru lizards "spanning four seasons over three additional years," and none in 29 from Pod Kopište.  But three Pod Mrčaru lizards examined in summer 2013 "did not have clear epithelial folds (valves) in their HGs."¹  HG is their abbreviation for hindgut.  They concluded that "although usually present, the HG valves in P. siculus may be flexible in their morphology or even in their presence, and they are not fixed features in the Pod Mrčaru lizards."¹  Herrel is a co-author of that sentence.

On the digestive differences as a whole, the same paper states the open question in plain terms:  "We currently do not know whether the observed changes represent flexibility (Piersma and Drent 2003) or evolved differences with underlying genetic changes (Garland and Adolph 1991)."¹  It adds that better digestion of plants "may be mediated through the microbiome of the HG (i.e., not necessarily an evolved difference in the lizards themselves; Garland and Adolph 1991)."¹

Test 3, 2024: raise them side by side

The experiment that the 2008 paper asked for has now been done for the head and the bite.  Iva Sabolić, Anamaria Štambuk, Herrel and twelve others combined genome data with a crossing experiment and a common garden, which means raising animals from both populations under the same conditions.⁶  Their abstract reports:

"Diverged traits related to head size and shape or bite force showed moderate heritability in a crossing experiment, but between-population differences in these traits did not persist in a common garden environment. Our results confirm the existence of sufficient additive genetic variance for traits to evolve under selection while also demonstrating that phenotypic plasticity and/or genotype by environment interactions are the main drivers of population differentiation at this early evolutionary stage."⁶

The head and bite differences that made the 2008 paper famous did not persist when lizards from both islets grew up in the same place.  Head size can be inherited within a population, but the difference between the islands is mainly a response to how the animals live.  Only the abstract of this paper could be read.  It does not say whether guts or valves were scored in the common garden.  The full text sits behind a publisher paywall, and Sabolić's 2021 doctoral thesis, which reports the same work, sits behind an automated robot check on the University of Zagreb repository.  Reading one of them is the most important open task for this article.

Test 4, 2023: look at the genome

Stéphanie Sherpa and colleagues read about 82,000 short stretches of DNA from 24 lizards on Sušac, Pod Kopište and Pod Mrčaru.  They confirmed the founder event:  Pod Mrčaru has the least variation and the most alleles fixed by chance.⁷  Their scan "identified just a handful of loci showing large allelic shifts between ecologically divergent populations," and they drew the conclusion that the differences "are determined by a small number of large‐effect loci and/or that phenotypic plasticity plays a major role."⁷  Of 29 outlier loci across all their comparisons, seven lay in or near known genes.  Six were inside introns and one lay upstream of a gene.⁷  None has been tied to the valves or to any measured trait.  They also found one Pod Mrčaru lizard of mixed ancestry, "potentially indicating very recent gene flow with Sušac."⁷  Sušac is a larger island whose wall lizards also eat plants.  So Pod Mrčaru may not be a closed population.

Guts are flexible in other animals

A gut that remodels itself with diet would be no surprise.  J. Matthias Starck's review opens:  "The vertebrate gastrointestinal tract is a flexible system that can individually be modified to account for changes in amount and quality of food as well as changes in internal demands."¹⁷  In reptiles, he adds, enlarging the gut lining "does not involve cell proliferation, thus it is presumably an energetically cheap process."¹⁷  Kohl and colleagues ran the forward experiment in a South American lizard, Liolaemus ruibali.  Animals fed 90% plants grew "significantly longer small intestines and larger hindguts, demonstrating that gut morphology is phenotypically flexible."¹⁸  Across ten wall-lizard populations in the Skyros islands of Greece, islet lizards had longer guts and slower digestion, and the authors called "digestive plasticity" a key mechanism.¹⁹  None of these studies examined valves.  Among the sources read here, the Pod Mrčaru valve is the only one whose loss on an animal diet has been reported.

What has been shown, and what has not

The experiment that would settle the valve question is simple to state.  Hatch eggs from both islets in one laboratory.  Raise half of each clutch on insects and half on plants.  Open the guts at the same age.  If valves appear only in plant-fed lizards from both islets, the valve is plastic and the capacity is shared.  If they appear in plant-fed Pod Mrčaru lizards but never in plant-fed Pod Kopište lizards, the populations differ genetically in the ability to make them.  If they appear in Pod Mrčaru lizards on either diet, the trait has become fixed.  The third result is the only one that would make this a case of an inherited new structure.

Part 4: How the case is used

The popular account

The university press release that went out with the paper set the tone.  ScienceDaily ran it on 18 April 2008 under the headline "Lizards Undergo Rapid Evolution After Introduction To A New Home."  It quotes Irschick:  "Our data shows that evolution of novel structures can occur on extremely short time scales."²⁰  It does not mention that the genetic basis was untested.  It also contains two errors.  It says DNA tests showed the lizards "were genetically identical to the source population," when two short mitochondrial fragments were compared.  And it calls the gut worms "microorganisms called nematodes that break down cellulose."²⁰  Nematodes are worms.  Oddly, its own description of the valves is a plasticity claim:  "Eating more plants caused the development of new structures called cecal valves."²⁰

Kimberly Johnson's National Geographic story three days later ran the deck "The changes should take millions of years-but these creatures are doing it in mere decades," and described the valves as "muscles between the large and small intestine," which they are not.²¹  To its credit, it is the one popular piece found that reports the open question.  It quotes Andrew Hendry of McGill University, who was not involved:  "All of this might be evolution," and "The logical next step would be to confirm the genetic basis for these changes."²¹

P. Z. Myers, on his blog Pharyngula, headed his post "Still just a lizard" and answered the expected creationist reply before it came:  "yes, this population of Podarcis sicula is still made up of lizards, but they’re a different kind of lizard now. Evolution works."²²  The post does not mention plasticity or the untested genetics.

Richard Dawkins gave the lizards a section of chapter 5 of The Greatest Show on Earth in 2009, "The lizards of Pod Mrcaru."  He wrote that "those valves have actually started to evolve in the population of P. sicula on Pod Mrcaru," and called the story "evolution before our very eyes."²³  He did not mention the untested genetic basis.  He also dated the return visit to 2008, when it ran from 2004 to 2006, and he counted generations differently from the paper:  "The average generation turnover of those lizards is about two years, so the evolutionary change observed on Pod Mrcaru represents only about eighteen or nineteen generations."²³  Dawkins wrote a year before the plasticity test was published.  Nothing in the later literature has been folded into the popular account, and in 2026 Christoph Adami's book The Evolution of Biological Information was still presenting the valve as the evolution of a new organ, according to a review of it by Winston Ewert.²⁴  Adami's book was not read for this article.

Jerry Coyne's blog was searched for the case and returned no post on it.

The creationist and design replies

The young-earth and design replies mostly argued, before any test had been run, that the capacity was already there and the diet switched it on.

Answers in Genesis, 2008.  Jean Lightner, a veterinarian writing for AiG, called the study "an excellent test—and an astounding confirmation—of the creation model."²⁵  Her account of the paper is accurate.  She reports the diet figures correctly, notes that "only mitochondrial DNA was sequenced," and says the authors themselves call for work on plasticity and maternal effects.  On the valves she wrote that the structure "is present in other herbivorous lizards in this family (Lacertidae)" and "may reappear if conditions change and it then becomes important to survival."²⁵  She also wrote that "36 years is hardly enough time to suspect that a beneficial random mutation might have occurred."²⁵  Part 6 prices that claim.  It holds for any one named mutation and fails for the population as a whole.

Answers in Genesis, News to Know.  An unsigned AiG piece quoted the anatomist David Menton.  He said the lizards "did have the ability to digest plant material," that the valve "is simply an enlargement of muscles already present in the gut wall at this juncture," and that "if the lizards were returned to their original habitat, the cecal valve feature may dwindle as the lizards returned to an insectivorous diet."⁸  The anatomy is wrong in detail.  Menton took "muscles" from National Geographic.  Iverson describes colic valves as infoldings of the gut wall, and Wehrle's team describes them as folds of the lining.^{3 1}  But the prediction came true in substance.  A year later Vervust's twenty lizards on an insect diet had no valves.  Menton predicted it for a return to the old island, and the test was a laboratory diet, but the result is the one he forecast.

Creation Ministries International, 2014.  Calvin Smith quoted CMI's Don Batten:  "It is significant that the cecal valve is present in other herbivorous lizards in this family (Lacertidae), so it is not surprising that this particular species has the ability to produce cecal valves under certain conditions."²⁶  Batten's point comes straight from Herrel's paper and is sound.  Smith's own summary overstates twice.  The lizards did not go "from predominantly carnivorous to vegetarian";  plants were a third to three-fifths of their food.  And the inference that "the DNA sequence of the newer lizards seemed identical to the parent population" rests on two mitochondrial fragments, which say nothing about the nuclear genes that would build a valve.²⁶

Evolution News, 2026.  Winston Ewert argued that "The invading species already had the information necessary to produce the valve," switched off in insect eaters and on in plant eaters, whether "due to a genetic mutation, an epigenetic marker, or environmental triggers."²⁴  That conclusion fits the later evidence well.  His facts are loose.  He says the lizards' "genomes" were sequenced, calls the valve "a muscle," and says "many other members of the same family have this structure," where Herrel says it is "rare in this family."²⁴  He does not cite Vervust, Wehrle or Sabolić, the three papers that most support him.

Uncommon Descent, 2008.  A writer using the name PaV used Motoo Kimura's substitution formula to argue that the change "cannot even begin to be explained using the Darwinian account."²⁷  The calculation fails on its own terms.  It starts from "12" founders, when there were ten.  It asserts "we know from the paper that many more fixed genes were involved," when the paper reports no nuclear genes at all.  It treats years as generations.  And its arithmetic drops the factor of 4 in its own formula.  Solving the formula as written gives 2.3, not 9.3, which is still impossible, because the deeper error is the model.  Kimura's formula prices new mutations at one site.  A population founded by ten wild animals starts with twenty copies of every gene and the variation among them, and that standing variation, not new mutation, is the ordinary route to fast change.²⁸

Institute for Creation Research.  No ICR article on Pod Mrčaru was found.  ICR's model of continuous environmental tracking, in which organisms sense conditions and adjust themselves, is the natural home for this case, and the stickleback and speciation articles discuss it.  See Sticklebacks and The Creation Model of Rapid Speciation.

How the claims score against the papers

The creationist writers read the 2008 paper more carefully than the press did.  Lightner and AiG both reported that the genetic basis was untested, and the press release and Dawkins did not.  Their central forecast, that the valves reflect a capacity switched on by diet, has so far held up better than the popular claim of a new organ evolved.  Their weaknesses are loose anatomy, overstatement of what identical mitochondrial DNA shows, and a habit of not citing the follow-up papers that help them.

Part 5: How typical is this?

Fast change after animals reach a new place is common.  The useful question for each case is the one Pod Mrčaru has not yet answered:  was the difference still there when animals from both places were raised side by side?

Bahamian anoles, limb length.  Brown anoles, Anolis sagrei, were introduced onto small Bahamian islands from a nearby source.  Jonathan Losos, Kenneth Warheit and Thomas Schoener reported in 1997 that the populations "differentiated from each other rapidly over a 10–14-year period," and that "The more different the recipient island's vegetation from that of the source, the greater the magnitude of differentiation."²⁹  Losos's own group then tested plasticity.  They raised hatchlings in cages with only broad or only narrow perches, and "lizards in the broad treatment had relatively longer hindlimbs than lizards in the narrow treatment."  They concluded that "not only is hindlimb length a plastic trait in these lizards, but that this plasticity leads to the production of phenotypes appropriate to particular environments."¹⁰  They added that the plastic range "is limited compared to the diversity shown throughout the anole radiation."¹⁰  A later experiment founded seven islands with one male and one female each and found that founder effects "persisted throughout the course of the experiment despite all populations adapting in the predicted direction-shorter hindlimbs."³⁰  Common garden:  the limb difference is partly plastic.  This is the closest parallel to Pod Mrčaru.

Florida anoles, toepads.  When brown anoles reached small islands in Florida in the mid-1990s, the native green anole, Anolis carolinensis, moved higher in the trees.  Stuart, Losos and colleagues found that it "adaptively evolved larger toepads after only 20 generations," at 0.091 standard deviations per generation for toepad area and 0.077 for the number of scales on the toe.⁹  They took pregnant females from four invaded and four uninvaded islands, hatched the eggs in the laboratory and raised the young identically.  "The effect of A. sagrei invasion on A. carolinensis toepad characteristics persisted in the common garden," which suggests a genetic difference, "though we cannot rule out trans-generational plasticity."⁹  Common garden:  the difference persisted.  This is what a demonstrated case looks like.

Cane toads, legs and speed.  Cane toads were introduced to Australia about seventy years before Phillips and colleagues reported in 2006 that the invasion front had sped up about fivefold, and that "toads with longer legs can not only move faster and are the first to arrive in new areas, but also that those at the front have longer legs than toads in older (long-established) populations."³¹  That 2006 paper did not test inheritance.  A 2016 study by Hudson, Shine and colleagues reports that the front-line toads' "offspring inherit that rapid dispersal rate," and that their skeletons have changed shape.³²  Common garden:  dispersal was shown to be inherited in later work;  the leg-length difference itself was not re-read here.

Trinidad guppies, life history.  David Reznick, Heather Bryga and John Endler moved a natural population of guppies from a place where predators eat adults to one where they eat the young.  They reported in 1990 that this "resulted in significant life-history evolution in the predicted direction after 11 years, or 30–60 generations," and that such population differences "are heritable."³³  In 1997 Reznick's team put the rate "up to seven orders of magnitude greater than rates inferred from the paleontological record," and found that males changed faster "largely because males had more genetic variation upon which natural selection could act."³⁴  Common garden:  heritability is stated in the abstracts;  the rearing design was not read for this article.  The selection coefficients for the guppy case are discussed in Haldane's Dilemma.

Sticklebacks and peppered moths.  The two best-studied cases in this series show the same pattern.  Freshwater sticklebacks lose armour in a few decades by reusing an allele already present at low frequency in the sea, and Barrett's pond experiment followed that allele through the generations.  The peppered moth's dark form is caused by an inherited insertion of mobile DNA.  Both are inherited changes from existing variation.  See Sticklebacks and Peppered Moths.  Blind cave fish show both at once:  some cave traits appear within one generation by plasticity, and eye loss is inherited.  See Blind Cave Fish.

Pod Mrčaru is typical in how fast it changed and unusual in how little of the change has been shown to be inherited.  The field has known since at least 2000 that a fast change in a lizard's body can be partly plastic, and the one case in this group that passed a common garden, the Florida toepads, is the one that should be quoted as observed inherited change in a lizard.

Part 6: Scoring and the ledger

The score

Measuring Biological Information scores a change by what happened to a coded element:  gain, loss, modification, or transfer of function.  It also says that "a change that has not been shown to help the organism is not scored at all."³⁵  Every class describes a change in DNA.  None of them has been shown here.

"Not yet scoreable" cuts both ways.  The case cannot be cited as the observed evolution of a new structure, because no inherited change has been shown, and the one experiment on the valves showed them disappearing when the diet changed.  But it cannot be dismissed as nothing either.  A fold of gut wall that forms when a lizard eats leaves, slows food, houses fermenting microbes, and goes away when the leaves stop is a working, conditional piece of engineering.  The capacity to build it is real, it was there before 1971, and it needs an explanation of its own.  Mainstream biology explains such capacities as the product of older evolution, pointing to valves arising again and again in plant-eating lizards.  The design view explains them as built-in flexibility.  Pod Mrčaru does not decide between those two, because it shows the capacity being used, not being made.

The ledger

The series asks how many mutations a population had the chance to try, and what it bought.  For Pod Mrčaru the inputs are these.

The working is in the note.³⁶  The totals are upper bounds, because the population grew from ten and was smaller for most of the period.

Two things follow.  First, Lightner's claim that 36 years "is hardly enough time to suspect that a beneficial random mutation might have occurred" is half right.  Any one named mutation very probably never arose on the islet.  But the population as a whole produced something like a million new mutations, and some of those may have been useful.  More important, fast change in a founded population does not need new mutations.  The ten founders brought twenty copies of every gene, with whatever variation Pod Kopište held, and the stickleback case shows that such standing variation is the usual raw material.  Second, the empty last cell is not a zero.  It is empty because nothing has yet been tied to a gene.  If the valves turn out to be inherited, the cell will have to be filled, and the ledger is a lower bound on what the population produced, which is the direction that would cut against the young-earth side.  For how a per-lineage count like this one is used across the series, see Benchmarking Evolution.

What the young-earth model wants from this case

Young-earth models need animals to change fast within a created kind, because hundreds of species must come from a few pairs within a few thousand years.  See The Creation Model of Rapid Speciation.  Pod Mrčaru shows that a lizard population can transform its diet, head, gut, numbers and behaviour within a few decades.  That fits the need for speed.

Which mechanism serves the model better is less simple than it looks.  Plasticity fits the claim that creatures were made able to adapt, and the creationist writers who predicted it have been borne out so far.  But plasticity by itself is not inherited.  A plastic change vanishes when the conditions change, as the valves did in Antwerp.  Diversification into lasting species after the Flood needs inherited differences that stay put.  For that, the model needs change from built-in genetic variation, the stickleback pattern, and Pod Mrčaru has not yet shown any.  So the case supports the claim of built-in flexibility and is neutral on the claim of rapid inherited diversification.  Mainstream biology has its own route from the first to the second, in which plasticity leads and genetic change follows.  Pod Mrčaru, at about 35 years, may be too young to have reached the second step.  That is the question the next common garden will answer.

Objections and responses

"The valves were in a hatchling with its umbilical scar.  That proves they are inborn."  It shows they can form early, not that they are coded differently.  A hatchling on Pod Mrčaru comes from a Pod Mrčaru mother, and what she ate while making the egg may matter.  Herrel's own paper names maternal effects as a possibility that needed testing.²  Only hatchlings from Pod Kopište eggs raised on plants, or Pod Mrčaru eggs raised on insects, can separate the causes.

"Plasticity is itself evolved, so this is still evolution."  Plasticity can evolve, and mainstream biology holds that the ability to grow valves on a plant diet arose in lizard ancestors long ago.  But that ancient event was not watched on Pod Mrčaru.  What was watched is a population using a capacity it already had.  The series counts what was observed, and an old capacity switched on is not an observed origin.

"Genome scans found real genetic change, so something evolved."  Allele frequencies did change, mostly by drift after the founding, which is evolution in the population-genetic sense and is not disputed here.  Sherpa's team found "just a handful of loci" that stood out and could not tie any to a trait.⁷  Change in allele frequency is not the same as a new function, and the scoring rules do not score a change with no shown benefit.

"Vervust's lizards were adults in a laboratory.  A lab diet for 15 weeks is not an island."  True.  The result shows that the valves can disappear, not what would happen over generations.  It is one experiment, in one direction, on 20 animals, with a discrepancy between 15 weeks in the text and 6 months in the table notes.⁵  It is still the only experiment, and its authors and the later Wehrle study both read the valves as flexible.

"Sabolić's common garden did not test the valves, so the valves may still be genetic."  Correct as far as the abstract shows.  This article therefore scores the valves "not yet scoreable," not "plastic."  If the full paper or a new study shows valves forming in Pod Mrčaru lizards raised on insects, the verdict here changes.

"Creationists just say 'it was already there' about everything."  In this case they said it in 2008 and 2009, before the first test, and made a checkable forecast:  Menton's "the cecal valve feature may dwindle" on an insect diet.⁸  The next year's experiment came out his way.  The claim was testable, and it passed its first test.

"The creationist writers get the anatomy and genetics wrong."  Several do.  Menton and Ewert call the valve a muscle, Ewert says genomes were sequenced in 2008, Smith reads identical mitochondrial DNA as showing the nuclear information was already present, and PaV's arithmetic is wrong.  Those errors are real, and a reader should not take their facts on trust.  Their central forecast survives them.

"Other introductions did show inherited change, so this one probably will too."  Florida's green anoles kept their larger toepads in a common garden.⁹  Bahamian brown anoles' legs turned out partly plastic.¹⁰  Pod Mrčaru's heads and bites turned out mainly plastic.⁶  The record in lizards is mixed, so no forecast is safe.

Issues and unanswered questions

1. The full text of Sabolić et al. 2024 was not read. Whether the common garden included gut dissection is the most important open fact in this article. The paper is paywalled, and the 2021 Zagreb thesis sits behind a robot check that this project's rules do not allow passing on a non-account site. Help from Eric, or an email to the authors, would settle it.
2. Herrel, Vanhooydonck and Van Damme 2004 was read only in abstract. It is the source for valves in other lacertids and for their absence in other P. siculus. Which lacertids, and how many animals, is not known here. The same holds for Sagonas 2015, the Lacerta trilineata valve counts.
3. No report of valves in Gallotia, Podarcis lilfordi or P. pityusensis was found. That is a gap in the search, not a finding.
4. Nobody has tested the forward direction: whether Pod Kopište lizards fed plants grow valves.
5. Generation time. Herrel's about 30 generations and Dawkins's 18 to 19 imply generation times of 1.2 and 2 years. No study found gives the age at first breeding on these islets.
6. Island areas differ tenfold between Herrel 2008 and Taverne 2019, and the rate figure in Herrel's text, 8,593 darwins, is not in his table. Nevo 1972's full text, which might give the original areas and counts, was not read.
7. The reciprocal half of Nevo's 1971 experiment, P. melisellensis taken to Pod Kopište, is announced in the 1972 summary. No later paper found says what became of it.
8. Gene flow. One Pod Mrčaru lizard carried ancestry from Sušac.⁷ If lizards from Sušac, which also eat plants, have reached Pod Mrčaru, some of the difference could be imported rather than produced.

Verdict

Pod Mrčaru is a real and remarkable case of fast change after an introduction:  ten founders, about 35 years, a new diet, bigger heads, gut valves, new microbes, five times the density and no territories.  Every one of those observations stands.

What has not been shown is that any of it is inherited.  The discoverers said the genetic basis was untested.  The only experiment on the valves found them gone after lizards ate insects for 15 weeks.  A later study by the same group found the valves "not fixed features."  A common garden found that the head and bite differences did not persist.  A genome scan found a handful of outlying loci and tied none to a trait.  So the case scores as not yet scoreable.  It cannot be cited as the observed evolution of a new organ.  It cannot be waved away either, because a gut structure that appears when needed and goes away when it is not is a capacity that needs explaining.  The honest summary is that Pod Mrčaru shows how much a lizard can change without new genes, and has not yet shown how much it can change with them.

Sources