Vertebrate

A member of the chordate lineage that includes jawless and jawed vertebrates. Features of the head, neural crest and internal skeleton vary across its descendant groups.

Mabuya heathi is the historical scientific name of a Brazilian skink now listed as Brasiliscincus heathi. The name used in older reproductive studies identifies the same species under an earlier generic classification.^[ref]

The species is important in the study of viviparity and placental matrotrophy. In a 1984 study, Daniel G. Blackburn and colleagues compared the small ovulated eggs with developing offspring and examined the reproductive tissues. They reported eggs about one millimetre across and estimated that placental transfer supplied more than $99\%$ of fetal dry mass near birth. The estimate concerns dry material accumulated during development, rather than simply uptake of water.^[ref]

The comparison demonstrates that live birth can involve extensive maternal nutrient supply after ovulation. Its similarity to nutritional support in placental mammals is an example of convergent evolution; it does not place the skink within the mammalian lineage.

A Cambrian fossil animal with evidence of segmented musculature, sensory anatomy and serial branchial structures.

A Chengjiang fossil animal described with Haikouichthys in studies of early vertebrate organization.

A Chengjiang fossil animal important to interpretations of early vertebrate anatomy.

A traditional assemblage of armoured jawless fossil vertebrates. It is not a complete clade, and many members are stem relatives of gnathostomes, rather than early cyclostomes.

A member of the jawed-vertebrate lineage. Crown usage includes the ancestor of living cartilaginous and bony vertebrates and all its descendants; a total-group usage additionally includes jawless stem relatives. The usage must be stated when discussing early fossils.

A member of an extinct armoured jawless lineage with differentiated pectoral fins and usually recovered close to jawed vertebrates.

A member of an extinct armoured jawless lineage commonly placed on the gnathostome stem outside osteostracans and jawed vertebrates.

A Silurian jawless vertebrate preserving continuous paired ventrolateral fins, interpreted as a precursor to more localized paired appendages.

An Early Devonian stem jawed vertebrate combining an extensively ossified dermal roof with informative braincase anatomy.

An armoured jawed fish in a traditional Palaeozoic assemblage. Whether the assemblage is monophyletic or a succession of branches affects ancestral state reconstruction.

An Early Devonian Siberian jawed vertebrate whose braincase–shoulder relationship supplies evidence for a posterior pharyngeal component of the pectoral girdle.

An Early Devonian Mongolian jawed vertebrate whose braincase contains a trabecular internal tissue interpreted as endochondral bone. Implications for ancestral bone depend on both tissue identity and phylogenetic placement.

A Silurian armoured jawed vertebrate with large marginal jaw bones previously associated with bony vertebrates.

A member of a traditionally recognized assemblage of Palaeozoic spiny jawed fishes. Many analyses resolve these taxa as a succession on the Chondrichthyes stem rather than a separate complete clade.

A fossil acanthodian whose preserved braincase informs early jawed-vertebrate comparisons.

The cartilaginous-fish clade, including sharks, ray fish and chimaeras. Extant lack of extensive bone does not demonstrate that their ancestors never possessed it.

The little skate, used experimentally to compare branchial and appendage patterning.

A living holocephalan cartilaginous fish, distinct from a genetic chimera.

A member of the batoid cartilaginous-fish lineage.

A shark is a cartilaginous fish within Chondrichthyes. Its paired fins, jaws and sensory systems support diverse ways of life. The group includes both active predators and filter-feeding forms, so one feeding stereotype does not define it.

An Early Silurian jawed vertebrate known from mineralized tooth whorls, illustrating the information carried by isolated hard parts.

The bony-vertebrate lineage including ray-finned vertebrates and Sarcopterygii; descendants remain members even after extensive modification of their ancestral skeleton.

A ray-finned vertebrate whose skin expression of enamel-matrix genes contributes to comparisons of enamel and ganoine evolution.

A fossil bony vertebrate whose dermal odontodes preserve enamel despite teeth without an enamel coating, important for separating tissue origin from oral deployment.

In crown-based usage, the clade comprising the most recent common ancestor of living coelacanths, lungfish and tetrapods, and all its descendants. Lobed, internally supported paired appendages provide a traditional diagnostic character; inherited organization can persist despite conversion of fins into limbs.

A finned Devonian relative of digit-bearing vertebrates. Its pectoral skeleton combines retained fin rays with jointed internal structures relevant to limb evolution.

A member of the lungfish lineage, one of the living branches included in Sarcopterygii.

A member of the coelacanth lineage of sarcopterygians. Living representatives are specialized descendants, not unchanged ancestral fish.

A member of the lineage uniting living hagfish and lampreys. Jawlessness alone cannot assign an extinct vertebrate to this lineage.

A jawless vertebrate lineage with a specialized oral apparatus. Living lamprey anatomy is informative without being a direct reconstruction of the ancestral vertebrate.

A Late Devonian fossil lamprey from South Africa preserving diagnostic anatomy in an exceptional soft-tissue window.

A jawless vertebrate with a strongly specialized feeding apparatus and slime-producing system. Soft-tissue anatomy and secondary modifications make the ordinary fossil record difficult to interpret.

A Cretaceous fossil hagfish preserving evidence of specialized soft-tissue anatomy, including slime-producing structures.

A vertebrate belonging to the lineage of four-limbed vertebrates, including descendants whose limbs were reduced or lost.

A Devonian digit-bearing vertebrate with eight anatomical digits in the forelimb, demonstrating that a five-digit condition was not universal in early limbs.

The posterior paired limb of the tetrapod body plan. A reduced limb can lose its locomotor role while retaining other functions.

The anterior paired limb of the tetrapod body plan, modified into structures including arms, wings and flippers.

The shoulder is the region connecting the proximal forelimb to the trunk. In human anatomy, movement involves several linked articulations and the shoulder girdle, rather than only the joint between humerus and scapula.

The elbow is the joint region between upper arm and forearm, involving the humerus, radius and ulna. Its geometry supports flexion and extension while interacting with forearm rotation. A named joint region can therefore contain more than one articular relation.

The olecranon is the proximal projection of the ulna forming the point of the elbow. Attachment of the triceps brachii connects contraction to elbow extension, illustrating how a bony projection supplies a mechanical lever.

Elbow extension increases the angle between upper arm and forearm at the elbow. The triceps brachii is its principal muscular contributor. The resulting movement depends on joint position, external load and the line of pull.

Elbow flexion decreases the angle between upper arm and forearm at the elbow. Contributors include the brachialis and biceps brachii. Their mechanical contributions change with joint position and, for the latter, forearm orientation.

A tetrapod in the lineage including synapsids and sauropsids, characterized ancestrally by embryonic membranes including an amnion.

The reptile-side amniote lineage, including birds. A traditional assemblage excluding birds does not contain all of this ancestry’s descendants.

A fossil reptile proposed as an early turtle relative. Its skull anatomy, ontogeny and phylogenetic placement have been used to test the history of temporal roof closure.

A reptile lineage with a two-opening temporal skull organization inferred in its ancestry. Modified descendants can lose or close these openings; turtles illustrate why present-day hole counts do not determine ancestry.

A snake is a member of an elongated, predominantly limbless reptile lineage within Squamata. Limb reduction is one feature of its evolutionary history, not evidence that the lineage has no relationship to limbed ancestors. Different snakes retain different remnants of the ancestral limb-development system.

A study of a limb regulatory enhancer found progressive functional loss in snake sequences. Replacing the corresponding enhancer in mice with a snake-derived sequence produced severe limb reduction, linking a regulatory DNA change to developmental output. ^[ref] This identifies a contributing mechanism; it does not show that one sequence change alone explains every anatomical specialization of snakes.

A lizard is a member of the reptile radiation that also contains snakes. The familiar four-legged form is not universal: some lineages have reduced limbs. Evolutionary relationships therefore cannot be inferred from limb presence alone.

The Komodo dragon, Varanus komodoensis, is a large predatory lizard native to Indonesian islands. It takes animal prey and also scavenges. The scientific name identifies the species, while the familiar name does not denote a separate evolutionary group from other lizards.^[ref]

Its reproduction provides a striking example of facultative parthenogenesis: production of offspring without fertilization can occur in a species that also reproduces sexually. A 2006 study examined offspring from two females isolated from males at separate institutions. Microsatellite genotypes and reproductive histories supported maternal-only inheritance; one female later also produced sexually conceived offspring.^[ref]

Genetic comparison matters because the absence of a male at laying does not alone exclude earlier mating or stored sperm. As a simplified example, a mother carrying alleles $A$ and $B$ at a locus might produce a parthenogenetic offspring carrying two copies of $A$. That offspring contains only a maternal allele but is not an identical genetic copy of its mother. Evidence from several loci is more informative than this single-locus illustration.

The observation therefore distinguishes parthenogenesis from simple cloning and shows that reproductive mode can vary within a species. It does not establish that all isolated females must reproduce this way or that every form of animal parthenogenesis has the same genetic mechanism.

A member of the turtle lineage, whose shell and skull underwent substantial modification. Molecular and fossil evidence support a diapsid origin, with living turtles commonly recovered as sister to archosaurs.

A Middle Triassic stem relative of turtles combining modified ribs and ventral skeletal elements with temporal anatomy supporting a diapsid origin.

The clade comprising the most recent common ancestor of living crocodilians and birds and all its descendants, including extinct dinosaurs. Being sister to this clade does not make turtles archosaurs.

A pterosaur is a member of an extinct group of flying archosaurs. Its wing membrane was supported chiefly by an elongated fourth finger. That construction differs from the feathered wing of a bird and the multi-digit membrane support of a bat.

The avian total group: Aves and all extinct archosaurs more closely related to them than to living crocodilians. Under standard relationships this includes pterosaurs and nonavian dinosaurs.

A member of the dinosaur lineage, including birds. Restricting the word to extinct nonavian forms excludes descendants.

Megalosaurus is a genus of extinct predatory theropod dinosaurs known from Jurassic remains in England. Fossil jaw and limb material from Stonesfield contributed to its early recognition. William Buckland published a description in 1824, before the later name Dinosauria had been introduced. ^[ref]

The preserved lower jaw illustrates how specific observations constrain reconstruction. Serrated, blade-like teeth support a meat-eating interpretation, while imaging can reveal replacement teeth hidden within the bone. A fossil specimen and a complete life reconstruction contain different amounts of direct evidence: posture, missing anatomy and evolutionary relationships require comparisons beyond the isolated jaw.

A member of the theropod lineage of dinosaurs, within which birds are nested. The hands and integument of nonavian relatives inform comparisons of avian origins.

A name used for a more restricted avian-lineage clade than Panaves. Definitions have included a branch-based bird-versus-deinonychosaur rule and an apomorphy-based flight rule; the definition must be specified in comparative work.

A nonavian tyrannosauroid dinosaur represented by skeletons with preserved filamentous feathers.

A member of the avian lineage of theropod dinosaurs. The extent of the named group can differ between crown-based and broader fossil usages.

The white-rumped munia, Lonchura striata, is a songbird whose domesticated descendant is the Bengalese finch. The two forms are useful for studying how behavior differs between a wild lineage and a lineage maintained under domestication. A domesticated form is not automatically a separate species.^[ref]

A 2021 experiment compared feeding when an unfamiliar object was present. Groups of white-rumped munias were less likely to feed and waited longer than Bengalese finch groups under the novel-object conditions. This was interpreted as a difference in neophobia, avoidance of unfamiliar things. The wild-caught white-rumped munias had already spent more than a year in laboratory housing before testing, so wild origin and immediate testing environment were distinct features of the design.^[ref]

A longer feeding delay does not alone identify a uniquely inherited fear mechanism. Prior experience, housing and the social group used in a test can affect behavior. The comparison supplies a measured behavioral contrast, while the full evolutionary explanation requires evidence beyond the contrast itself.

The Seychelles warbler, Acrocephalus sechellensis, is a small songbird of the Seychelles islands. Its natural geographic restriction is an example of endemism. Long-term field monitoring has made it a useful subject for studying cooperative breeding, in which additional individuals help care for young.^[ref]

In this species, older offspring can remain with their parents and help defend the breeding area or feed younger offspring. That observation raises a causal question: do helpers increase reproductive success, or do productive breeding areas simply produce and retain more helpers? Jan Komdeur's 1994 removal experiment addressed this by reducing groups and controlling comparisons for area quality, initial group size and breeder age.^[ref]

One additional caregiver improved parental reproductive success under the tested conditions. Two or more in medium-quality areas could instead reduce success relative to one. Competition over reproduction or resources were proposed explanations, rather than established universal causes. The experiment therefore shows why counting helpers alone cannot establish a steadily increasing benefit: the effect depends on ecological and social context.^[ref]

The Bengalese finch is a domesticated songbird derived from the white-rumped munia, Lonchura striata. Research papers often designate it Lonchura striata var. domestica. The name identifies a domesticated form, rather than by itself establishing a separate wild species. Its learned birdsong makes it useful for experiments on auditory feedback and behavioral change.^[ref]

In 2007, Evren C. Tumer and Michael S. Brainard tested whether small variations in otherwise stable adult birdsong could support learning. A computerized system monitored the pitch of selected song elements and disrupted auditory feedback for a selected subset of renditions. The birds shifted the targeted pitch to avoid the disruption, while other song features were relatively preserved. Delaying the feedback prevented the adaptive shift in the reported comparison.^[ref]

For an illustrative version of the logic, suppose feedback is disrupted only when one chosen song element falls below a preset pitch. Later singing above that threshold would reduce disruption. The key comparison is the distribution of that element before and after contingent feedback, together with appropriate feedback controls. A shift need not mean the bird acquired an entirely new song; it can reveal adjustment of one feature within a practiced behavior.

Passeriformes is the order of birds commonly called passerines or perching birds. Its major living branches include Acanthisittidae, the New Zealand wrens; Tyranni, the suboscines; and Passeri, the songbirds. Thus songbirds form an important branch within Passeriformes, rather than being an exact synonym for the entire order.^[ref]

The group illustrates how evolutionary histories are tested with independent kinds of evidence. A 2019 study analyzed 4060 nuclear DNA regions across 137 passerine families and used 13 fossils to calibrate the resulting phylogenetic tree. DNA comparisons estimate relationships and relative amounts of change; a fossil supplies an age constraint, rather than a complete date label for every branch.^[ref]

An illustrative consequence is that two visually similar birds need not be closest relatives: similar ways of feeding or moving can favor similar shapes. Conversely, related branches can evolve very different appearances. A well-sampled phylogenetic tree lets investigators ask whether a repeated feature was inherited once or arose independently, while making explicit which taxa and genes were compared. The 2019 reconstruction tested explanations for diversification instead of assuming that a rise in global temperature must automatically produce a matching rise in speciation.

Acanthisittidae is the New Zealand wren family, a major branch of Passeriformes distinct from both Tyranni and the songbirds of Passeri. The rifleman is one representative. The familiar word “wren” is not by itself evidence that these birds belong with every other small bird given that common name.^[ref]

The rifleman searches trunks and branches for small invertebrates and makes short flights between feeding sites. Such observations describe how a living member uses its environment; they are a different kind of evidence from the DNA comparisons used to establish the family's evolutionary position.^[ref]

A useful classification test therefore asks two separate questions: does another bird share a similar body shape or feeding habit, and does it share the ancestry indicated by a phylogenetic tree? Similar ecological demands can produce similar appearances in different branches. Broad genomic sampling places Acanthisittidae in the passerine tree without defining the group merely by small size, a short tail or an English common name.

The rifleman, Acanthisitta chloris, is a small New Zealand forest bird in Acanthisittidae. It has short wings, a very short tail and a slender, slightly upturned bill. Males and females differ in plumage: males have green upperparts, while females are more brown and speckled, an example of sexual dimorphism.^[ref]

Its way of feeding is closely tied to the forest's surfaces. A rifleman searches tree trunks, bark and foliage for small invertebrates and usually moves between feeding sites by short flights. Watching whether a bird probes beneath bark, picks prey from a leaf or catches it in the air distinguishes different foraging actions rather than treating all insect eating as the same behavior.^[ref]

The species also illustrates cooperative breeding. Related young can help feed a later brood alongside the breeding pair. To describe this correctly, an observer must identify individuals and record who delivers food: counting visits to a nest alone cannot distinguish parental care from help supplied by other group members. Such observations connect the family group to the survival of young without assuming that every bird at a nest is a parent.^[ref]

Tyranni, the suboscines, is a major branch of Passeriformes distinct from Acanthisittidae and from the songbirds of Passeri. It includes the eastern phoebe. The taxonomic distinction makes comparisons of vocal development especially informative: a pattern established in an oscine songbird need not apply to every passerine.^[ref]

A 1991 experiment tested whether the eastern phoebe required hearing its own voice to develop normal song. Four birds were collected as nestlings and hand-reared; before they began singing, they were deafened. Their later songs were compared with those of intact laboratory birds and wild males and were judged normal. In these tested birds, normal song development therefore did not require auditory feedback.^[ref]

The experiment separates a developmental requirement from a correlation: hearing song and producing song normally occur together, but removing feedback tests whether it is necessary. The result concerns the tested species and conditions. It should not be generalized into a claim that no member of Tyranni can ever modify a vocalization through learning.

The eastern phoebe, Sayornis phoebe, is a North American flycatcher belonging to Tyranni, the suboscine branch of Passeriformes. It often watches for flying insects from a low perch, then makes a short flight to capture prey. Sheltered ledges, including those on bridges and buildings, provide nesting sites. Feeding behavior and nest placement help identify the living species alongside its appearance and voice.^[ref]

Its vocal development provides a controlled test of learning. In a 1991 study, four nestlings collected at 10–12 days were hand-reared and deafened at about 35 days, before they began singing. Their later songs were judged normal in comparison with wild males and intact laboratory birds. The tested birds could therefore develop the species-typical song without auditory feedback from their own voices.^[ref]

The result is narrower than saying that the brain or experience has no role in singing. It tests whether intact hearing during this developmental interval is necessary for the measured song pattern. Comparing it with a songbird experiment in which removing auditory experience disrupts song makes the experimental question clear: similar adult behavior can arise through different developmental requirements.

A songbird, or oscine, is a bird of Passeri, a major branch of Passeriformes. The name denotes evolutionary membership, rather than a judgment about whether a particular call is melodious. The chaffinch is one familiar example. Relationships among these birds are studied using DNA and phylogenetic trees, alongside their anatomy and behavior.^[ref]

Songbirds provide particularly clear experiments on learning. In W. H. Thorpe's chaffinch experiments, birds raised without normal song exposure developed simpler songs, whereas birds with early experience could retain much more normal patterns. An apparatus called the Song Tutor replayed constructed sequences of notes, letting the investigator change what a juvenile heard rather than merely observe wild adults. Some artificial patterns were learned, provided their sound characteristics were sufficiently appropriate.^[ref]

The comparison separates three questions: what vocal patterns a bird can physically produce, what experience supplies, and when that experience is effective. A sensitive period means that the timing of exposure matters. These results establish a role for experience in the tested species; they do not imply that every songbird learns the same repertoire at the same age.

The chaffinch studied in classic song-learning experiments is Fringilla coelebs, a songbird. Its male song combines inherited organization with details acquired through learning.

In his 1958 experiments, W. H. Thorpe compared wild birds, juveniles isolated after time in the wild, and birds hand-reared in auditory isolation from the fifth day of life. The early-isolated birds produced much simpler songs; birds isolated only in their first autumn retained nearly normal patterns. Groups of early-isolated birds could develop elaborate but abnormal shared songs by hearing and copying one another.^[ref]

A playback apparatus called the Song Tutor presented artificial patterns assembled from chaffinch notes. Birds learned some patterns when their tonal qualities were sufficiently similar to normal song. These results distinguish inherited dispositions from the auditory experience needed for local song details. Learning was also concentrated in a sensitive period, principally within the first thirteen months in these experiments.^[ref]

The common nightingale, Luscinia megarhynchos, is a songbird associated with dense low vegetation in its breeding habitat. Its familiar song and its habitat requirements provide different ways to study the same organism: vocal behaviour concerns communication, while settlement and movement reveal responses to vegetation structure.

A woodland experiment created eight paired areas in which deer were either excluded or allowed to browse. Territory mapping and radio tracking showed strong nightingale use of the protected regrowth. ^[ref] Tracked birds spent 69% of their time in the protected 6% of the study area. This links browsing to habitat use through an intervention, rather than inferring the mechanism solely from parallel population trends.

A toothed latest Cretaceous bird outside the living-bird crown group. Its preserved pterygoid bone supplies evidence for a mobile palatal arrangement before the crown divergence.

A toothed Late Cretaceous bird outside Aves in crown usage, important for comparisons of the avian skull.

A Late Jurassic avian-lineage theropod dinosaur outside the living-bird crown group under standard relationships.

Used here for the crown group of birds: the last common ancestor of living palaeognaths and neognaths and all descendants. Broader historical definitions also occur and should be stated explicitly.

The crown-bird lineage containing Galloanserae and other living birds outside Palaeognathae. Taxonomic membership and possession of a particular mobile palate are different claims.

A sparrow is a small perching bird; the common name is applied to more than one evolutionary group. An ecological description such as seed eating should therefore be separated from a claim that every bird called a sparrow belongs to one narrow lineage.

The clade containing landfowl and waterfowl.

Waterfowl are birds of the duck, goose and swan group. Together with landfowl they form Galloanserae. Aquatic habits are common, but the evolutionary grouping is more specific than all birds that live near water.

Landfowl are birds of the chicken, pheasant and related group. They form one branch of Galloanserae, alongside waterfowl. The name identifies a lineage rather than every ground-feeding bird.

The crown-bird lineage containing ostriches, emus, kiwis, tinamous and relatives. Its palaeognathous palate is an anatomical specialization, not evidence that its members are ancestors of neognaths.

A tinamou is a palaeognath bird native to the Americas. Tinamous can fly, showing that membership in this evolutionary group is not equivalent to flightlessness.

An ostrich is a large flightless palaeognath bird with strong running adaptations. Its reduced flight apparatus and powerful hindlimbs illustrate evolutionary modification of the shared avian body plan.

The great tit, Parus major, is a small perching bird widely studied in ecology and animal behaviour. Observations of breeding, foraging and survival can connect variation among individuals to their changing environment.

A corvid used in studies of food caching and experience-dependent cache protection.

A corvid, including Corvus corax, used in studies of social information and caching.

A radiation of Galápagos and nearby finches important in studies of ecological divergence, song and hybridization.

Geospiza conirostris is the large cactus finch, a member of Darwin's finches. Its feeding structures provide material for studying the relation between ecology, inherited variation and evolutionary change.

Geospiza fortis is the medium ground finch, one of Darwin's finches. Variation in its beak and food resources has made it an important system for studying natural selection across environmental changes.

The owl called Tyto alba in the cited sibling-negotiation experiments.

The bird studied in experiments on flexible deceptive alarm mimicry.

A member of the living crocodilian lineage, including crocodiles and their closest living relatives. birds are its living sister lineage within Archosauria.

A member of the lineage containing the tuatara and squamates such as lizards and snakes.

Squamata is the reptile lineage containing lizards, snakes and amphisbaenians. It belongs within lepidosaurs, whose other living branch is represented by the tuatara. Snakes are nested among lineages commonly called lizards, so excluding snakes does not leave a complete evolutionary branch.

The relationships require evidence beyond superficial similarity: elongated, limbless bodies can arise in different lineages. A 2015 analysis combined hundreds of anatomical characters with dozens of genes from living and fossil groups to compare competing arrangements.^[ref] This makes Squamata a useful example of how phylogenetics tests anatomical interpretations against molecular evidence while retaining fossils that have no recoverable sequence data.

Lacertoidea is a branch of Squamata containing Lacertidae, Teiidae, Gymnophthalmidae and amphisbaenians. It groups lineages by common ancestry, despite striking differences in their visible bodies. In particular, a burrowing, nearly limbless amphisbaenian can be more closely related to a normally limbed member of Lacertidae than superficial resemblance to a snake suggests.^[ref]

This grouping is a useful test of convergent evolution. Imagine recording only whether an animal has external legs: many otherwise unrelated burrowing forms would receive the same score. Recording hundreds of anatomical characters and comparing many genes supplies additional, partly independent evidence. The 2015 integrated analysis used 691 morphological characters and 46 genes, and included fossils as well as living species. Its combined analyses recovered Lacertoidea, while some deeper relationships depended on uncertain placements of fossil taxa.^[ref]

Gymnophthalmidae is a family of lizards within Lacertoidea, often called microteiids. It is distinct from Teiidae, despite that historical common name. Its members include forms with fairly conventional limbs and forms with strongly reduced limbs and elongated bodies, making the family useful for studying how a body plan changes during evolution.^[ref]

A morphological study distinguished modest reductions, such as losing one or two finger or toe bones, from much more extensive reductions that can leave no externally distinct adult limb. The investigators compared digit arrangements and also examined gene expression in developing reduced limbs. Adult anatomy and embryonic development consequently provide complementary observations of the same evolutionary problem.^[ref]

For an anatomical example, two feet can each have four visible digits while differing in which digit is absent or in the number of bones within each remaining digit. Merely counting visible toes collapses those possibilities into one score. Recording the detailed bone pattern and placing it on a phylogenetic tree gives a more informative test of whether superficially similar reductions share an evolutionary origin.

Teiidae is a family of lizards within Lacertoidea, including the whiptails of Aspidoscelis. It is a separate lineage from Gymnophthalmidae. Beyond their place in reptile classification, some whiptails offer a striking example of parthenogenesis: females produce offspring without fertilization.^[ref]

In a 2010 study of parthenogenetic Aspidoscelis, investigators asked how eggs could retain the parental complement of chromosomes and preserve heterozygosity. Chromosome imaging and fluorescent probes showed an extra premeiotic doubling, followed by pairing of genetically identical chromosome copies. The cells still underwent meiosis and chromosomal crossover; reproduction did not simply bypass the entire meiotic program.^[ref]

A simplified genetic example explains the significance. Begin with distinguishable chromosome versions $A$ and $B$. After copying, matching $A$ with another $A$ and $B$ with another $B$ permits exchange between equivalent copies while retaining both parental versions in the unreduced egg. This differs from mechanisms that discard one version and thereby lose heterozygosity. The finding concerns particular parthenogenetic species; it does not make every member of Teiidae an asexual animal.

Aspidoscelis is a taxonomic genus of whiptail lizards in Teiidae. It contains both sexually reproducing species and all-female lineages that reproduce by parthenogenesis. This variation makes a closely related set of animals useful for studying how reproduction and chromosome organization interact.^[ref]

In a 2011 experiment, researchers crossed a triploid parthenogenetic female lineage with males of a sexually reproducing lineage. The resulting four-copy hybrid offspring were checked with molecular and chromosome measurements. Some female offspring themselves reproduced by parthenogenesis, establishing laboratory lineages able to reproduce without repeating the original cross.^[ref]

A chromosome-set calculation clarifies the change: an unreduced egg carrying three sets can combine with a sperm carrying one set to make a four-set offspring, $3n+n=4n$. This is tetraploidy, rather than merely four copies of one selected gene. Producing one viable hybrid is also different from demonstrating that its descendants can reproduce. The experiment tested both steps, linking the immediate effects of a cross to the persistence of a new reproductive lineage.

Lacertidae is a family of lizards within Lacertoidea, distinct from Teiidae, Gymnophthalmidae and amphisbaenians. It provides an important comparison for the latter group: common ancestry can connect normally limbed lizards with highly modified burrowing forms, so a wormlike body is not by itself a reliable map of evolutionary relationships.^[ref]

Lacertidae is also a useful system for investigating thermoregulation. A 2019 comparative study related environmental temperatures to thermal physiology and to the evolutionary histories inferred from genomic data. Behavioral choices, such as basking or using shaded shelter, are relevant because the air temperature at a site is not automatically an animal's body temperature.^[ref]

For an illustrative measurement design, offer a lizard a range of warm and cool positions and record both its chosen position and body temperature. Repeating the assay at different background temperatures distinguishes an available environment from the temperature the animal actively selects. Comparisons between species then require the same measurement protocol and an account of their relatedness; otherwise a difference caused by the assay or shared ancestry can be mistaken for an independent adaptation.