COMPARISONS, HOMOLOGY AND PHYLOGENY OF VERTEBRATES
Chordates traditionally include vertebrates, lancelets Chordate characters indicated on the phylogeny that unite tunicates with .. Fibrillar collagen gene relationships based on the major triple helix amino acid sequence. Perhaps the best informati on is known about the phylogeny of mammals during the Some of the basic similarities of Chordates and Echinodermata are. A chordate is an animal belonging to the phylum Chordata; chordates possess a notochord, In the case of vertebrate chordates, the notochord is usually replaced by a vertebral column during .. Phylogenetic tree of the Chordate phylum.
The dorsal neural tube is a chordate characteristic at one time thought to be present in hemichordates, albeit in a more limited form, namely the collar nerve cord Bateson Hemichordates have a diffuse nervous system composed of a middorsal nerve cord, a midventral nerve cord in the collar and trunk, as well as an epidermal nerve net throughout all body regions Knight-Jones The dorsal nerve cord in the collar region has been hypothesized to have homology with the chordate neural tube because it is hollow and forms by rolling up into a tube during development Morgan However, Lowe et al.
These results suggest that the deuterostome ancestor may have had an epidermal nerve net rather than a CNS, yet the anterior-posterior patterning of the ectoderm was already present Holland In contrast, three of the chordate characteristics are present in hemichordates, a ventral postanal tail, an endostyle, and pharyngeal gill slits. A ventral postanal tail is present in juvenile harrimaniid hemichordate worms Bateson ; Burdon-Jones ; Cameronand it has been proposed to be homologous to the chordate postanal tail due to posterior Hox gene expression data Lowe et al.
Furthermore, they share specific amino acid motifs with the echinoderms Petersonstrongly supporting their sister-group relationship Zeng and Swalla It is interesting that a postanal tail has only been reported in species of the direct-developing Harrimaniidae Bateson ; Burdon-Jones ; Cameronnot in the Ptychoderidae Urata and Yamaguchi These two families are paraphyletic with 18S rDNA analyses, with harrimaniids a sister group to the colonial pterobranchs Halanych ; Cameron, Garey, and Swalla ; however, theses two families are monophyletic when a 28S rDNA Winchell et al.
A postanal tail not being present in the indirect-developing ptychoderid acorn worms means that either it was lost in ptychoderids or the direct-developing harrimaniid worms more closely resemble the chordate ancestor. The endostyle is an iodine-binding organ present in the tunicate and cephalochordate pharynx and is considered by many researchers to be a homologous organ to the vertebrate thyroid Sasaki et al.
The hemichordate epibranchial ridge, like the chordate endostyle, is composed of specialized secretory cells Ruppert, Cameron, and Frickand these cells bind iodine. However, iodine binding is not restricted to this region but instead occurs all throughout the pharynx Ruppert Likewise, the hemichordate homolog of the gene NK2. Based on these data, it appears that the endostyle function in chordates is accomplished broadly by the pharynx in hemichordates.
The most convincing homology between hemichordates and chordates is the pharyngeal gills.Paleo 103: Early Vertebrate Evolution
Hollandlamprey Ogasawara et al. The expression of the most anterior Hox gene, Hox1, is first seen in vertebrates in the second pharyngeal slit and the expression of Hox1 in hemichordates is seen at the level between the first and second gill slits, suggesting that the location of the gill slits along the anterior-posterior axis is also homologous Lowe et al. Additionally, the pharyngeal skeletal elements of hemichordates and cephalochordates are strikingly similar in appearance Hyman ; Schaeffer Both have been reported to contain fibrillar collagen, as seen by transmission electron microscopy TEM studies Rahr ; Pardos and Benito Pharyngeal gills appear to function primarily for feeding and possibly for oxygen absorption, and thus in burrowing animals, cartilaginous elements may play an important structural role.
All deuterostomes with gills, except for tunicates, have structural elements associated with pharyngeal gills.
Are protochordates chordates? | Biological Journal of the Linnean Society | Oxford Academic
Phylogeny results have shown that it is likely that the ancestral deuterostome was a worm that had pharyngeal gills supported by a collagenous acellular skeleton Cameron, Garey, and Swalla ; Bourlat et al. Neither Xenoturbella nor Echinodermata has been reported to express this transcription factor. The most abundant and studied members of the collagen family are the fibrillar collagens. Each fibrillar collagen possesses the following characteristics: In this article, we construct gene trees with invertebrate and vertebrate triple helical domain sequences to show that hemichordates and lancelets have at least one fibrillar collagen and that it is similar to type I and type II vertebrate collagens.
The phenotypic features resulting in ecological isolation insure that individual or ganisms of a species taxa are functional, integrated whole organisms that are viable in the normal environment occupied the species taxon. These features can be grouped together under the general heading of the adaptive features of the species taxon.
Species taxa in asexually reproducing organisms are characterized only by the last property, and the limits of these species taxa are largely arbitrary, corresponding roughly to the ecological limits of species taxa in sexually reproducing ta xa.
Geographical extrinsic barriers are essential for th e process of speciation. Such barriers are not a intrinsic isolating mechanism and must be carefully distinguished from the set of intrinsic isolating mechanisms. The result of the genetic intrinsic isolating mechanisms is prevention of gene flow between two species. Individuals of different species may mate and even produce hybrid offspring, but without resulting gene flow exchange of genetic materia l between the two species.
Reproductive isolating mechanisms prevent reproduction between members of different species. The basic external barrier in sexually reproducing animals and many plants is by the existence of a geographical barrier which separates the original species into two allopatric segments. Evolution of intrinsic genetic isolating mechanisms occurs as a byproduct of other evolutionary changes. Intrinsic genetic isolating mechanisms are never favored by selective demands acting to improve them as intrinsic isolating mechanisms.
R eproductive isolating mechanisms can evolve as the result of selective demands if the two species are separated by genetic isolating mechanisms. Selection of intrinsic isolating mechani sms after the two newly evolved species become sympatric is not to improve them as isolating mechanisms, but to reduce their reproductive cost -- that is selective demands act on these intrinsic isolating mechanisms only as reproductive isolating mechanis ms.
Post-mating intrinsic isolating mechanisms are as effective as pre-mating intrinsic isolating mechanisms, but involve a much higher reproductive cost. Selective demands can act to reduce this cost and hence modify the isolating mechanisms after the in itial sympatry, but these selective demands are not for isolating mechanisms as such.
Speciation is not completed with the breakdown of the geographical barrier, but has a sympatric phase during which the two newly appeared species exert mutual selective demands on each other because of ecological interactions and breeding int eractions, with resulting divergence of the two species.
The mutual selective demands each species exert on the other during their initial period of sympatry may be strong and the rate of divergence is high. As the species become more different, they no l onger interact during the breeding season no longer attempt to form interspecific pair bonds and no longer compete; hence the mutual selective demands decline to zero.
B Phylogeny is the history of life. It is the lineages of organisms arranged chronologically, showing the branching of phyletic lines and the sequences of events and changes in the characteristics of the organisms. Phylogeny is not th e same as evolution, but is the result of the evolution of organisms. Obviously only correct phylogeny exists for any group of organisms, but no one has ever observed a phylogeny directly.
Paleontologists and neontologists must use the same types of obser vations and make the same conclusions in studies of phylogeny. Any statement about the presumed phylogeny of a group of organisms is a scientific hypothesis that is offered for disproof; statements about phylogeny can never be proven. If repeated attempts to disprove a particular phylogeny have failed, then one's confidence in that particular phylogeny is strengthened.
The confidence in a particular phylogeny depends upon the types of observations used in the attempts to disprove it as well as the continu ed failure to disprove the phylogeny. Phylogenies are ascertained by determination of homologies, arrangements of morphological characteristics into sequences, attempts to ascertain primitive and advanced features, recognition of unique paradaptations, correlations of features an d sequences and so forth.
All methods depend upon the consequences of evolutionary mechanisms and phenomena. C Homology is a concept applied to features of organisms and connotes a certain type of relationship, namely features that stem from the same feature in the common ancestor and possess certain shared characteristics because of this de scent from the same feature in the common ancestor.
Hence, homology means common phylogenetic origin and may be defined as: This is a phylogenetic definition of homology and is strictly noncircular in that homology is defined in terms of phylogeny and phylogeny is defined in terms of evolution.
Please note that in this definition of homology that: The definition of homology has nothing to do with the similarity of features. Degree of resemblance and common origin are quite distinct problems of phylogenetic study and must be kept separate.
Homology is not an intrinsic property of a feat ure, such as its color or mass, but a relationship depending upon the existence of corresponding features in other organisms.
Homology can be applied to any attribute of organisms - morphological features, developmental sequences, behavioral displays and so forth - so long as the definition applies to them. Nonhomology, the opposite of homology, applies to features or conditions in two or more organisms that do not stem phylogenetically from the same feature or condition in the immediate common ancestor of these organisms. Frequently the term an alogy is used in place of the term nonhomology, but such a usage should be discouraged because of past and continued ambiguity and multiple use of analogy.
Thus homologous features can be analogous and nonhomologous features can be analogous. This term has the identical or a similar meaning as convergence which is unambiguous and should be used for features that have become more similar to one another during the course of their evolution. And, if one examines the use of the term analogy, it is always used in the meaning of convergence.
Statements about homology should never be put in the form "The skull of the human is homologous" which does not express a relationship, or in the form "The arm of the gorilla is homologous to the arm of the chimpanzee" or "The quadrate of bir ds is homologous to the incus of mammals" which are meaningless because they do not state what the homologous relationship is.
A proper statement about homology must always include a "conditional phrase" which states the conditions of the homology. Thus t he correct form of a statement on homology is: The conditional phrase in a statement about homologous features describes the presumed characteristics of the feature in the presumed common ancestor.
The homologous features in different organisms would agree in sharing these properties. The oretically, one would have to describe in detail the characteristics in the conditional phrase, which is actually done when one knows little about the phylogeny and classification of the group of organisms under study.
In better known groups, a short cut is used if the conditional phrase is given "as the tetrapod forelimb", and this abbreviated style should be recognized as such. After a proper comparative study, decisions must be reached whether features in different organisms are homologous. Repeated determinations of homologues must be undertaken using more and more restricted conditional phrases until one has a hi erarchial series of homologies.
Any decision about homologous or nonhomologous features is a scientific statement that may be disproven, but can never be proven. Decisions must be made about the homology of features with the realization that many of them may be in error. The lack of a sharp demarkation between homologous and nonhomologous features must also be recognized. A broad gray zone exists between the two limits, but one does not have the luxury of deciding that features are "probably homologous", or "probably nonhomologous".
The methods by which homologous features are recognized must be in agreement with the general principles of evolution and phylogeny. But these methods cannot be based upon earlier conclusions on the presumed phylogeny or the relationships of organisms under study, Any such methods for recognizing homologues would result in circular reasoning. The basic methods for recognizing homologous features are founded on similarities of various sorts between the features.
Thus morphological similarity, functional similarity, the same ontogenetic development, a similar pattern of relationships to other features e. The rationale for using methods based upon similarities for determining homologues is based upon the argument that if two features stemmed phylogenetically from the same feature in their common ancestor, then these features were identical at that time.
Each feature might change somewhat during the evolution of each lineage from the common ancestor.
But the features will still be the same for those aspects that did not change during the evolution of either line. And these shared similar aspects would provide evidence for the homology of the features. Moreover, the longer the period of time between the species under study and their common ancestor, the more chance for change.
Therefore, the conditional phrase would be less detailed. Thus, if one concludes that the features are still homologous with increasingly restrictive conditional phrases, then one can conclude that the common ancestor had been more recent in time.
And in general, species belonging to the same taxon of a lower categorical rank e. The pattern of study in ascertaining presumed phylogenies is: A large number of homologies must be ascertained because of the low resolving power of the methods, and hen ce the good chance of erring in both directions.
Each homologous feature must be tested independently of the testing of other homologues. Presumably the pattern of individual homologues should agree with the phylogeny of the species. Conflicts will be found in almost all studies which can be resolved in several ways. A larger number of homologous features can be used in the phylogenetic study with the notion that decision on the phylogeny can be reached by a correlation between the numbers of homologues that show the same pattern of distribu tion.
Or each feature can be studied in more detail to discover which homologues appear to be valid i. Statements about phylogenies are scientific statements that may be disproven, but never proven.
Confidence in a phylogeny is gained after repeated failed attempts to disprove the phylogeny using features and comparisons best designed to dispr ove the phylogeny. The study of actual phylogenies is one of the most demanding of biological disciplines.
In spite of the two hundred years of work on the phylogeny and classification of organisms, great gaps in our knowledge still exist. One should, for examp le, examine most statements about the phylogeny of vertebrates with great care.
Once one attempts statements any more precise than reptiles arose from amphibians and gave rise to birds and reptiles, difficulties start to appear.
Perhaps the best informati on is known about the phylogeny of mammals during the Cenozoic for which a superb fossil record is available. In biology, comparisons of species should be separated into two types.
The observations, conclusions and interpretations which may result from each of these comparisons are different, and those of horizontal comparisons are not necessarily applicable to vertical comparisons and vice versa.
At the onset of a comparative study, one does not know which comparisons are horizontal ones and which are vertical ones. These conclusions must be reached after a proper study of the features and whether their properties are ones that may be regarded to represent different paradaptations. E Classification is the arrangement of species into monophyletic taxonomic groups or taxa taxon, singular.
These taxa are arranged into hierarchial, non-overlapping sets with each taxon containing all members of the contained taxa o f the next lower level. The level of taxa correspond to one categorical level of the taxonomic hierarchy. A category is a level in this hierarchy such as genus, family or order; it is not a group of organisms.
Categories are defined e. Taxa are natural groups of organisms and hence are diagnosed, not defined. Monophyletic is applied to a group of organisms that descended from a single common ancestor. A monophyletic taxa contains species that descended from an ancestral taxa of the same or lower categorical level.
Thus a monophyletic taxon contains species that descended from an ancestral order, family, genus, or species. If possible, the breadth of the ancestral taxon is restricted as much as possible. Thus it is preferable to recognize an order that descended from an ancestral genus than an ancestral order. It is not necessary to include all descendants of an ancestral taxon in a monophyletic descendant taxon If one lineage becomes markedly different, then it may be set off as another taxon of equal rank.
A polyphyletic taxon is an unnatural or artificial group and contains species which have descended from two or more ancestral taxa of the same or higher categorical rank. Hence if members of an order descended from two ancestral ordersthan that taxon would be polyphyletic.
It should be clear that monophyletic and polyphyletic taxa are not separated by a sharp boundary, but grade into one another with a large gray intermediate zone. This gradual gradation between monophyly and polyphyly corresponds exactly to th e gradual gradation between homology and nonhomology. F Phylogeny and classification of the Chordata: You are responsible for knowing the taxa that are in bold.
You should know their major characteristic s including the diagnostic attributeswhen these features appeared and underwent major modifications in each group during the history of the chordates, the phylogenetic relationships of these taxa and when they appeared in the fossil record.
You should have some notion of the major factors in the evolution of vertebrate features. The phylum closest to the Chordata is the Echinodermata. The Hemichordata are often considered to be a separate phylum, but will be treated as a subphylum of the Chordata in this course.
The Hemichordata contain two main groups, the Enteropneusta or acorn worms and the Pterobranchia "moss animals". Another group that may belong here are the Pogonophora, a group that has become much better known in recent years. A pecu liar group of fossil animals -- the Calcichordata -- are sometimes placed in the Echinodermata and sometimes in the Chordata, but are perhaps better placed in the Echinodermata. Some workers have argued that these fossils may represent the intermed iate stock between echinoderms and chordates.
Urochordates are unknown as fossils. Cephalochordata are also known as amphioxus and lancelets. The group contains only about 20 species of sand-burrowing marine creatures. The Cambrian fossils Yunnanozoon and Pikaia are likely related to modern cephalochordates. During the Ordovician Period - Ma jawless or agnathan fishes appeared and diversified. These are the earliest known members of Vertebrata, the chordate subgroup that is most familiar to us. Fossils representing most major lineages of fish-like vertebrates and the earliest tetrapods Amphibia were in existence before the end of the Devonian Period Ma.
Reptile-like tetrapods originated during the Carboniferous - Mamammals differentiated before the end of the Triassic Ma and birds before the end of the Jurassic Ma. The smallest chordates e.
Characteristics The notochord is an elongate, rod-like, skeletal structure dorsal to the gut tube and ventral to the nerve cord. The notochord should not be confused with the backbone or vertebral column of most adult vertebrates.
The notochord appears early in embryogeny and plays an important role in promoting or organizing the embryonic development of nearby structures. In most adult chordates the notochord disappears or becomes highly modified. In some non-vertebrate chordates and fishes the notochord persists as a laterally flexible but incompressible skeletal rod that prevents telescopic collapse of the body during swimming.
The nerve cord of chordates develops dorsally in the body as a hollow tube above the notochord.
In most species it differentiates in embryogeny into the brain anteriorly and spinal cord that runs through the trunk and tail. Together the brain and spinal cord are the central nervous system to which peripheral sensory and motor nerves connect. The visceral also called pharyngeal or gill clefts and arches are located in the pharyngeal part of the digestive tract behind the oral cavity and anterior to the esophagus.
The visceral clefts appear as several pairs of pouches that push outward from the lateral walls of the pharynx eventually to reach the surface to form the clefts.
Thus the clefts are continuous, slit-like passages connecting the pharynx to the exterior. The soft and skeletal tissues between adjacent clefts are the visceral arches.