Chronospecies

A chronospecies is a species derived from a sequential development pattern which involves continual and uniform changes from an extinct ancestral form on an evolutionary scale. This sequence of alterations eventually produces a population which is physically, morphologically, and/or genetically distinct from the original ancestors. Throughout this change, there is only one species in the lineage at any point in time, as opposed to cases where divergent evolution produces contemporary species with a common ancestor. The related term paleospecies (or palaeospecies) indicates an extinct species only identified with fossil material. This identification relies on distinct similarities between the earlier fossil specimens and some proposed descendant, although the exact relationship to the later species is not always defined. In particular, the range of variation within all the early fossil specimens does not exceed the observed range which exists in the later species.

A paleosubspecies (or palaeosubspecies) identifies an extinct subspecies which evolved into the currently existing form. This connection with relatively recent variations, usually from the Late Pleistocene, often relies on the additional information available in subfossil material. Most of the current species have changed in size adapting to the climatic changes during the last ice age (see Bergmann's Rule).

The further identification of fossil specimens as part of a "chronospecies" relies on additional similarities which more strongly indicate a specific relationship with a known species. For example,[1] relatively recent specimens – hundreds of thousands to a few million years old – with consistent variations (e.g. always smaller but with the same proportions) as a living species might represent the final step in a chronospecies. This possible identification of the immediate ancestor of the living taxon may also rely on stratigraphic information to establish the age of the specimens.

The concept of chronospecies is related to the phyletic gradualism model of evolution, and also relies on an extensive fossil record, since morphological changes accumulate over time and two very different organisms could be connected by a series of intermediaries.

Chronospecies
In palaeontology, the evidence for species and evolution comes mainly from the comparative anatomy of fossils. A chronospecies is defined in a single lineage (solid line) whose morphology changes with time. At some point, palaeontologists judge that enough change has occurred that two forms (A and B), separated in time and anatomy, once existed. If only sporadic examples of each survive in the fossil record, then the forms will appear sharply distinct.

Examples

References

  1. ^ Howard, Hildegarde (1947). "An ancestral Golden Eagle raises a question in taxonomy" (PDF). The Auk. 64 (2): 287–291. doi:10.2307/4080550.

Further reading

  • Evolutionary species vs. chronospecies from Dr. Steven M. Carr, Memorial University of Newfoundland biology department
  • Stanley, S. M. (1978) "Chronospecies' longevities, the origin of genera, and the punctuational model of evolution," Paleobiology, 4, 26–40.
Ailuropoda

Ailuropoda is the only extant genus in the ursid (bear) subfamily Ailuropodinae. It contains one living and three fossil species of panda.Only one species—Ailuropoda melanoleuca—currently exists; the other four species are prehistoric chronospecies. Despite its taxonomic classification as a carnivoran, the giant panda has a diet that is primarily herbivorous, which consists almost exclusively of bamboo.

Giant pandas have descended from Ailurarctos, which lived during the late Miocene.In 2011 fossil teeth from over 11 mya found in the Iberian peninsula were identified as belonging to a previously unidentified species in the Ailuropodinae subfamily This species was named Agriarctos beatrix (now Kretzoiarctos).

Anagenesis

Anagenesis is the gradual evolution of a species that continues to exist as an interbreeding population. This contrasts with cladogenesis, which occurs when there is branching or splitting, leading to two or more lineages and resulting in separate species. Anagenesis does not always lead to the formation of a new species from an ancestral species. When speciation does occur as different lineages branch off and cease to interbreed, a core group may continue to be defined as the original species. The evolution of this group, without extinction or species selection, is anagenesis.

Ardipithecus kadabba

Ardipithecus kadabba is the scientific classification given to fossil remains "known only from teeth and bits and pieces of skeletal bones," originally estimated to be 5.8 to 5.2 million years old, and later revised to 5.77 to 5.54 million years. According to the first description, these fossils are close to the common ancestor of chimps and humans. Their development lines are estimated to have parted 6.5–5.5 million years ago. It has been described as a "probable chronospecies" (i.e. ancestor) of A. ramidus. Although originally considered a subspecies of A. ramidus, in 2004 anthropologists Yohannes Haile-Selassie, Gen Suwa, and Tim D. White published an article elevating A. kadabba to species level on the basis of newly discovered teeth from Ethiopia. These teeth show "primitive morphology and wear pattern" which demonstrate that A. kadabba is a distinct species from A. ramidus.The specific name comes from the Afar word for "basal family ancestor".

Aroeira 3

Aroeira 3 is a 400,000 year old Homo heidelbergensis hominid skull which was discovered in the Aroeira cave, Portugal. It is the earliest human trace in Portugal. H. heidelbergensis existed at the transition between Homo erectus and early Neanderthals and used both stone tools and fire. The skull was damaged during the 2014 excavation but was restored in the following two years. In 2017 the description of the skull was published in PNAS. It is on display in the National Archaeology Museum (Lisbon).

Bison occidentalis

Bison occidentalis is an extinct species of bison that lived in North America and the Japanese archipelago from about 11,000 to 5,000 years ago, spanning the end of the Pleistocene to the mid-Holocene. Likely evolving from Bison antiquus, B. occidentalis was smaller overall from its ancestor and other species such as the steppe bison. B. occidentalis had a highly variable morphology, and their horns, which pointed rearward, were much thinner and pointed than other Pleistocene species of bison. Around 5,000 years ago, B. occidentalis was replaced by today's smaller Bison bison.

B. occidentalis, like other bison species, may have existed in small, scattered populations and been unable to increase in numbers until after the Pleistocene epoch ended 10,000 years ago because of competition with other large grazers during the Pleistocene. More recently ancient DNA studies have proven interbreeding between B. occidentalis and ancestral modern bison, so B. occidentalis was proposed to have been a localized offshoot of B. antiquus and part of the transition from that chronospecies to modern bison.

Craniidae

The Craniidae are a family of brachiopods, commonly known as lamp shells. Although it belongs to a subdivision called the inarticulata which have shells where the mineral content consist of calcium phosphate, the Craniidae have shells that consist of calcium carbonate. Other special characteristics of this family are that no outgrowths are developed to form a hinge between both valves, nor is there any support for the lophophore. As adults, craniids either lived free on the ocean floor or, more commonly, were attached to a hard object with all or part of the ventral valve. All other brachiopods are supposed to have a stalk or pedicle, at least as an adolescent, but in craniids a pedicle is not known from any development stage.They are the only members of the order Craniida and the monotypic suborder Craniidina and superfamily Cranioidea; consequently, the latter two taxa are presently redundant and not used very often. Valdiviathyris and Neoancistrocrania were sometimes separated in a family Valdiviathyrididae but this has turned out to be unjustified.Most Craniidae are long extinct forms known only from fossils like all other Craniforma. However, some 20 species of this 470-million-year-old lineage are surviving today. They include Valdiviathyris quenstedti which has remained essentially unchanged for the last 35 million years or so. Although some minimal evolution would obviously have taken place in the meantime, this was essentially silent mutations and marginal adaptations to cooler habitat. Present-day Valdiviathyris are all but inseparable from those of the Late Eocene and the genus cannot even be divided into chronospecies. Thus, V. quenstedti is a true living fossil and one of the oldest and most long-lived species known to science.

Curlew

The curlews (), genus Numenius, are a group of eight species of birds, characterised by long, slender, downcurved bills and mottled brown plumage. The English name is imitative of the Eurasian curlew's call, but may have been influenced by the Old French corliu, "messenger", from courir , "to run". It was first recorded in 1377 in Langland's Piers Plowman "Fissch to lyue in þe flode..Þe corlue by kynde of þe eyre". In Europe "curlew" usually refers to one species, the Eurasian curlew Numenius arquata.

They are one of the most ancient lineages of scolopacid waders, together with the godwits which look similar but have straight bills.Curlews feed on mud or very soft ground, searching for worms and other invertebrates with their long bills. They will also take crabs and similar items.

Curlews enjoy a worldwide distribution. Most species show strong migratory habits and consequently one or more species can be encountered at different times of the year in Europe, Ireland, Britain, Iberia, Iceland, Africa, Southeast Asia, Siberia, North America, South America and Australasia.

The distribution of curlews has altered considerably in the past hundred years as a result of changing agricultural practices. Reclamation and drainage of marshy fields and moorland, and afforestation of the latter, have led to local decreases, while conversion of forest to grassland in some parts of Scandinavia has led to increases there.The stone-curlews are not true curlews (family Scolopacidae) but members of the family Burhinidae, which is in the same order Charadriiformes, but only distantly related within that.

Homo antecessor

Homo antecessor is a proposed archaic human species of the Lower Paleolithic, known to have been present in Western Europe (Spain, England and France) between about 1.2 million and 0.8 million years ago (Mya). It was described in 1997 by Eudald Carbonell, Juan Luis Arsuaga and J. M. Bermúdez de Castro, who based on its "unique mix of modern and primitive traits" classified it as a previously unknown archaic human species.The fossils associated with Homo antecessor represent the oldest direct fossil record of the presence of Homo in Europe. The species name antecessor proposed in 1997 is a Latin word meaning "predecessor", or "vanguard, scout, pioneer". Authors who do not accept H. antecessor as a separate species consider the fossils in question an early form of H. heidelbergensis or as a European variety of H. erectus.

Homo erectus

Homo erectus (meaning 'upright man') is a species of archaic humans that lived throughout most of the Pleistocene geological epoch.

Its earliest fossil evidence dates to 1.8 million years ago (discovered 1991 in Dmanisi, Georgia).A debate regarding the classification, ancestry, and progeny of H. erectus, especially in relation to Homo ergaster, is ongoing, with two major positions:

1) H. erectus is the same species as H. ergaster, and thereby H. erectus is a direct ancestor of the later hominins including Homo heidelbergensis, Homo antecessor, Homo neanderthalensis, Homo denisova, and Homo sapiens; or,

2) it is in fact an Asian species or subspecies distinct from African H. ergaster.Some paleoanthropologists consider H. ergaster to be a variety, that is, the "African" variety, of H. erectus; the labels "Homo erectus sensu stricto" (strict sense) for the Asian species and "Homo erectus sensu lato" (broad sense) have been offered for the greater species comprising both Asian and African populations.H. erectus eventually became extinct throughout its range in Africa, Europe and Asia, but developed into derived species, notably Homo heidelbergensis.

As a chronospecies, the time of its disappearance is thus a matter of contention. The species name proposed in 1950

defines Java Man as the type specimen (now H. e. erectus). Since then, there has been a trend in palaeoanthropology of reducing the number of proposed species of Homo, to the point where H. erectus includes all

early (Lower Paleolithic) forms of Homo sufficiently derived from H. habilis and

distinct from early H. heidelbergensis (in Africa also known as H. rhodesiensis). In this wider sense, H. erectus had mostly been replaced by H. heidelbergensis by about 300,000 years ago, with possible late survival in Java as late as 70,000 years ago.

The discovery of the morphologically divergent Dmanisi skull 5 in 2013 has reinforced the trend of subsuming fossils formerly given separate species names under H. erectus considered as a wide-ranging, polymorphous species. Thus, H. ergaster is now well within the accepted morphological range of H. erectus, and it has been suggested that even H. rudolfensis and H. habilis (alternatively suggested as late forms of Australopithecus rather than early Homo)

should be considered early varieties of H. erectus.

Homo ergaster

Homo ergaster , also Homo erectus ergaster or African Homo erectus is an extinct chronospecies of the genus Homo that lived in eastern and southern Africa during the early Pleistocene, between about 1.9 million and 1.4 million years ago.

Originally proposed as a separate species, H. ergaster is now mostly considered either an early form, or an African variety, of H. erectus.The binomial name was published in 1975 by Groves and Mazák. The specific epithet, "ergaster", is derived from the Ancient Greek ἐργαστήρ ergastḗr - "workman", in reference to the advanced lithic technology developed by the species, thereby introducing the Acheulean industry.

KNM-ER 2598, a "H. erectus-like" occipital bone stands as the earliest evidence for H. erectus in Africa at approximately 1.9 million years ago (contemporary with Homo rudolfensis). There is a fossil gap between 1.9 and 1.6 million years ago, KNM-ER 3733 is the oldest known H. ergaster skull dated to about 1.6 million years ago. Its survival past 1.4 million years ago is uncertain, again due to a fossil gap, the next available African fossils allowing reliable morphological analysis are those of Homo rhodesiensis (African H. heidelbergensis), at 0.6 million years ago.

Homo heidelbergensis

Homo heidelbergensis is an extinct species or subspecies of archaic humans in the genus Homo, which radiated in the Middle Pleistocene from about 700,000 to 300,000 years ago, known from fossils found in Southern Africa, East Africa and Europe. African H. heidelbergensis has several subspecies. The subspecies are Homo heidelbergensis heidelbergensis, Homo heidelbergensis daliensis, Homo rhodesiensis, and Homo heidelbergensis steinheimensi. The derivation of Homo sapiens from Homo rhodesiensis has often been proposed, but is obscured by a fossil gap from 400–260 kya. The species was originally named Homo heidelbergensis due to the skeleton's first discovery near Heidelberg, Germany.The first discovery—a mandible—was made in 1907 by Otto Schoetensack. The skulls of this species share features with both Homo erectus and the anatomically modern Homo sapiens; its brain was nearly as large as that of Homo sapiens. The Sima de los Huesos cave at Atapuerca in northern Spain holds particularly rich layers of deposits where excavations were still in progress as of 2018.H. heidelbergensis was dispersed throughout Eastern and Southern Africa (Ethiopia, Namibia, Southern Africa) as well as Europe (England, France, Germany, Greece, Hungary, Italy, Portugal, Spain). Its exact relation both to the earlier Homo antecessor and Homo ergaster, and to the later lineages of Neanderthals, Denisovans, and modern humans is unclear.Homo sapiens has been proposed as derived from H. heidelbergensis via Homo rhodesiensis, present in East and North Africa from around 400,000 years ago. The correct assignment of many fossils to a particular chronospecies is difficult and often differences in opinion ensue among paleoanthropologists due to the absence of universally accepted dividing lines (autapomorphies) between Homo erectus, Homo heidelbergensis, Homo rhodesiensis and Neanderthals.

It is uncertain whether H. heidelbergensis is ancestral to Homo sapiens, as a fossil gap in Africa between 400,000 and 260,000 years ago obscures the presumed derivation of H. sapiens from H. rhodesiensis. Genetic analysis of the Sima de los Huesos fossils (Meyer et al. 2016) seems to suggest that H. heidelbergensis in its entirety should be included in the Neanderthal lineage, as "pre-Neanderthal" or "archaic Neanderthal" or "early Neanderthal", while the divergence time between the Neanderthal and modern lineages has been pushed back to before the emergence of H. heidelbergensis, to about 600,000 to 800,000 years ago, the approximate time of disappearance of Homo antecessor.The delineation between early H. heidelbergensis and H. erectus is also unclear.

Index of evolutionary biology articles

This is a list of topics in evolutionary biology.

Olduvai Hominid 9

Olduvai Hominid number 9 (OH 9) is a fossilized skull cap of an early hominin, found in LLK II, Olduvai Gorge by Louis S. B. Leakey in 1960. It is believed to be around 1.4 to 1.5 million years old and is the first hominid to have a brain size larger than 1,000 cubic centimetres (61 in3). Leakey named it "Chellean Man", in reference to the Oldowan tools found at the site, which were then referred to by the now-obsolete name Chellean. Heberer (1963) provisionally named a new species Homo leakeyi based on the specimen in honour of Leakey, but most subsequent workers have regarded it as H. ergaster, or as H. erectus where H. ergaster is considered a junior synonym. Phillip Tobias provisionally named a new subspecies, H. erectus olduvaiensis, in 1968 based on the specimen, but this has not seen continued use. To the extent that proponents of the use of H. ergaster define ergaster as a separate lineage, equivalent to "African H. erectus", rather than a pure chronospecies, the assignment of OH 9 to H. erectus sensu stricto by Colin Groves supports subsuming H. ergaster into H. erectus.

Paraphyly

In taxonomy, a group is paraphyletic if it consists of the group's last common ancestor and all descendants of that ancestor excluding a few—typically only one or two—monophyletic subgroups. The group is said to be paraphyletic with respect to the excluded subgroups. The arrangement of the members of a paraphyletic group is called a paraphyly. The term is commonly used in phylogenetics (a subfield of biology) and in linguistics.

The term was coined to apply to well-known taxa like Reptilia (reptiles) which, as commonly named and traditionally defined, is paraphyletic with respect to mammals and birds. Reptilia contains the last common ancestor of reptiles and all descendants of that ancestor, including all extant reptiles as well as the extinct synapsids, except for mammals and birds. Other commonly recognized paraphyletic groups include fish, monkeys, and lizards.If many subgroups are missing from the named group, it is said to be polyparaphyletic. A paraphyletic group cannot be a clade, or monophyletic group, which is any group of species that includes a common ancestor and all of its descendants. Formally, a paraphyletic group is the relative complement of one or more subclades within a clade: removing one or more subclades leaves a paraphyletic group.

Pseudoextinction

Pseudoextinction (or phyletic extinction) of a species occurs when all members of the species are extinct, but members of a daughter species remain alive. The term pseudoextinction refers to the evolution of a species into a new form, with the resultant disappearance of the ancestral form. Pseudoextinction results in the relationship between ancestor and descendant still existing even though the ancestor species no longer exists.The classic example is that of the non-avian dinosaurs. While the non-avian dinosaurs of the Mesozoic died out, their descendants, birds, live on today. As of now it is accepted that modern birds evolved from Archaeopteryx, a small dinosaur with flight feathers from the Mesozoic period. Many other families of bird-like dinosaurs also died out as the heirs of the dinosaurs continued to evolve, but because their birds continue to thrive in the world today their ancestors are only pseudoextinct.

Species

In biology, a species ( (listen)) is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. Other ways of defining species include their karyotype, DNA sequence, morphology, behaviour or ecological niche. In addition, paleontologists use the concept of the chronospecies since fossil reproduction cannot be examined. While these definitions may seem adequate, when looked at more closely they represent problematic species concepts. For example, the boundaries between closely related species become unclear with hybridisation, in a species complex of hundreds of similar microspecies, and in a ring species. Also, among organisms that reproduce only asexually, the concept of a reproductive species breaks down, and each clone is potentially a microspecies.

All species (except viruses) are given a two-part name, a "binomial". The first part of a binomial is the genus to which the species belongs. The second part is called the specific name or the specific epithet (in botanical nomenclature, also sometimes in zoological nomenclature). For example, Boa constrictor is one of four species of the genus Boa.

None of these are entirely satisfactory definitions, but scientists and conservationists need a species definition which allows them to work, regardless of the theoretical difficulties. If species were fixed and clearly distinct from one another, there would be no problem, but evolutionary processes cause species to change continually, and to grade into one another.

Species were seen from the time of Aristotle until the 18th century as fixed kinds that could be arranged in a hierarchy, the great chain of being. In the 19th century, biologists grasped that species could evolve given sufficient time. Charles Darwin's 1859 book The Origin of Species explained how species could arise by natural selection. That understanding was greatly extended in the 20th century through genetics and population ecology. Genetic variability arises from mutations and recombination, while organisms themselves are mobile, leading to geographical isolation and genetic drift with varying selection pressures. Genes can sometimes be exchanged between species by horizontal gene transfer; new species can arise rapidly through hybridisation and polyploidy; and species may become extinct for a variety of reasons. Viruses are a special case, driven by a balance of mutation and selection, and can be treated as quasispecies.

Thalassocnus

Thalassocnus is an extinct genus of semiaquatic ground sloths from the Miocene and Pliocene of the Pacific South American coast. The five species—T. antiquus, T. natans, T. littoralis, T. carolomartini, and T. yuacensis—represent a chronospecies, a population gradually adapting to marine life in one direct lineage. They are the only known aquatic sloths. They have been found in the Pisco Formation of Peru and the Bahía Inglesa, Coquimbo, and Horcón formations of Chile. Thalassocnus are placed in the subfamily Thalassocninae of the family Megatheriidae.

Thalassocnus evolved several marine adaptations over the course of 4 million years, such as dense and heavy bones to counteract buoyancy, the internal nostrils migrating farther into the head to help with breathing while completely submerged, the snout becoming wider and more elongated to consume aquatic plants better, and the head angling farther and farther downwards to aid in bottom feeding. The long tail was probably used for diving and balance similarly to the modern day beaver (Castor spp.) and platypus (Ornithorhynchus anatinus).

Thalassocnus probably walked across the seafloor and dug up food with its claws. They probably could not do high-powered swimming, relying on paddling if necessary. Early Thalassocnus were probably generalist grazers eating seaweed and seagrasses close to shore, whereas later species specialized on seagrasses farther off the coast. They were probably preyed upon by sharks and macroraptorial sperm whales such as Acrophyseter. Thalassocnus were found in formations with large marine mammal and shark assemblages.

Transmutation of species

Transmutation of species and transformism are 19th-century evolutionary ideas for the altering of one species into another that preceded Charles Darwin's theory of natural selection. The French Transformisme was a term used by Jean Baptiste Lamarck in 1809 for his theory, and other 19th century proponents of pre-Darwinian evolutionary ideas included Étienne Geoffroy Saint-Hilaire, Robert Grant, and Robert Chambers, the anonymous author of the book Vestiges of the Natural History of Creation. Opposition in the scientific community to these early theories of evolution, led by influential scientists like the anatomists Georges Cuvier and Richard Owen and the geologist Charles Lyell, was intense. The debate over them was an important stage in the history of evolutionary thought and would influence the subsequent reaction to Darwin's theory.

Xenocyon

Xenocyon ("strange wolf") is an extinct subgenus of Canis. The group includes Canis (Xenocyon) africanus, Canis (Xenocyon) antonii and Canis (Xenocyon) falconeri that gave rise to Canis (Xenocyon) lycanoides. The hypercarnivore Xenocyon gave rise to the modern dhole and the African wild dog.

This page is based on a Wikipedia article written by authors (here).
Text is available under the CC BY-SA 3.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.