In biology, a hybrid is the offspring resulting from combining the qualities of two organisms of different breeds, species or genera through sexual reproduction. Hybrids are not always intermediates between their parents, but can show hybrid vigour, sometimes growing larger or taller than either parent; the concept of a hybrid is interpreted differently in animal and plant breeding, where there is interest in the individual parentage. In genetics, attention is focused on the numbers of chromosomes. In taxonomy, a key question is how related the parent species are. Species are reproductively isolated by strong barriers to hybridisation, which include morphological differences, differing times of fertility, mating behaviors and cues, physiological rejection of sperm cells or the developing embryo; some act before fertilization and others after it. Similar barriers exist in plants, with differences in flowering times, pollen vectors, inhibition of pollen tube growth, somatoplastic sterility, cytoplasmic-genic male sterility and the structure of the chromosomes.
A few animal species and many plant species, are the result of hybrid speciation, including important crop plants such as wheat, where the number of chromosomes has been doubled. Human impact on the environment has resulted in an increase in the interbreeding between regional species, the proliferation of introduced species worldwide has resulted in an increase in hybridisation; this genetic mixing may threaten many species with extinction, while genetic erosion in crop plants may be damaging the gene pools of many species for future breeding. A form of intentional human-mediated hybridisation is the crossing of wild and domesticated species; this is common in modern agriculture. One such flower, Oenothera lamarckiana, was central to early genetics research into mutationism and polyploidy, it is more done in the livestock and pet trades. Human selective breeding of domesticated animals and plants has resulted is the development of distinct breeds. Hybrid humans existed in prehistory. For example and anatomically modern humans are thought to have interbred as as 40,000 years ago.
Mythological hybrids appear in human culture in forms as diverse as the Minotaur, blends of animals and mythical beasts such as centaurs and sphinxes, the Nephilim of the Biblical apocrypha described as the wicked sons of fallen angels and attractive women. The term hybrid is derived from Latin hybrida, used for crosses such as of a tame sow and a wild boar; the term came into popular use in English in the 19th century, though examples of its use have been found from the early 17th century. Conspicuous hybrids are popularly named with portmanteau words, starting in the 1920s with the breeding of tiger–lion hybrids. From the point of view of animal and plant breeders, there are several kinds of hybrid formed from crosses within a species, such as between different breeds. Single cross hybrids result from the cross between two true-breeding organisms which produces an F1 hybrid; the cross between two different homozygous lines produces an F1 hybrid, heterozygous. The F1 generation is phenotypically homogeneous, producing offspring that are all similar to each other.
Double cross hybrids result from the cross between two different F1 hybrids. Three-way cross hybrids result from the cross between an inbred line. Triple cross hybrids result from the crossing of two different three-way cross hybrids. Top cross hybrids result from the crossing of a top quality or pure-bred male and a lower quality female, intended to improve the quality of the offspring, on average. Population hybrids result from the crossing of plants or animals in one population with those of another population; these crosses between different breeds. In horticulture, the term stable hybrid is used to describe an annual plant that, if grown and bred in a small monoculture free of external pollen produces offspring that are "true to type" with respect to phenotype. Hybridisation can occur in the hybrid zones where the geographical ranges of species, subspecies, or distinct genetic lineages overlap. For example, the butterfly Limenitis arthemis has two major subspecies in North America, L. a. arthemis and L. a. astyanax.
The white admiral has a bright, white band on its wings, while the red-spotted purple has cooler blue-green shades. Hybridisation occurs between a narrow area across New England, southern Ontario, the Great Lakes, the "suture region", it is at these regions. Other hybrid zones have formed between described species of animals. From the point of view of genetics, several different kinds of hybrid can be distinguished. A genetic hybrid carries two different alleles of the same gene, where for instance one allele may code for a lighter coat colour than the other. A structural hybrid results from the fusion of gametes that have differing structure in at least one chromosome, as a result of structural abnormalities. A numerical hybrid results from the fusion of gamet
The masked julie is a species of cichlid endemic to Lake Tanganyika in Africa where it is found only along the northwestern shore preferring areas with rocky substrates. They eat benthic invertebrates found in the algae growth in the wild; this species reaches a length of 7 centimetres TL. List of freshwater aquarium fish species
Julidochromis marksmithi is a species of cichlid from the tribe Lamprologini of the subfamily Pseudocrenilabrinae, endemic to Lake Tanganyika where it occurs on the Tanzanian shore around Kiplipi in Nkasi District. The specific name of this fish honours the aquarist Mark Smith
Julidochromis dickfeldi is a species of cichlid endemic to Lake Tanganyika in Africa where it is only known from the southwestern portion. This species inhabits areas with rock/rubble substrates, each fish maintaining a territory around a crevice or crack; this species reaches a length of 11 centimetres TL. The specific name honours the German fishkeeper Alf Dickfeld who proposed the expedition on which the type was collected. List of freshwater aquarium fish species
A chordate is an animal constituting the phylum Chordata. During some period of their life cycle, chordates possess a notochord, a dorsal nerve cord, pharyngeal slits, an endostyle, a post-anal tail: these five anatomical features define this phylum. Chordates are bilaterally symmetric; the Chordata and Ambulacraria together form the superphylum Deuterostomia. Chordates are divided into three subphyla: Vertebrata. There are extinct taxa such as the Vetulicolia. Hemichordata has been presented as a fourth chordate subphylum, but now is treated as a separate phylum: hemichordates and Echinodermata form the Ambulacraria, the sister phylum of the Chordates. Of the more than 65,000 living species of chordates, about half are bony fish that are members of the superclass Osteichthyes. Chordate fossils have been found from as early as the Cambrian explosion, 541 million years ago. Cladistically, vertebrates - chordates with the notochord replaced by a vertebral column during development - are considered to be a subgroup of the clade Craniata, which consists of chordates with a skull.
The Craniata and Tunicata compose the clade Olfactores. Chordates form a phylum of animals that are defined by having at some stage in their lives all of the following anatomical features: A notochord, a stiff rod of cartilage that extends along the inside of the body. Among the vertebrate sub-group of chordates the notochord develops into the spine, in wholly aquatic species this helps the animal to swim by flexing its tail. A dorsal neural tube. In fish and other vertebrates, this develops into the spinal cord, the main communications trunk of the nervous system. Pharyngeal slits; the pharynx is the part of the throat behind the mouth. In fish, the slits are modified to form gills, but in some other chordates they are part of a filter-feeding system that extracts particles of food from the water in which the animals live. Post-anal tail. A muscular tail that extends backwards behind the anus. An endostyle; this is a groove in the ventral wall of the pharynx. In filter-feeding species it produces mucus to gather food particles, which helps in transporting food to the esophagus.
It stores iodine, may be a precursor of the vertebrate thyroid gland. There are soft constraints that separate chordates from certain other biological lineages, but are not part of the formal definition: All chordates are deuterostomes; this means. All chordates are based on a bilateral body plan. All chordates are coelomates, have a fluid filled body cavity called a coelom with a complete lining called peritoneum derived from mesoderm; the following schema is from the third edition of Vertebrate Palaeontology. The invertebrate chordate classes are from Fishes of the World. While it is structured so as to reflect evolutionary relationships, it retains the traditional ranks used in Linnaean taxonomy. Phylum Chordata †Vetulicolia? Subphylum Cephalochordata – Class Leptocardii Clade Olfactores Subphylum Tunicata – Class Ascidiacea Class Thaliacea Class Appendicularia Class Sorberacea Subphylum Vertebrata Infraphylum incertae sedis Cyclostomata Superclass'Agnatha' paraphyletic Class Myxini Class Petromyzontida or Hyperoartia Class †Conodonta Class †Myllokunmingiida Class †Pteraspidomorphi Class †Thelodonti Class †Anaspida Class †Cephalaspidomorphi Infraphylum Gnathostomata Class †Placodermi Class Chondrichthyes Class †Acanthodii Superclass Osteichthyes Class Actinopterygii Class Sarcopterygii Superclass Tetrapoda Class Amphibia Class Sauropsida Class Synapsida Craniates, one of the three subdivisions of chordates, all have distinct skulls.
They include the hagfish. Michael J. Benton commented that "craniates are characterized by their heads, just as chordates, or all deuterostomes, are by their tails". Most craniates are vertebrates; these consist of a series of bony or cartilaginous cylindrical vertebrae with neural arches that protect the spinal cord, with projections that link the vertebrae. However hagfish have incomplete braincases and no vertebrae, are therefore not regarded as vertebrates, but as members of the craniates, the group from which vertebrates are thought to have evolved; however the cladistic exclusion of hagfish from the vertebrates is controversial, as they ma
In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring by sexual reproduction. Other ways of defining species include their karyotype, DNA sequence, 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 they represent problematic species concepts. For example, the boundaries between related species become unclear with hybridisation, in a species complex of hundreds of similar microspecies, in a ring species. Among organisms that reproduce only asexually, the concept of a reproductive species breaks down, each clone is a microspecies. All species are given a two-part name, a "binomial"; the first part of a binomial is the genus.
The second part is called the specific epithet. For example, Boa constrictor is one of four species of the genus Boa. None of these is 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 distinct from one another, there would be no problem, but evolutionary processes cause species to change continually, 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. Charles Darwin's 1859 book The Origin of Species explained how species could arise by natural selection; that understanding was 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. Viruses are a special case, driven by a balance of mutation and selection, can be treated as quasispecies. Biologists and taxonomists have made many attempts to define species, beginning from morphology and moving towards genetics. Early taxonomists such as Linnaeus had no option but to describe what they saw: this was formalised as the typological or morphological species concept. Ernst Mayr emphasised reproductive isolation, but this, like other species concepts, is hard or impossible to test. Biologists have tried to refine Mayr's definition with the recognition and cohesion concepts, among others. Many of the concepts are quite similar or overlap, so they are not easy to count: the biologist R. L. Mayden recorded about 24 concepts, the philosopher of science John Wilkins counted 26. Wilkins further grouped the species concepts into seven basic kinds of concepts: agamospecies for asexual organisms biospecies for reproductively isolated sexual organisms ecospecies based on ecological niches evolutionary species based on lineage genetic species based on gene pool morphospecies based on form or phenotype and taxonomic species, a species as determined by a taxonomist.
A typological species is a group of organisms in which individuals conform to certain fixed properties, so that pre-literate people recognise the same taxon as do modern taxonomists. The clusters of variations or phenotypes within specimens would differentiate the species; this method was used as a "classical" method of determining species, such as with Linnaeus early in evolutionary theory. However, different phenotypes are not different species. Species named in this manner are called morphospecies. In the 1970s, Robert R. Sokal, Theodore J. Crovello and Peter Sneath proposed a variation on this, a phenetic species, defined as a set of organisms with a similar phenotype to each other, but a different phenotype from other sets of organisms, it differs from the morphological species concept in including a numerical measure of distance or similarity to cluster entities based on multivariate comparisons of a reasonably large number of phenotypic traits. A mate-recognition species is a group of sexually reproducing organisms that recognize one another as potential mates.
Expanding on this to allow for post-mating isolation, a cohesion species is the most inclusive population of individuals having the potential for phenotypic cohesion through intrinsic cohesion mechanisms. A further development of the recognition concept is provided by the biosemiotic concept of species. In microbiology, genes can move even between distantly related bacteria extending to the whole bacterial domain; as a rule of thumb, microbiologists have assumed that kinds of Bacteria or Archaea with 16S ribosomal RNA gene sequences more similar than 97% to each other need to be checked by DNA-DNA hybridisation to decide if they belong to the same species or not. This concept was narrowed in 2006 to a similarity of 98.7%. DNA-DNA hybri
The convict julie is a cichlid species in the subfamily Pseudocrenilabrinae family endemic to Lake Tanganyika. Hence it is found in Burundi, the Democratic Republic of the Congo and Zambia; the fish is named after Charles Tate Regan. This species is related to Julidochromis marlieri. A male common ancestor of these two hybridised with some female Telmatochromis ancestor in their evolutionary past. Julidochromis regani is a small ray-finned fish, its pale to golden yellow body is elongated. A varying black stripe patterns, depending on which part of the lake the fish originates from, is present. However, Julidochromis regani all have four slender lateral black stripes that run the length of the body, although some variations have the fourth stripe on the head. On the dorsal fin the stripes are vertical; the outer edge of the caudal and anal fins is white, while the pectoral fins are yellow. Sexing is difficult, but males do have a small genital papilla and females are plumper than the male, it is omnivorous.
Pairs are monogamous, though instances of polyandry, with the female taking more than one mate, have been recorded in both the wild and the aquarium. It is a secretive biparental substrate spawner, retreating to caves or rock crevices for protection and breeding. Julidochromis regani are small-growing dwarf cichlids and easy to spawn and care for if their basic needs are fulfilled. Like all Lake Tanganyika cichlids, Julidochromis regani are best maintained in hard alkaline water, with a pH of 8.5 - 9.0 and a hardness of 12-14 kH, in aquaria no smaller than 60 - 80 litres. Only one species of Julidochromis should be kept in any single aquarium, as the species within this genus tend to hybridise easily. Hybridisation with Chalinochromis and Telmatochromis is suspected, it is common enough in Lamprologini to better avoid keeping more than one species of this tribe per aquarium; the tank should be decorated with rocks to form passageways as shelter. It is therefore best to keep them with other cichlids to defuse their rowdy behavior.
Fry should be fed protein-rich foods such as baby brine shrimp. Parents can sometimes be found leading their fry around the aquarium. Breeding is, as usual in these social and intelligent fishes, easiest achieved by acquiring a stock of juveniles, letting them grow up together until pair-bonds are established. Barlow, G. W.: The Cichlid Fishes - Nature's grand experiment in evolution. Basic Books. Konings, A.: Tanganyikan cichlids in their natural habitat. Cichlid Press. Smith, M. P.: Lake Tanganyikan Cichlids - a complete pet owners manual. Barron's Educational. Care information for the home aquarium at the Aquarium Wiki