Misplaced Pages

#359640

101-640: The kipunji ( Rungwecebus kipunji ), also known as the highland mangabey , is a species of Old World monkey that lives in the highland forests of Tanzania . It was independently discovered by researchers from the Wildlife Conservation Society , the University of Georgia , and Conservation International , in December 2003 and July 2004, making it the first new African monkey species discovered since 1984. Originally assigned to

202-423: A mutation–selection balance . It is predicted that a viral quasispecies at a low but evolutionarily neutral and highly connected (that is, flat) region in the fitness landscape will outcompete a quasispecies located at a higher but narrower fitness peak in which the surrounding mutants are unfit, "the quasispecies effect" or the "survival of the flattest". There is no suggestion that a viral quasispecies resembles

303-472: A positive feedback known as a Fisherian runaway . Thus sexual reproduction, as a form of natural selection , has an effect on evolution . Sexual dimorphism is where the basic phenotypic traits vary between males and females of the same species . Dimorphism is found in both sex organs and in secondary sex characteristics , body size, physical strength and morphology, biological ornamentation , behavior and other bodily traits. However, sexual selection

404-400: A "classical" method of determining species, such as with Linnaeus, early in evolutionary theory. However, different phenotypes are not necessarily different species (e.g. a four-winged Drosophila born to a two-winged mother is not a different species). Species named in this manner are called morphospecies . In the 1970s, Robert R. Sokal , Theodore J. Crovello and Peter Sneath proposed

505-424: A 'smallest clade' idea" (a phylogenetic species concept). Mishler and Wilkins and others concur with this approach, even though this would raise difficulties in biological nomenclature. Wilkins cited the ichthyologist Charles Tate Regan 's early 20th century remark that "a species is whatever a suitably qualified biologist chooses to call a species". Wilkins noted that the philosopher Philip Kitcher called this

606-472: A bacterium to bind, take up, and recombine exogenous DNA into its chromosome, it must enter a special physiological state referred to as competence (see Natural competence ). Sexual reproduction in early single-celled eukaryotes may have evolved from bacterial transformation, or from a similar process in archaea (see below). On the other hand, bacterial conjugation is a type of direct transfer of DNA between two bacteria mediated by an external appendage called

707-428: A connected series of neighbouring populations, each of which can sexually interbreed with adjacent related populations, but for which there exist at least two "end" populations in the series, which are too distantly related to interbreed, though there is a potential gene flow between each "linked" population. Such non-breeding, though genetically connected, "end" populations may co-exist in the same region thus closing

808-432: A different species from its ancestors. Viruses have enormous populations, are doubtfully living since they consist of little more than a string of DNA or RNA in a protein coat, and mutate rapidly. All of these factors make conventional species concepts largely inapplicable. A viral quasispecies is a group of genotypes related by similar mutations, competing within a highly mutagenic environment, and hence governed by

909-508: A genetic boundary suitable for defining a species concept is present. DNA barcoding has been proposed as a way to distinguish species suitable even for non-specialists to use. One of the barcodes is a region of mitochondrial DNA within the gene for cytochrome c oxidase . A database, Barcode of Life Data System , contains DNA barcode sequences from over 190,000 species. However, scientists such as Rob DeSalle have expressed concern that classical taxonomy and DNA barcoding, which they consider

1010-403: A haploid spore that grows into the dominant gametophyte form, which is a multicellular haploid body with leaf-like structures that photosynthesize . Haploid gametes are produced in antheridia (male) and archegonia (female) by mitosis. The sperm released from the antheridia respond to chemicals released by ripe archegonia and swim to them in a film of water and fertilize the egg cells thus producing

1111-465: A lineage should be divided into multiple chronospecies , or when populations have diverged to have enough distinct character states to be described as cladistic species. Species and higher taxa were seen from the time of Aristotle until the 18th century as categories 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 On

SECTION 10

#1732787744360

1212-492: A misnomer, need to be reconciled, as they delimit species differently. Genetic introgression mediated by endosymbionts and other vectors can further make barcodes ineffective in the identification of species. A phylogenetic or cladistic species is "the smallest aggregation of populations (sexual) or lineages (asexual) diagnosable by a unique combination of character states in comparable individuals (semaphoronts)". The empirical basis – observed character states – provides

1313-440: A multicellular gametophyte phase that produces gametes at maturity. The gametophytes of different groups of plants vary in size. Mosses and other pteridophytic plants may have gametophytes consisting of several million cells, while angiosperms have as few as three cells in each pollen grain. Flowering plants are the dominant plant form on land and they reproduce either sexually or asexually. Often their most distinctive feature

1414-449: A particular species, including which genus (and higher taxa) it is placed in, is a hypothesis about the evolutionary relationships and distinguishability of that group of organisms. As further information comes to hand, the hypothesis may be corroborated or refuted. Sometimes, especially in the past when communication was more difficult, taxonomists working in isolation have given two distinct names to individual organisms later identified as

1515-400: A perfect model of life, it is still a useful tool to scientists and conservationists for studying life on Earth, 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. This obliges taxonomists to decide, for example, when enough change has occurred to declare that

1616-531: A population because they are better at securing mates for sexual reproduction. It has been described as "a powerful evolutionary force that does not exist in asexual populations". The first fossilized evidence of sexual reproduction in eukaryotes is from the Stenian period, about 1.05 billion years old. Biologists studying evolution propose several explanations for the development of sexual reproduction and its maintenance. These reasons include reducing

1717-400: A short way of saying that something applies to many species within a genus, but not to all. If scientists mean that something applies to all species within a genus, they use the genus name without the specific name or epithet. The names of genera and species are usually printed in italics . However, abbreviations such as "sp." should not be italicised. When a species' identity is not clear,

1818-404: A specialist may use "cf." before the epithet to indicate that confirmation is required. The abbreviations "nr." (near) or "aff." (affine) may be used when the identity is unclear but when the species appears to be similar to the species mentioned after. With the rise of online databases, codes have been devised to provide identifiers for species that are already defined, including: The naming of

1919-523: A species as groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups. It has been argued that this definition is a natural consequence of the effect of sexual reproduction on the dynamics of natural selection. Mayr's use of the adjective "potentially" has been a point of debate; some interpretations exclude unusual or artificial matings that occur only in captivity, or that involve animals capable of mating but that do not normally do so in

2020-400: A species as determined by a taxonomist. A typological species is a group of organisms in which individuals conform to certain fixed properties (a type), so that even pre-literate people often recognise the same taxon as do modern taxonomists. The clusters of variations or phenotypes within specimens (such as longer or shorter tails) would differentiate the species. This method was used as

2121-491: A species. All species definitions assume that an organism acquires its genes from one or two parents very like the "daughter" organism, but that is not what happens in HGT. There is strong evidence of HGT between very dissimilar groups of prokaryotes , and at least occasionally between dissimilar groups of eukaryotes , including some crustaceans and echinoderms . The evolutionary biologist James Mallet concludes that there

SECTION 20

#1732787744360

2222-685: A species. Generally the term includes the unknown element of a distinct act of creation. Many authors have argued that a simple textbook definition, following Mayr's concept, works well for most multi-celled organisms , but breaks down in several situations: Species identification is made difficult by discordance between molecular and morphological investigations; these can be categorised as two types: (i) one morphology, multiple lineages (e.g. morphological convergence , cryptic species ) and (ii) one lineage, multiple morphologies (e.g. phenotypic plasticity , multiple life-cycle stages). In addition, horizontal gene transfer (HGT) makes it difficult to define

2323-528: A taxonomic decision at the discretion of cognizant specialists, is not governed by the Codes of Zoological or Botanical Nomenclature, in contrast to the PhyloCode , and contrary to what is done in several other fields, in which the definitions of technical terms, like geochronological units and geopolitical entities, are explicitly delimited. The nomenclatural codes that guide the naming of species, including

2424-517: A traditional biological species. The International Committee on Taxonomy of Viruses has since 1962 developed a universal taxonomic scheme for viruses; this has stabilised viral taxonomy. Most modern textbooks make use of Ernst Mayr 's 1942 definition, known as the Biological Species Concept as a basis for further discussion on the definition of species. It is also called a reproductive or isolation concept. This defines

2525-447: A variation on the morphological species concept, 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

2626-515: A variety of reasons. Viruses are a special case, driven by a balance of mutation and selection , and 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 later formalised as the typological or morphological species concept. Ernst Mayr emphasised reproductive isolation, but this, like other species concepts,

2727-407: A zygote. The zygote divides by mitotic division and grows into a multicellular, diploid sporophyte. The sporophyte produces spore capsules ( sporangia ), which are connected by stalks ( setae ) to the archegonia. The spore capsules produce spores by meiosis and when ripe the capsules burst open to release the spores. Bryophytes show considerable variation in their reproductive structures and the above

2828-438: Is "an entity composed of organisms which maintains its identity from other such entities through time and over space, and which has its own independent evolutionary fate and historical tendencies". This differs from the biological species concept in embodying persistence over time. Wiley and Mayden stated that they see the evolutionary species concept as "identical" to Willi Hennig 's species-as-lineages concept, and asserted that

2929-444: Is a fitness disadvantage of sexual reproduction. The two-fold cost of sex includes this cost and the fact that any organism can only pass on 50% of its own genes to its offspring. However, one definite advantage of sexual reproduction is that it increases genetic diversity and impedes the accumulation of harmful genetic mutations . Sexual selection is a mode of natural selection in which some individuals out-reproduce others of

3030-400: Is a basic outline. Also in some species each plant is one sex ( dioicous ) while other species produce both sexes on the same plant ( monoicous ). Fungi are classified by the methods of sexual reproduction they employ. The outcome of sexual reproduction most often is the production of resting spores that are used to survive inclement times and to spread. There are typically three phases in

3131-400: Is a group of sexually reproducing organisms that recognise 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; no matter whether populations can hybridise successfully, they are still distinct cohesion species if

Kipunji - Misplaced Pages Continue

3232-452: Is a population of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring , typically by sexual reproduction . It is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity . Other ways of defining species include their karyotype , DNA sequence, morphology , behaviour, or ecological niche . In addition, paleontologists use

3333-458: Is a set of organisms adapted to a particular set of resources, called a niche, in the environment. According to this concept, populations form the discrete phenetic clusters that we recognise as species because the ecological and evolutionary processes controlling how resources are divided up tend to produce those clusters. A genetic species as defined by Robert Baker and Robert Bradley is a set of genetically isolated interbreeding populations. This

3434-508: Is a transfer of plasmid DNA between bacteria, but the plasmids are rarely incorporated into the bacterial chromosome; and gene transfer and genetic exchange in archaea . Bacterial transformation involves the recombination of genetic material and its function is mainly associated with DNA repair . Bacterial transformation is a complex process encoded by numerous bacterial genes, and is a bacterial adaptation for DNA transfer. This process occurs naturally in at least 40 bacterial species. For

3535-574: Is a unisex species that uses a form of parthenogenesis called gynogenesis , where unfertilized eggs develop into embryos that produce female offspring. Poecilia formosa mate with males of other fish species that use internal fertilization, the sperm does not fertilize the eggs but stimulates the growth of the eggs which develops into embryos. Animals have life cycles with a single diploid multicellular phase that produces haploid gametes directly by meiosis. Male gametes are called sperm, and female gametes are called eggs or ova. In animals, fertilization of

3636-568: Is believed to have developed in an ancient eukaryotic ancestor. In eukaryotes, diploid precursor cells divide to produce haploid cells in a process called meiosis . In meiosis, DNA is replicated to produce a total of four copies of each chromosome. This is followed by two cell divisions to generate haploid gametes. After the DNA is replicated in meiosis, the homologous chromosomes pair up so that their DNA sequences are aligned with each other. During this period before cell divisions, genetic information

3737-414: Is called speciation . Charles Darwin was the first to describe the role of natural selection in speciation in his 1859 book The Origin of Species . Speciation depends on a measure of reproductive isolation , a reduced gene flow. This occurs most easily in allopatric speciation, where populations are separated geographically and can diverge gradually as mutations accumulate. Reproductive isolation

3838-412: Is called the specific name or the specific epithet (in botanical nomenclature , also sometimes in zoological nomenclature ). For example, Boa constrictor is one of the species of the genus Boa , with constrictor being the species' epithet. While the definitions given above may seem adequate at first glance, when looked at more closely they represent problematic species concepts. For example,

3939-403: Is described formally, in a publication that assigns it a unique scientific name. The description typically provides means for identifying the new species, which may not be based solely on morphology (see cryptic species ), differentiating it from other previously described and related or confusable species and provides a validly published name (in botany) or an available name (in zoology) when

4040-399: Is entirely beneficial. Larger populations appear to respond more quickly to some of the benefits obtained through sexual reproduction than do smaller population sizes. However, newer models presented in recent years suggest a basic advantage for sexual reproduction in slowly reproducing complex organisms . Sexual reproduction allows these species to exhibit characteristics that depend on

4141-431: Is exchanged between homologous chromosomes in genetic recombination . Homologous chromosomes contain highly similar but not identical information, and by exchanging similar but not identical regions, genetic recombination increases genetic diversity among future generations. During sexual reproduction, two haploid gametes combine into one diploid cell known as a zygote in a process called fertilization . The nuclei from

Kipunji - Misplaced Pages Continue

4242-671: Is further weakened by the existence of microspecies , groups of organisms, including many plants, with very little genetic variability, usually forming species aggregates . For example, the dandelion Taraxacum officinale and the blackberry Rubus fruticosus are aggregates with many microspecies—perhaps 400 in the case of the blackberry and over 200 in the dandelion, complicated by hybridisation , apomixis and polyploidy , making gene flow between populations difficult to determine, and their taxonomy debatable. Species complexes occur in insects such as Heliconius butterflies, vertebrates such as Hypsiboas treefrogs, and fungi such as

4343-726: Is hard or even impossible to test. Later 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, and the philosopher of science John Wilkins counted 26. Wilkins further grouped the species concepts into seven basic kinds of concepts: (1) agamospecies for asexual organisms (2) biospecies for reproductively isolated sexual organisms (3) ecospecies based on ecological niches (4) evolutionary species based on lineage (5) genetic species based on gene pool (6) morphospecies based on form or phenotype and (7) taxonomic species,

4444-766: Is in better shape, but the population there is smaller. The kipunji is classified as an endangered species on the IUCN Red List . It was included in the list of " The World's 25 Most Endangered Primates " in 2006 and 2008. Its range is restricted to 17.7 km (6.8 sq mi) of forest in the two isolated regions, the Ndundulu forest and the Rungwe-Livingstone forest. The kipunji faces multiple threats, primarily stemming from human activities such as logging , charcoal making , illegal hunting, and unmanaged resource extraction have been commonplace in

4545-403: Is no easy way to tell whether related geographic or temporal forms belong to the same or different species. Species gaps can be verified only locally and at a point of time. One is forced to admit that Darwin's insight is correct: any local reality or integrity of species is greatly reduced over large geographic ranges and time periods. The botanist Brent Mishler argued that the species concept

4646-478: Is not valid, notably because gene flux decreases gradually rather than in discrete steps, which hampers objective delimitation of species. Indeed, complex and unstable patterns of gene flux have been observed in cichlid teleosts of the East African Great Lakes . Wilkins argued that "if we were being true to evolution and the consequent phylogenetic approach to taxa, we should replace it with

4747-595: Is only implied over an extended period of time leading to sexual dimorphism. A few arthropods, such as barnacles , are hermaphroditic , that is, each can have the organs of both sexes . However, individuals of most species remain of one sex their entire lives. A few species of insects and crustaceans can reproduce by parthenogenesis , especially if conditions favor a "population explosion". However, most arthropods rely on sexual reproduction, and parthenogenetic species often revert to sexual reproduction when conditions become less favorable. The ability to undergo meiosis

4848-586: Is similar to Mayr's Biological Species Concept, but stresses genetic rather than reproductive isolation. In the 21st century, a genetic species could be established by comparing DNA sequences. Earlier, other methods were available, such as comparing karyotypes (sets of chromosomes ) and allozymes ( enzyme variants). An evolutionarily significant unit (ESU) or "wildlife species" is a population of organisms considered distinct for purposes of conservation. In palaeontology , with only comparative anatomy (morphology) and histology from fossils as evidence,

4949-423: Is sometimes an important source of genetic variation. Viruses can transfer genes between species. Bacteria can exchange plasmids with bacteria of other species, including some apparently distantly related ones in different phylogenetic domains , making analysis of their relationships difficult, and weakening the concept of a bacterial species. Sexual reproduction In placental mammals , sperm cells exit

5050-538: Is the broad crest of hair on the crown of their heads. Around 1,100 of the animals live in the highland Ndundulu Forest Reserve, adjacent to Udzungwa Mountains National Park , and in a disjunct population 250 miles away on Mount Rungwe and in Kitulo National Park , which is adjacent to it. The forest at Rungwe is highly degraded, and fragmentation of the remaining forest threatens to split that population into three smaller populations. The Ndundulu forest

5151-412: Is their reproductive organs, commonly called flowers. The anther produces pollen grains which contain the male gametophytes that produce sperm nuclei. For pollination to occur, pollen grains must attach to the stigma of the female reproductive structure ( carpel ), where the female gametophytes are located within ovules enclose within the ovary . After the pollen tube grows through the carpel's style,

SECTION 50

#1732787744360

5252-594: Is threatened by hybridisation, but this can be selected against once a pair of populations have incompatible alleles of the same gene, as described in the Bateson–Dobzhansky–Muller model . A different mechanism, phyletic speciation, involves one lineage gradually changing over time into a new and distinct form (a chronospecies ), without increasing the number of resultant species. Horizontal gene transfer between organisms of different species, either through hybridisation , antigenic shift , or reassortment ,

5353-459: Is widespread among arthropods including both those that reproduce sexually and those that reproduce parthenogenetically . Although meiosis is a major characteristic of arthropods, understanding of its fundamental adaptive benefit has long been regarded as an unresolved problem, that appears to have remained unsettled. Aquatic arthropods may breed by external fertilization, as for example horseshoe crabs do, or by internal fertilization , where

5454-530: The ICZN for animals and the ICN for plants, do not make rules for defining the boundaries of the species. Research can change the boundaries, also known as circumscription, based on new evidence. Species may then need to be distinguished by the boundary definitions used, and in such cases the names may be qualified with sensu stricto ("in the narrow sense") to denote usage in the exact meaning given by an author such as

5555-399: The fly agaric . Natural hybridisation presents a challenge to the concept of a reproductively isolated species, as fertile hybrids permit gene flow between two populations. For example, the carrion crow Corvus corone and the hooded crow Corvus cornix appear and are classified as separate species, yet they can hybridise where their geographical ranges overlap. A ring species is

5656-560: The gametophyte , which produces gametes directly by mitosis. This type of life cycle, involving alternation between two multicellular phases, the sexual haploid gametophyte and asexual diploid sporophyte, is known as alternation of generations . The evolution of sexual reproduction is considered paradoxical, because asexual reproduction should be able to outperform it as every young organism created can bear its own young. This implies that an asexual population has an intrinsic capacity to grow more rapidly with each generation. This 50% cost

5757-507: The jaguar ( Panthera onca ) of Latin America or the leopard ( Panthera pardus ) of Africa and Asia. In contrast, the scientific names of species are chosen to be unique and universal (except for some inter-code homonyms ); they are in two parts used together : the genus as in Puma , and the specific epithet as in concolor . A species is given a taxonomic name when a type specimen

5858-410: The ova remain in the female's body and the sperm must somehow be inserted. All known terrestrial arthropods use internal fertilization. Opiliones (harvestmen), millipedes , and some crustaceans use modified appendages such as gonopods or penises to transfer the sperm directly to the female. However, most male terrestrial arthropods produce spermatophores , waterproof packets of sperm , which

5959-896: The penis through the male urethra and enter the vagina during copulation , while egg cells enter the uterus through the oviduct . Other vertebrates of both sexes possess a cloaca for the release of sperm or egg cells. Sexual reproduction is the most common life cycle in multicellular eukaryotes, such as animals , fungi and plants . Sexual reproduction also occurs in some unicellular eukaryotes. Sexual reproduction does not occur in prokaryotes , unicellular organisms without cell nuclei , such as bacteria and archaea . However, some processes in bacteria, including bacterial conjugation , transformation and transduction , may be considered analogous to sexual reproduction in that they incorporate new genetic information. Some proteins and other features that are key for sexual reproduction may have arisen in bacteria, but sexual reproduction

6060-420: The sex cell nuclei from the pollen grain migrate into the ovule to fertilize the egg cell and endosperm nuclei within the female gametophyte in a process termed double fertilization . The resulting zygote develops into an embryo, while the triploid endosperm (one sperm cell plus two female cells) and female tissues of the ovule give rise to the surrounding tissues in the developing seed. The ovary, which produced

6161-507: The sex organs present although not reproductively functional. After several months or years, depending on the species, the sex organs develop further to maturity and the animal becomes sexually mature . Most female mammals are only fertile during certain periods during their estrous cycle, at which point they are ready to mate. For most mammals, males and females exchange sexual partners throughout their adult lives . The vast majority of fish species lay eggs that are then fertilized by

SECTION 60

#1732787744360

6262-406: The "cynical species concept", and arguing that far from being cynical, it usefully leads to an empirical taxonomy for any given group, based on taxonomists' experience. Other biologists have gone further and argued that we should abandon species entirely, and refer to the "Least Inclusive Taxonomic Units" (LITUs), a view that would be coherent with current evolutionary theory. The species concept

6363-626: 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

6464-589: The Rungwe-Kitulo forests, leading to habitat fragmentation and degradation. The narrow corridors linking different forest sections are encroached by farmers, further exacerbating the fragmentation of population units. Kipunjis are hunted using log traps and dogs, mainly during January to April in retribution for crop-raiding incidents. Both Kilombero Nature Forest Reserve and Mount Rungwe Nature Reserves lack adequate management resources to counter these threats. Species A species ( pl. : species)

6565-405: The abbreviation "sp." in the singular or "spp." (standing for species pluralis , Latin for "multiple species") in the plural in place of the specific name or epithet (e.g. Canis sp.). This commonly occurs when authors are confident that some individuals belong to a particular genus but are not sure to which exact species they belong, as is common in paleontology . Authors may also use "spp." as

6666-570: The amount of hybridisation is insufficient to completely mix their respective gene pools . A further development of the recognition concept is provided by the biosemiotic concept of species. In microbiology , genes can move freely even between distantly related bacteria, possibly extending to the whole bacterial domain. As a rule of thumb, microbiologists have assumed that members 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

6767-474: The biological species concept, "the several versions" of the phylogenetic species concept, and the idea that species are of the same kind as higher taxa are not suitable for biodiversity studies (with the intention of estimating the number of species accurately). They further suggested that the concept works for both asexual and sexually-reproducing species. A version of the concept is Kevin de Queiroz 's "General Lineage Concept of Species". An ecological species

6868-505: The biological species concept, a cladistic species does not rely on reproductive isolation – its criteria are independent of processes that are integral in other concepts. Therefore, it applies to asexual lineages. However, it does not always provide clear cut and intuitively satisfying boundaries between taxa, and may require multiple sources of evidence, such as more than one polymorphic locus, to give plausible results. An evolutionary species, suggested by George Gaylord Simpson in 1951,

6969-428: 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. Although none of these are entirely satisfactory definitions, and while the concept of species may not be

7070-433: The concept of a chronospecies can be applied. During anagenesis (evolution, not necessarily involving branching), some palaeontologists seek to identify a sequence of species, each one derived from the phyletically extinct one before through continuous, slow and more or less uniform change. In such a time sequence, some palaeontologists assess how much change is required for a morphologically distinct form to be considered

7171-416: The concept of the chronospecies since fossil reproduction cannot be examined. The most recent rigorous estimate for the total number of species of eukaryotes is between 8 and 8.7 million. About 14% of these had been described by 2011. All species (except viruses ) are given a two-part name , called a "binomial". The first part of a binomial is the genus to which the species belongs. The second part

7272-783: The conjugation pilus. Bacterial conjugation is controlled by plasmid genes that are adapted for spreading copies of the plasmid between bacteria. The infrequent integration of a plasmid into a host bacterial chromosome, and the subsequent transfer of a part of the host chromosome to another cell do not appear to be bacterial adaptations. Exposure of hyperthermophilic archaeal Sulfolobus species to DNA damaging conditions induces cellular aggregation accompanied by high frequency genetic marker exchange Ajon et al. hypothesized that this cellular aggregation enhances species-specific DNA repair by homologous recombination. DNA transfer in Sulfolobus may be an early form of sexual interaction similar to

7373-464: The eggs are fertilized within the female and the eggs simply hatch within the female body, or in seahorses , the male carries the developing young within a pouch, and gives birth to live young. Fishes can also be viviparous , where the female supplies nourishment to the internally growing offspring. Some fish are hermaphrodites , where a single fish is both male and female and can produce eggs and sperm. In hermaphroditic fish, some are male and female at

7474-556: The end of the tail and the ventrum . The pelage close to the hands and feet tends to be a medium to dark brown. Its hands, feet, and face are all black. These primates do not appear to show any sexual dimorphism in relation to pelage coloration. Adult male kipunjis have been observed at a typical length of 85–90 cm (33–35 in) and are estimated to weigh between 10 and 16 kg (22 and 35 lb). One feature, in combination with their pelage coloration, that helps to separate kipunjis from their Cercocebus and Lophocebus relatives

7575-435: The evidence to support hypotheses about evolutionarily divergent lineages that have maintained their hereditary integrity through time and space. Molecular markers may be used to determine diagnostic genetic differences in the nuclear or mitochondrial DNA of various species. For example, in a study done on fungi , studying the nucleotide characters using cladistic species produced the most accurate results in recognising

7676-479: The existence of the kipunji until its discovery, as traditional tales of the Nyakyusa people described the monkey as both real and mythical. It has a unique call, described as a 'honk-bark', which distinguishes it from its relatives, the grey-cheeked mangabey and the black crested mangabey , whose calls are described as 'whoop-gobbles'. The kipunji's relatively long pelage is light or medium brown with white on

7777-481: The female gametophyte(s), then grows into a fruit , which surrounds the seed(s). Plants may either self-pollinate or cross-pollinate . In 2013, flowers dating from the Cretaceous (100 million years before present) were found encased in amber, the oldest evidence of sexual reproduction in a flowering plant. Microscopic images showed tubes growing out of pollen and penetrating the flower's stigma. The pollen

7878-494: The females take into their bodies. A few such species rely on females to find spermatophores that have already been deposited on the ground, but in most cases males only deposit spermatophores when complex courtship rituals look likely to be successful. Insect species make up more than two-thirds of all extant animal species. Most insect species reproduce sexually, though some species are facultatively parthenogenetic . Many insect species have sexual dimorphism , while in others

7979-405: The formation of a zygote, and varying degrees of development, in many species the eggs are deposited outside the female; while in others, they develop further within the female and the young are born live. There are three extant kinds of mammals: monotremes , placentals and marsupials , all with internal fertilization. In placental mammals, offspring are born as juveniles: complete animals with

8080-409: The gametes fuse, and each gamete contributes half of the genetic material of the zygote. Multiple cell divisions by mitosis (without change in the number of chromosomes) then develop into a multicellular diploid phase or generation. In plants, the diploid phase, known as the sporophyte , produces spores by meiosis. These spores then germinate and divide by mitosis to form a haploid multicellular phase,

8181-412: The genus Lophocebus , genetic and morphological data showed that it is more closely related to the baboons (genus Papio ) than to the other mangabeys in the genus Lophocebus . Scientists subsequently assigned it to a new genus, Rungwecebus , named after Mount Rungwe . The kipunji is the first new monkey genus discovered since Allen's swamp monkey in 1923. Zoologists were initially skeptical of

8282-421: The likelihood of the accumulation of deleterious mutations, increasing rate of adaptation to changing environments , dealing with competition , DNA repair , masking deleterious mutations, and reducing genetic variation on the genomic level. All of these ideas about why sexual reproduction has been maintained are generally supported, but ultimately the size of the population determines if sexual reproduction

8383-454: The male. Some species lay their eggs on a substrate like a rock or on plants, while others scatter their eggs and the eggs are fertilized as they drift or sink in the water column. Some fish species use internal fertilization and then disperse the developing eggs or give birth to live offspring. Fish that have live-bearing offspring include the guppy and mollies or Poecilia . Fishes that give birth to live young can be ovoviviparous , where

8484-414: The most plausible reason for maintaining this capability is the benefit of repairing DNA damage , caused by a variety of stresses, through recombination that occurs during meiosis . Three distinct processes in prokaryotes are regarded as similar to eukaryotic sex : bacterial transformation , which involves the incorporation of foreign DNA into the bacterial chromosome; bacterial conjugation , which

8585-483: The numerous fungi species of all the concepts studied. Versions of the phylogenetic species concept that emphasise monophyly or diagnosability may lead to splitting of existing species, for example in Bovidae , by recognising old subspecies as species, despite the fact that there are no reproductive barriers, and populations may intergrade morphologically. Others have called this approach taxonomic inflation , diluting

8686-453: The ovum by a sperm results in the formation of a diploid zygote that develops by repeated mitotic divisions into a diploid adult. Plants have two multicellular life-cycle phases, resulting in an alternation of generations . Plant zygotes germinate and divide repeatedly by mitosis to produce a diploid multicellular organism known as the sporophyte. The mature sporophyte produces haploid spores by meiosis that germinate and divide by mitosis to form

8787-593: The paper is accepted for publication. The type material is usually held in a permanent repository, often the research collection of a major museum or university, that allows independent verification and the means to compare specimens. Describers of new species are asked to choose names that, in the words of the International Code of Zoological Nomenclature , are "appropriate, compact, euphonious, memorable, and do not cause offence". Books and articles sometimes intentionally do not identify species fully, using

8888-674: The person who named the species, while the antonym sensu lato ("in the broad sense") denotes a wider usage, for instance including other subspecies. Other abbreviations such as "auct." ("author"), and qualifiers such as "non" ("not") may be used to further clarify the sense in which the specified authors delineated or described the species. Species are subject to change, whether by evolving into new species, exchanging genes with other species, merging with other species or by becoming extinct. The evolutionary process by which biological populations of sexually-reproducing organisms evolve to become distinct or reproductively isolated as species

8989-487: The result of misclassification leading to questions on whether there really are any ring species. The commonly used names for kinds of organisms are often ambiguous: "cat" could mean the domestic cat, Felis catus , or the cat family, Felidae . Another problem with common names is that they often vary from place to place, so that puma, cougar, catamount, panther, painter and mountain lion all mean Puma concolor in various parts of America, while "panther" may also mean

9090-586: The ring. Ring species thus present a difficulty for any species concept that relies on reproductive isolation. However, ring species are at best rare. Proposed examples include the herring gull – lesser black-backed gull complex around the North pole, the Ensatina eschscholtzii group of 19 populations of salamanders in America, and the greenish warbler in Asia, but many so-called ring species have turned out to be

9191-415: The same or different plants. After rains or when dew deposits a film of water, the motile sperm are splashed away from the antheridia, which are normally produced on the top side of the thallus, and swim in the film of water to the archegonia where they fertilize the egg. To promote out crossing or cross fertilization the sperm are released before the eggs are receptive of the sperm, making it more likely that

9292-508: The same species. This concept was narrowed in 2006 to a similarity of 98.7%. The average nucleotide identity (ANI) method quantifies genetic distance between entire genomes , using regions of about 10,000 base pairs . With enough data from genomes of one genus, algorithms can be used to categorize species, as for Pseudomonas avellanae in 2013, and for all sequenced bacteria and archaea since 2020. Observed ANI values among sequences appear to have an "ANI gap" at 85–95%, suggesting that

9393-529: The same species. When two species names are discovered to apply to the same species, the older species name is given priority and usually retained, and the newer name considered as a junior synonym, a process called synonymy . Dividing a taxon into multiple, often new, taxa is called splitting . Taxonomists are often referred to as "lumpers" or "splitters" by their colleagues, depending on their personal approach to recognising differences or commonalities between organisms. The circumscription of taxa, considered

9494-402: The same time while in other fish they are serially hermaphroditic; starting as one sex and changing to the other. In at least one hermaphroditic species, self-fertilization occurs when the eggs and sperm are released together. Internal self-fertilization may occur in some other species. One fish species does not reproduce by sexual reproduction but uses sex to produce offspring; Poecilia formosa

9595-450: The sexes look nearly identical. Typically they have two sexes with males producing spermatozoa and females ova. The ova develop into eggs that have a covering called the chorion , which forms before internal fertilization. Insects have very diverse mating and reproductive strategies most often resulting in the male depositing a spermatophore within the female, which she stores until she is ready for egg fertilization. After fertilization, and

9696-477: The sexual reproduction of fungi: plasmogamy , karyogamy and meiosis . The cytoplasm of two parent cells fuse during plasmogamy and the nuclei fuse during karyogamy. New haploid gametes are formed during meiosis and develop into spores. The adaptive basis for the maintenance of sexual reproduction in the Ascomycota and Basidiomycota ( dikaryon ) fungi was reviewed by Wallen and Perlin. They concluded that

9797-506: The species concept and making taxonomy unstable. Yet others defend this approach, considering "taxonomic inflation" pejorative and labelling the opposing view as "taxonomic conservatism"; claiming it is politically expedient to split species and recognise smaller populations at the species level, because this means they can more easily be included as endangered in the IUCN red list and can attract conservation legislation and funding. Unlike

9898-405: The specific environment that they inhabit, and the particular survival strategies that they employ. In order to reproduce sexually, both males and females need to find a mate . Generally in animals mate choice is made by females while males compete to be chosen. This can lead organisms to extreme efforts in order to reproduce, such as combat and display, or produce extreme features caused by

9999-541: The sperm will fertilize the eggs of different thallus. After fertilization, a zygote is formed which grows into a new sporophytic plant. The condition of having separate sporophyte and gametophyte plants is called alternation of generations . The bryophytes , which include liverworts , hornworts and mosses , reproduce both sexually and vegetatively . They are small plants found growing in moist locations and like ferns, have motile sperm with flagella and need water to facilitate sexual reproduction. These plants start as

10100-540: The wild. It is difficult to define a species in a way that applies to all organisms. The debate about species concepts is called the species problem. The problem was recognised even in 1859, when Darwin wrote in On the Origin of Species : I was much struck how entirely vague and arbitrary is the distinction between species and varieties. He went on to write: No one definition has satisfied all naturalists; yet every naturalist knows vaguely what he means when he speaks of

10201-424: Was sticky, suggesting it was carried by insects. Ferns produce large diploid sporophytes with rhizomes , roots and leaves. Fertile leaves produce sporangia that contain haploid spores . The spores are released and germinate to produce small, thin gametophytes that are typically heart shaped and green in color. The gametophyte prothalli , produce motile sperm in the antheridia and egg cells in archegonia on

#359640