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62-615: The chestnut-headed partridge ( Arborophila cambodiana ) is a bird species in the family Phasianidae . It is found in highland forest in Cambodia , specifically the Cardamom Mountains . Some taxonomists consider the Siamese partridge ( A. diversa ) conspecific. The chestnut-headed partridge has two recognized subspecies: This Galliformes article is a stub . You can help Misplaced Pages by expanding it . Bird Birds are

124-472: 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

186-472: 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

248-732: A better sense of smell. A third stage of bird evolution starting with Ornithothoraces (the "bird-chested" avialans) can be associated with the refining of aerodynamics and flight capabilities, and the loss or co-ossification of several skeletal features. Particularly significant are the development of an enlarged, keeled sternum and the alula , and the loss of grasping hands. † Anchiornis † Archaeopteryx † Xiaotingia † Rahonavis † Jeholornis † Jixiangornis † Balaur † Zhongjianornis † Sapeornis † Confuciusornithiformes † Protopteryx † Pengornis Ornithothoraces † Enantiornithes Sexual reproduction In placental mammals , sperm cells exit

310-473: A definition similar to "all theropods closer to birds than to Deinonychus ", with Troodon being sometimes added as a second external specifier in case it is closer to birds than to Deinonychus . Avialae is also occasionally defined as an apomorphy-based clade (that is, one based on physical characteristics). Jacques Gauthier , who named Avialae in 1986, re-defined it in 2001 as all dinosaurs that possessed feathered wings used in flapping flight , and

372-442: A group called Paraves . Some basal members of Deinonychosauria, such as Microraptor , have features which may have enabled them to glide or fly. The most basal deinonychosaurs were very small. This evidence raises the possibility that the ancestor of all paravians may have been arboreal , have been able to glide, or both. Unlike Archaeopteryx and the non-avialan feathered dinosaurs, who primarily ate meat, studies suggest that

434-634: A group of warm-blooded vertebrates constituting the class Aves ( Latin: [ˈaveːs] ), characterised by feathers , toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart , and a strong yet lightweight skeleton . Birds live worldwide and range in size from the 5.5 cm (2.2 in) bee hummingbird to the 2.8 m (9 ft 2 in) common ostrich . There are over 11,000 living species and they are split into 44 orders . More than half are passerine or "perching" birds. Birds have wings whose development varies according to species;

496-403: 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

558-440: 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

620-526: 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

682-406: A sister group, the order Crocodilia , contain the only living representatives of the reptile clade Archosauria . During the late 1990s, Aves was most commonly defined phylogenetically as all descendants of the most recent common ancestor of modern birds and Archaeopteryx lithographica . However, an earlier definition proposed by Jacques Gauthier gained wide currency in the 21st century, and

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744-717: A time, sometimes for years, and rarely for life. Other species have breeding systems that are polygynous (one male with many females) or, rarely, polyandrous (one female with many males). Birds produce offspring by laying eggs which are fertilised through sexual reproduction . They are usually laid in a nest and incubated by the parents. Most birds have an extended period of parental care after hatching. Many species of birds are economically important as food for human consumption and raw material in manufacturing, with domesticated and undomesticated birds being important sources of eggs, meat, and feathers. Songbirds , parrots, and other species are popular as pets. Guano (bird excrement)

806-407: 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

868-444: 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

930-400: 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

992-508: 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

1054-574: 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

1116-565: 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

1178-533: Is called ornithology . Birds are feathered theropod dinosaurs and constitute the only known living dinosaurs . Likewise, birds are considered reptiles in the modern cladistic sense of the term, and their closest living relatives are the crocodilians . Birds are descendants of the primitive avialans (whose members include Archaeopteryx ) which first appeared during the Late Jurassic . According to recent estimates, modern birds ( Neornithes ) evolved in

1240-399: 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

1302-431: 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

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1364-429: Is harvested for use as a fertiliser. Birds figure throughout human culture. About 120 to 130 species have become extinct due to human activity since the 17th century, and hundreds more before then. Human activity threatens about 1,200 bird species with extinction, though efforts are underway to protect them. Recreational birdwatching is an important part of the ecotourism industry. The first classification of birds

1426-503: Is not considered a direct ancestor of birds, though it is possibly closely related to the true ancestor. Over 40% of key traits found in modern birds evolved during the 60 million year transition from the earliest bird-line archosaurs to the first maniraptoromorphs , i.e. the first dinosaurs closer to living birds than to Tyrannosaurus rex . The loss of osteoderms otherwise common in archosaurs and acquisition of primitive feathers might have occurred early during this phase. After

1488-595: 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

1550-516: Is synonymous to Avifilopluma. † Scansoriopterygidae † Eosinopteryx † Jinfengopteryx † Aurornis † Dromaeosauridae † Troodontidae Avialae Based on fossil and biological evidence, most scientists accept that birds are a specialised subgroup of theropod dinosaurs and, more specifically, members of Maniraptora , a group of theropods which includes dromaeosaurids and oviraptorosaurs , among others. As scientists have discovered more theropods closely related to birds,

1612-412: 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,

1674-530: Is used by many scientists including adherents to the PhyloCode . Gauthier defined Aves to include only the crown group of the set of modern birds. This was done by excluding most groups known only from fossils , and assigning them, instead, to the broader group Avialae, on the principle that a clade based on extant species should be limited to those extant species and their closest extinct relatives. Gauthier and de Queiroz identified four different definitions for

1736-459: 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

1798-715: The Late Cretaceous and diversified dramatically around the time of the Cretaceous–Paleogene extinction event 66 million years ago, which killed off the pterosaurs and all non- ornithuran dinosaurs. Many social species preserve knowledge across generations ( culture ). Birds are social, communicating with visual signals, calls, and songs , and participating in such behaviour as cooperative breeding and hunting, flocking , and mobbing of predators. The vast majority of bird species are socially (but not necessarily sexually) monogamous , usually for one breeding season at

1860-660: The Tiaojishan Formation of China, which has been dated to the late Jurassic period ( Oxfordian stage), about 160 million years ago. The avialan species from this time period include Anchiornis huxleyi , Xiaotingia zhengi , and Aurornis xui . The well-known probable early avialan, Archaeopteryx , dates from slightly later Jurassic rocks (about 155 million years old) from Germany . Many of these early avialans shared unusual anatomical features that may be ancestral to modern birds but were later lost during bird evolution. These features include enlarged claws on

1922-560: 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

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1984-410: 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

2046-896: 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

2108-420: 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

2170-507: 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

2232-428: The appearance of Maniraptoromorpha, the next 40 million years marked a continuous reduction of body size and the accumulation of neotenic (juvenile-like) characteristics. Hypercarnivory became increasingly less common while braincases enlarged and forelimbs became longer. The integument evolved into complex, pennaceous feathers . The oldest known paravian (and probably the earliest avialan) fossils come from

2294-509: The birds that descended from them. Despite being currently one of the most widely used, the crown-group definition of Aves has been criticised by some researchers. Lee and Spencer (1997) argued that, contrary to what Gauthier defended, this definition would not increase the stability of the clade and the exact content of Aves will always be uncertain because any defined clade (either crown or not) will have few synapomorphies distinguishing it from its closest relatives. Their alternative definition

2356-777: 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

2418-665: The earliest members of Aves, is removed from this group, becoming a non-avian dinosaur instead. These proposals have been adopted by many researchers in the field of palaeontology and bird evolution , though the exact definitions applied have been inconsistent. Avialae, initially proposed to replace the traditional fossil content of Aves, is often used synonymously with the vernacular term "bird" by these researchers. † Coelurus † Ornitholestes † Ornithomimosauria † Alvarezsauridae † Oviraptorosauria   Paraves Most researchers define Avialae as branch-based clade, though definitions vary. Many authors have used

2480-464: 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

2542-475: 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

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2604-494: 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

2666-451: The first avialans were omnivores . The Late Jurassic Archaeopteryx is well known as one of the first transitional fossils to be found, and it provided support for the theory of evolution in the late 19th century. Archaeopteryx was the first fossil to display both clearly traditional reptilian characteristics—teeth, clawed fingers, and a long, lizard-like tail—as well as wings with flight feathers similar to those of modern birds. It

2728-405: 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

2790-409: 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,

2852-421: 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

2914-454: 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

2976-414: 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

3038-523: The only known groups without wings are the extinct moa and elephant birds . Wings, which are modified forelimbs , gave birds the ability to fly, although further evolution has led to the loss of flight in some birds , including ratites , penguins , and diverse endemic island species. The digestive and respiratory systems of birds are also uniquely adapted for flight. Some bird species of aquatic environments, particularly seabirds and some waterbirds , have further evolved for swimming. The study of birds

3100-402: The outermost half) can be seen in the evolution of maniraptoromorphs, and this process culminated in the appearance of the pygostyle , an ossification of fused tail vertebrae. In the late Cretaceous, about 100 million years ago, the ancestors of all modern birds evolved a more open pelvis, allowing them to lay larger eggs compared to body size. Around 95 million years ago, they evolved

3162-453: 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

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3224-528: The previously clear distinction between non-birds and birds has become blurred. By the 2000s, discoveries in the Liaoning Province of northeast China, which demonstrated many small theropod feathered dinosaurs , contributed to this ambiguity. The consensus view in contemporary palaeontology is that the flying theropods, or avialans , are the closest relatives of the deinonychosaurs , which include dromaeosaurids and troodontids . Together, these form

3286-452: The same biological name "Aves", which is a problem. The authors proposed to reserve the term Aves only for the crown group consisting of the last common ancestor of all living birds and all of its descendants, which corresponds to meaning number 4 below. They assigned other names to the other groups.   Lizards & snakes   Turtles   Crocodiles   Birds Under the fourth definition Archaeopteryx , traditionally considered one of

3348-415: 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

3410-402: 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

3472-493: The second toe which may have been held clear of the ground in life, and long feathers or "hind wings" covering the hind limbs and feet, which may have been used in aerial maneuvering. Avialans diversified into a wide variety of forms during the Cretaceous period. Many groups retained primitive characteristics , such as clawed wings and teeth, though the latter were lost independently in a number of avialan groups, including modern birds (Aves). Increasingly stiff tails (especially

3534-450: 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

3596-471: 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

3658-405: 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

3720-541: 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

3782-461: Was developed by Francis Willughby and John Ray in their 1676 volume Ornithologiae . Carl Linnaeus modified that work in 1758 to devise the taxonomic classification system currently in use. Birds are categorised as the biological class Aves in Linnaean taxonomy . Phylogenetic taxonomy places Aves in the clade Theropoda as an infraclass or a subclass, more recently a subclass. Aves and

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3844-424: 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

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