Переходные формы в биологии: Переходные формы в эволюции животных ( к теме «Докозательства эволюции)

Переходная форма — Википедия. Что такое Переходная форма Лондонский экземпляр археоптерикса, переходного звена между рептилиями и птицами.

Переходная форма — организм с промежуточным состоянием, обязательно существующим при состоявшемся постепенном переходе от одного биологического типа строения к другому. Переходные формы характеризуются наличием более древних и примитивных (в значении первичных) черт, чем их более поздние родственники, но, в то же время, наличием более прогрессивных (в значении более поздних) черт, чем их предки. Как правило, говоря о промежуточных формах, имеют в виду ископаемые виды, хотя промежуточные виды вовсе не должны непременно вымирать. Известно множество переходных форм, иллюстрирующих происхождение четвероногих от рыб, рептилий от земноводных, птиц от динозавров, млекопитающих от териодонтов, китообразных от наземных млекопитающих, лошадей от пятипалого предка и человека от древних гоминид^[1].

Теория эволюции

Ко времени написания Чарльзом Дарвином «Происхождения видов» науке не были известны переходные формы жизни. Этот факт Дарвин считал одним из самых слабых мест своей теории. Однако уже через два года был обнаружен археоптерикс — переходное звено между рептилиями и птицами.

Переходные формы между двумя группами организмов не обязательно являются потомками одной группы и предками другой. По ископаемым останкам, как правило, невозможно достоверно установить, является ли один организм прямым предком другого. Кроме того, вероятность найти в палеонтологической летописи чьего-нибудь прямого предка очень мала, и по умолчанию среди биологов принято такую возможность вообще не рассматривать. Гораздо выше вероятность обнаружить относительно близких родственников этого предка, схожих с ним по строению. Поэтому любая переходная форма автоматически интерпретируется как боковая ветвь эволюции, а не «участок ствола».^[2]

Примеры

Меритерий, ранний предок слонов, больше похож на свинью, хотя уже имеет маленький хобот и бивни, свойственные хоботным.

В качестве примеров переходных форм можно привести амбулоцетуса — «ходячего кита» (переходная форма между наземными млекопитающими и китообразными), тиктаалика и ихтиостегу (переходные формы от рыб к земноводным), мезогиппуса (между ранними лошадиными и современной лошадью).

Переходные формы в эволюции человека

В настоящее время известно множество переходных форм между человеком разумным и его обезьяноподобными предками. Так, например, открыты останки сахелантропа, ардипитека, австралопитеков (африканского, афарского и других), человека умелого, человека работающего, человека прямоходящего, человека-предшественника, гейдельбергского человека, неандертальца и раннего человека разумного.

Спор о постепенности эволюционных преобразований

Переходные формы являются объектом исследований в споре между градуалистами и пунктуалистами. Градуалисты считают, что эволюция является постепенным, непрерывным процессом. Пунктуалисты, или сторонники теории прерывистого равновесия считают, что виды в течение долгого времени остаются неизменными, а изменения происходят в очень короткий промежуток времени. Переходные формы являются аргументом обеих сторон. Пунктуалисты утверждают, что большинство видов появляются внезапно и на протяжении всей своей истории меняются слабо. Градуалисты отмечают, что среди позвоночных нет ни одного бесспорного примера очень быстрого перехода от одного вида к другому, большинство переходов вообще плохо задокументированы ископаемыми. Также много хорошо описанных внутривидовых изменений. Например, описаны изменения среднекайнозойских кроликов, миоценовых грызунов, гиппарионов, эоценовых парнокопытных, переход между родами эоценовых приматов.^[3]

Недостающие звенья

1850
1900
1950
2002
Прогресс в изучении эволюции человека. Увеличение числа известных науке видов гоминин по времени. Каждый вид изображен в виде прямоугольника, показывающего границы, в которых варьировал объём черепа, и место вида в палеонтологической летописи. Видно постепенное заполнение промежутков между видами.

Многие переходные формы между живыми организмами пока не найдены. Обычно их называют недостающими звеньями. Редкость переходных форм объясняется неполнотой палеонтологической летописи. Сторонники теории прерывистого равновесия также указывают на быстроту эволюционных преобразований. Однако палеонтологи постоянно находят новые ископаемые, заполняя пробелы (например, в 2004 году был найден тиктаалик — ещё одно переходное звено между рыбами и земноводными).

Неполнота палеонтологической летописи

Далеко не каждая переходная форма существует в виде окаменелости из-за принципиальной неполноты палеонтологической летописи. Неполнота вызвана особенностями процесса фоссилизации, то есть перехода в окаменелое состояние. Для образования окаменелости необходимо, чтобы погибший организм был погребён под большим слоем осадочных пород. Из-за крайне медленной скорости осадконакопления на суше, чисто сухопутные виды редко переходят в окаменелое состояние и сохраняются. Кроме того, редко удаётся обнаружить виды, живущие в глубинах океана, из-за редкости поднятия на поверхность больших массивов дна. Таким образом, большинство известных ископаемых (а, соответственно, и переходных форм) — это либо виды, обитающие на мелководье, в морях и реках, либо наземные виды, ведущие полуводный образ жизни, либо обитающие недалеко от береговой линии. Закономерности процессов захоронения останков живых существ изучает особая отрасль палеонтологии — тафономия.

Филогенетические ряды

Филогенетический ряд лошади. Показана реконструкция ископаемых видов, полученных из последовательных слоёв породы. Справа — вид спереди передней левой ноги с выделенным третьим сегментом, а также зуб в продольном сечении^[4].

Филогенетические ряды − ряды видов, последовательно сменявших друг друга в процессе эволюции различных групп животных и растений.

Впервые были открыты В. О. Ковалевским, который показал, что современные однопалые копытные происходят от древних пятипалых мелких всеядных животных.

Креационизм

Сторонники креационизма заявляют, что переходных форм не обнаружено. Научное сообщество считает подобные заявления ложью и намеренным введением в заблуждение.

См. также

Примечания

Литература

• Дарвин, Чарльз. Происхождение видов путём естественного отбора: В 2 кн. — М.: ТЕРРА — Книжный клуб, 2009. — ISBN 978-5-275-02114-1.
• Кэрролл Р. Палеонтология и эволюция позвоночных: В 3-х т. — Мир, 1992. — 280 с. — ISBN 5-03-001819-0.

Ссылки

90000 List of transitional forms — RationalWiki 90001 90002 90003 90004 90005 90006 90004 90008 An editor believes this article contains duplicate material. 90009 90010 This article may have a content or subject overlap with 90008 Indohyus 90009. The pages could be 90008 merged 90009. You can discuss this at 90008 RationalWiki: Duplicate articles 90009. 90006 90018 90019 90020 A 90008 transitional form 90009 is an organism that has features intermediate of its ancestors and progeny.The term is most common in evolution to refer to organisms that show certain features (wings, feathers, gills and so on) partly in development. In theory, every fossil is a 90008 transitional form 90009 if it has descendants and each living creature is a transition between its parent and its offspring. However, evolution is about the features of populations rather than individuals, and the transition at the species level can be too small to notice in fossils. So the list below concentrates on broad transitional features and the genus or larger group.90025 90026 Definitions [edit] 90027 90020 A fossil (and the organism it came from) is usually considered transitional with respect to just two groups. (If more than two are considered, then the definition of «transitional» is fuzzier.) Individual organisms (or individual species), considered by themselves, can not be divided into «transitional» and «non-transitional», rather like how individual towns can be labeled «between» and «not between»; the term is relative. However, in cases where two groups are very populous or well-known, and all of the intermediate species have gone extinct, any intermediate organism’s transitional nature becomes noteworthy and noticeable, and may hence be called «transitional» all by itself.To return to the analogy of geography, the United States has a region called «the Midwest» or (somewhat derogatory) «flyover country». Of course 90029 every 90030 town and region in the US is in the middle and to the West of somewhere, and can be flown over. But the reason the Midwest is singled out as a «transitional» region is that it lies between the more populous and city-strewn East and West Coasts yet itself is denser than the Great Plains to its own west. 90025 90020 While some of the fossilized species in this list may be direct descendants and direct ancestors of the forms on either side of the transition, most are in fact close relatives of the «true» ancestral species.This is because evolution produces a tree of life on which there are many paths «up», but forward in time there is only one true path «down», through the various parent species. (A mother species can have many daughters, but each daughter has only one mother.) The number of fossils directly ancestral to, for example, modern birds decreases as one travels down the geologic column. Alongside each of these true bird ancestors are dozens of fossilized cousin bird species whose lines eventually died out altogether, but whose morphology provides clues about the nature of the ancestral species.90025 90020 Because of all this, the term 90029 transitional form 90030 is often used to mean organisms that lie between two groups as cousins, not just as ancestors. For example, the echidna is not ancestral to modern mammals, but retains features of the reptile-like mammals which other mammals (the therians) lost, and is therefore transitional between ancient reptiles and therian mammals. A generally interchangeable term is 90029 intermediate form 90030. Most iconic transitionals are the close descendants or cousins ​​of a directly ancestral species whose fossils have not been found, or which never fossilized to begin with.(Fossilization is extremely rare.) 90025 90026 According to creationists [edit] 90027 90020 According to creationists, each and every one of the following fossils is either misinterpreted by scientists, a fraud manufactured by scientists, or does not exist at all. They have been shown utterly wrong in this claim — so they keep claiming it anyway. 90025 90020 Creationists also 90029 define 90030 transitional forms in inherently absurd or vague ways that can not be met by the evidence. For example, they may insist that each of a proposed transitional organism’s parts (its legs, eyes, wings) should be «partially formed» instead of «fully formed», even if those parts have nothing to do with the transition.(For example, that an ape-human transitional form would have half-length limbs, or only one eye.) What «partially formed» means is not always clear. 90025 90020 Even more nebulously, it is argued that a true transitional would be an «incomplete creature», a fantasy that creationists will contrast with the known fossil record of «fully formed» or «fully functional» creatures. A similar idea is that «real» transitionals would be Frankenstein’s monsters, stitched-together assemblages of the parts of the two species.(It is akin to asserting that evolution could not be real unless the Piltdown hoax was also real, or asserting that if teenagers were 90029 really 90030 between children and adults, they would have baby heads on adult bodies.) In short, if biologists believe the species’s members were healthy enough to survive for more than a few days, then it can not be transitional, because it must have been «fully formed». The thinking here may be a product of biological essentialism — a thing is either A or B, and something «between» A and B is logically impossible or convoluted.90025 90020 Some creationists propose that the common ancestor of two species would necessarily look like a blend of them, such as Kirk Cameron’s infamous Crocoduck. This ignores that the evolutionary model involves traits arising without having to be present in the common ancestor. Otherwise, the world’s earliest life forms would somehow look like combinations of every species alive today. Also, a given distant ancestor species with living progeny (for example, the reptilian ancestor of ducks and crocodiles) will also have many different-looking descendants, not just two.Evolutionary lines do not «run out» of divergences any more than the surname Smith would run out of acts of reproduction. 90025 90020 Another mistake / lie made by creationist is that a transitional fossil has to be the direct descendant of one species and the direct ancestor of another. What they do not understand is the true definition of a transitional form. A transitional form 90008 illustrates 90009 an evolutionary link, as it can have features of two species, but have no other species as descendants.For example, your mother would be a ‘transitional form’ between you and your grandmother, as she shares traits with both of you. However, if your mother had a sister, she would also be a «transitional form» between you and your grandmother, having traits from both of you. 90025 90020 For example, apes and humans split from a common ancestor seven million years ago and both lineages are still around. 90025 90020 For this reason the concept of «missing link» is a misleading one. A transitional form does not need to be a perfect halfway house directly linking one group of organisms to another.»It merely needs to record aspects of evolutionary change that occurred as one lineage split from another.» [2] 90025 90020 It is also sometimes argued that if key transitional traits of an intermediate species (for example, the wing claws of 90029 Archaeopteryx 90030) have not completely disappeared today, the species is not transitional. (The ostrich has wing claws). Of course, biology is under no obligation to «clear the slate» and make it so that extinct species ‘traits are utterly foreign to us.To assume otherwise is to assume presentism, that there is something «special» about the modern day. Eventually, perhaps, ostrich claws will disappear — will this make them «retroactively» transitional? 90025 90020 When all hope is lost, creationists call the intermediate a «mosaic species» that «does not prove anything», and point out that God can create whatever he wants. 90025 90020 The species listed here are all thought to be close relatives of species descending from the first forms in the list and ancestral to the last ones.Each list represents some branch on the tree of life, its members being the sub-branches and «twigs» extending from this branch. 90025 90026 Transitions in vertebrates before the Cenozoic [edit] 90027 90072 90073 90029 This is a fairly incomplete list. Please help us expand it! 90030 90076 90077 90078 Invertebrate to Vertebrate [edit] 90079 90080 90081 Unnamed Upper (U.) Pre-Cambrian chordate — First to bear a primitive notochord; archaetypical chordate. 90082 90081 90029 90008 Pikaia gracilens 90009 90030 — Middle (M.) Cambrian chordate with lancelet-like morphology. 90082 90081 90029 90008 Haikouella 90009 90030 — Lower (L.) Cambrian chordate, first to bear a skull; archaetypical craniate. 90082 90081 90029 90008 Haikouichthys 90009 90030 — L. Cambrian quasi-vertebrate, intermediate in developing a vertebral column; archaetypical vertebrate. 90100 [1] 90101 90082 90081 90008 Conodonts 90009 — U. Cambrian to Triassic quasi-vertebrates with spinal cord; «Bug-eyed lampreys». 90082 90081 90029 90008 Myllokunmingia 90009 90030 — L.Cambrian vertebrate with primitive spinal column; oldest true crown-group vertebrate. 90082 90081 90029 90008 Arandaspis 90009 90030 — L. Ordovician vertebrate, armoured jawless fish (ostracoderm), oldest known vertebrate with hard parts known from (mostly) complete fossils. 90100 [2] 90101 90082 90121 90078 Jawless Fish to Jawed Vertebrate [edit] 90079 90080 90081 90029 90008 Birkenia 90009 90030 — Silurian primitive, jawless fish, a typical member of the Anaspida 90100 [3] 90101 90100 [4] 90101 90082 90081 90029 90008 Cephalaspis 90009 90030 — Silurian armoured jawless fish, archaetypical member of the «Osteostraca,» sister group to all jawed vertebrates.90082 90081 90029 90008 Shuyu 90009 90030 — Silurian to Devonian, armoured jawless fish belonging to Galeaspida, related to Osteostraca. Internal cranial anatomy very similar to the anatomy seen in basal jawed vertebrates 90100 [5] 90101. This similarity is directly implied with the translation of its name, «Dawn Fish,» with the implication that it represents the «dawn of jawed vertebrates.» 90082 90121 90078 Acanthodian to shark 90100 [6] 90101 [edit] 90079 90080 90081 90029 90008 Ptomacanthus 90009 90030 — sharklike fish, originally described as an acanthodian fish: brain anatomy demonstrates that it is an intermediate between acanthodians and sharks.90082 90081 90029 90008 Cladoselache 90009 90030 — primitive / basal shark. 90082 90081 90029 90008 Tristychius 90009 90030 — another sharklike fish. 90082 90081 90029 90008 Ctenacanthus 90009 90030 — primitive / basal shark. 90082 90081 90029 90008 Paleospinax 90009 90030 — sharklike jaw, primitive teeth. 90082 90081 90029 90008 Spathobatis 90009 90030 — Ray-like fish. 90082 90081 90029 90008 Protospinax 90009 90030 — Ancestral to both sharks and skates. 90082 90121 90078 Primitive jawed fish to bony fish 90100 [7] 90101 [edit] 90079 90080 90081 90029 90008 Acanthodians 90009 90030 — superficially similar to early bony fishes, and some have been identified as being the ancestors of sharks.90082 90081 90029 90008 Palaeoniscoids 90009 90030 — primitive bony fishes. 90082 90081 90029 90008 Canobius, Aeduella 90009 90030 — palaeoniscoids with more advanced jaws. 90082 90081 90029 90008 Parasemionotus 90009 90030 — combination of modern cheeks with more primitive features, like lungs 90082 90081 90029 90008 Oreochima 90009 90030 — first teleost fish 90082 90081 90029 90008 Leptolepids 90009 90030 — vaguely herring-like ancestors of modern teleost fish.Lung modified into swim bladder. 90082 90081 90029 90008 Amphistium and Heteronectes 90009 90030 — percomorphs that demonstrate the transition of the eye location of flatfishes. 90082 90121 90078 Fish to amphibian 90100 [8] 90101 [edit] 90079 90080 90081 90029 90008 Paleoniscoids 90009 90030 — both ancestral to modern fish and land vertebrates 90082 90081 90029 90008 Osteolepis 90009 90030 — modified limb bones, amphibian like skull and teeth 90082 90081 90029 90008 Kenichthys 90009 90030 — shows the position of exhaling nostrils moving from front to fish to throat in tetrapods in its halfway point, in the teeth 90082 90081 90029 90008 Eusthenopteron, Sterropterygion 90009 90030 — fin bones similarly structured to amphibian feet, but no toes yet, and still fishlike bodily proportions 90082 90081 90029 90008 Panderichthys, Elpistostege 90009 90030 — tetrapod-like bodily proportions.90082 90081 90029 90008 Obruchevichthys 90009 90030 — fragmented skeleton with intermediate characteristics, possible first tetrapod. 90082 90081 90029 90008 Tiktaalik 90009 90030 — a fish with developing legs. Also appearance of ribs and neck. 90082 90081 90029 90008 Acanthostega gunnari 90009 90030 -famous intermediate fossil. most primitive fossil that is known to be a tetrapod or four legged animal from the Upper Devonian of Greenland, which has shed significant light on the derivation and early evolution of tetrapods.It had legs and feet but was aquatic, not an amphibian. 90082 90081 90029 90008 Ichthyostega 90009 90030 — like Acanthostega, another fishlike amphibian 90082 90081 90029 90008 Hynerpeton 90009 90030 — A little more advanced then Acanthostega and Ichtyostega 90082 90081 90029 90008 Labyrinthodonts 90009 90030 — still many fishlike features, but tailfins have disappeared 90082 90081 90029 90008 Gars 90009 90030 — Fish with vascularized swim bladders that can function as lungs 90082 90081 90029 90008 Lungfish and Birchirs 90009 90030 — fish that have lungs 90082 90121 90078 Primitive to modern amphibians 90100 [9] 90101 [edit] 90079 90080 90081 90029 90008 Temnospondyls 90009 90030 90082 90081 90029 90008 Dendrerpeton acadianum 90009 90030 90082 90081 90029 90008 Archegosaurus decheni 90009 90030 90082 90081 90029 90008 Eryops megacephalus 90009 90030 90082 90081 90029 90008 Trematops 90009 90030 90082 90081 90029 90008 Amphibamus lyelli 90009 90030 90082 90081 90029 90008 Doleserpeton annectens 90009 90030 90082 90081 90029 90008 Triadobatrachus 90009 90030 — a primitive frog.90082 90081 90029 90008 Vieraella 90009 90030 — an early modern frog 90082 90081 90029 90008 Karaurus 90009 90030 — a primitive salamander 90082 90121 90078 Amphibian to reptile 90100 [10] 90101 [edit] 90079 90080 90081 90029 90008 Proterogyrinus 90009 90030 90082 90081 90029 90008 Limnoscelis 90009 90030 90082 90081 90029 90008 Tseajaia 90009 90030 90082 90081 90029 90008 Solenodonsaurus 90009 90030 90082 90081 90029 90008 Hylonomus 90009 90030 90082 90081 90029 90008 Paleothyris 90009 90030 90082 90121 90078 Early reptile to diapsid 90100 [11] 90101 [edit] 90079 90080 90081 90029 90008 Hylonomus 90009 90030 90082 90081 90029 90008 Paleothyris 90009 90030 90082 90081 90029 90008 Petrolacosaurus 90009 90030 90082 90081 90029 90008 Araeoscelis 90009 90030 90082 90081 90029 90008 Apsisaurus 90009 90030 90082 90081 90029 90008 Claudiosaurus 90009 90030 90082 90081 90029 90008 Planocephalosaurus 90009 90030 90082 90081 90029 90008 Protorosaurus 90009 90030 90082 90081 90029 90008 Prolacerta 90009 90030 90082 90081 90029 90008 Proterosuchus 90009 90030 90082 90081 90029 90008 Hyperodapedon 90009 90030 90082 90081 90029 90008 Trilophosaurus 90009 90030 90082 90121 90078 Early diapsid to turtle 90100 [12] 90101 [edit] 90079 90080 90081 90029 90008 Pappochelys rosinae 90009 90030 — diapsid skull with expanded ribs and fused gastralia 90082 90081 90029 90008 Odontochelys semitestacea 90009 90030 — secondary loss of temporal fenestrae, partial formation of a turtle shell, showing how the hard underbelly, or plastron, formed first.90100 [13] 90101 90082 90081 90029 90008 Deltavjatia vjatkensis 90009 90030 90082 90081 90029 90008 Proganochelys 90009 90030 90082 90121 90078 Early synapsid to mammal 90100 [14] 90101 [edit] 90079 90080 90081 90029 90008 Paleothyris 90009 90030 90082 90081 90029 90008 Protoclepsydrops haplous 90009 90030 90082 90081 90029 90008 Clepsydrops 90009 90030 90082 90081 90029 90008 Archaeothyris 90009 90030 90082 90081 90029 90008 Varanops 90009 90030 90082 90081 90029 90008 Haptodus 90009 90030 90082 90081 90029 90008 Dimetrodon 90009 90030 90082 90081 90029 90008 Sphenacodon 90009 90030 90082 90081 90029 90008 Biarmosuchia 90009 90030 90082 90081 90029 90008 Procynosuchus 90009 90030 90082 90081 90029 90008 Dvinia 90009 90030 90082 90081 90029 90008 Thrinaxodon 90009 90030 90082 90081 90029 90008 Cynognathus 90009 90030 90082 90081 90029 90008 Diademodon 90009 90030 90082 90081 90029 90008 Probelesodon 90009 90030 90082 90081 90029 90008 Probainognathus 90009 90030 90082 90081 90029 90008 Exaeretodon 90009 90030 90082 90081 90029 90008 Oligokyphus 90009 90030 90082 90081 90029 90008 Kayentatherium 90009 90030 90082 90081 90029 90008 Pachygenelus 90009 90030 90082 90081 90029 90008 Diarthrognathus 90009 90030 90082 90081 90029 90008 Adelobasileus cromptoni 90009 90030 90082 90081 90029 90008 Sinoconodon 90009 90030 90082 90081 90029 90008 Kuehneotherium 90009 90030 90082 90081 90029 90008 Eozostrodon 90009 90030 90082 90081 90029 90008 Morganucodon 90009 90030 — a transition between «proto mammals» and «true mammals».90082 90081 90029 90008 Haldanodon 90009 90030 90082 90081 90029 90008 Peramus 90009 90030 90082 90081 90029 90008 Endotherium 90009 90030 90082 90081 90029 90008 Kielantherium 90009 90030 90082 90081 90029 90008 Aegialodon 90009 90030 90082 90081 90029 90008 Steropodon galmani 90009 90030 90082 90081 90029 90008 Vincelestes neuquenianus 90009 90030 90082 90081 90029 90008 Pariadens kirklandi 90009 90030 90082 90081 90029 90008 Kennalestes 90009 90030 90082 90081 90029 90008 Asioryctes 90009 90030 90082 90081 90029 90008 Procerberus 90009 90030 90082 90081 90029 90008 Gypsonictops 90009 90030 90082 90081 90029 90008 Juramaia 90009 90030 90082 90081 90029 90008 Eomaia 90009 90030 90082 90081 90029 90008 Sinodelphys 90009 90030 90082 90121 90078 Dinosaur to bird 90100 [15] 90101 [edit] 90079 90080 90081 90029 90008 Kulindadromeus 90009 90030 — A basal neornithischian (Ya know, 90029 Triceratops 90030, 90029 Iguanadon 90030, 90029 Hypsilophodon 90030, and such) with feathers.90082 90081 90029 90008 Allosaurus 90009 90030 — A large theropod with a wishbone. 90082 90081 90029 90008 Aerosteon 90009 90030 — A large theropod of the same lineage as the aforementioned 90029 Allosaurus 90030 that was discovered to have air sacs supplementing lungs, like modern birds. 90082 90081 90029 90008 Compsognathus 90009 90030 — A small coeleurosaur with a wishbone. 90082 90081 90029 90008 Epidendrosaurus 90009 90030 90082 90081 90029 90008 Epidexipteryx 90009 90030 90082 90081 90029 90008 Scandoriopteryx 90009 90030 90082 90081 90029 90008 Gigantoraptor 90009 90030 — A large oviraptorosaur discovered brooding its nests in order to protect and incubate eggs.90082 90081 90029 90008 Gobivenator 90009 90030 90082 90081 90029 90008 Mei 90009 90030 — A troodont discovered sleeping with its head underneath its wing / 90082 90081 90029 90008 Saurornithoides 90009 90030 90082 90081 90029 90008 Sinovenator 90009 90030 90082 90081 90029 90008 Buitreraptor 90009 90030 90082 90081 90029 90008 Pyroraptor 90009 90030 90082 90081 90029 90008 Unenlagia 90009 90030 90082 90081 90029 90008 Graciliraptor 90009 90030 90082 90081 90029 90008 Bambiraptor 90009 90030 90082 90081 90029 90008 Balaur 90009 90030 — A large flightless bird that was wrongly thought to be a dromaeosaurid dinosaur.. 90082 90081 90029 90008 Tsaagan 90009 90030 90082 90081 90029 90008 Dromaeosaurus 90009 90030 90082 90081 90029 90008 Sinosauropteryx 90009 90030 — a basal coelurosaur discovered to be covered in feathers. It is also the first dinosaur to have its colour determined, thanks to preserved pigment structures in the feathers. 90082 90081 90029 90008 Protarchaeopteryx 90009 90030 90082 90081 90029 90008 Caudipteryx 90009 90030 90082 90081 90029 90008 Velociraptor 90100 90101 90009 90030 — a famous dromaeosaurid discovered to have quill knobs on its wrists.For some odd-like reason, sadly. Everyone sees these things as mutant dragon-like things. 90082 90081 90029 90008 Deinonychus 90100 90101 90009 90030 90082 90081 90029 90008 Utahraptor 90009 90030 90082 90081 90029 90008 Achillobator 90009 90030 90082 90081 90029 90008 Oviraptor 90009 90030 — the first dinosaur discovered to 90984 steal 90985 brood nests. 90082 90081 90029 90008 Sinovenator 90009 90030 90082 90081 90029 90008 Beipiaosaurus 90009 90030 90082 90081 90029 90008 Lisboasaurus 90009 90030 90082 90081 90029 90008 Sinornithosaurus 90009 90030 90082 90081 90029 90008 Microraptor 90009 90030 — a feathered bird with distinctly dinosaurian characteristics, such as its tail.90082 90081 90029 90008 Xiaotingia 90009 90030 — slightly earlier than Archaeopteryx, slightly more like a dinosaur and less like a bird 90082 90081 90029 90008 Archaeopteryx 90009 90030 — the famous bird-with-teeth. 90082 90081 90029 90008 Anchiornis 90009 90030 90082 90081 90029 90008 Baptornis 90009 90030 90082 90081 90029 90008 Rahonavis 90009 90030 90082 90081 90029 90008 Confuciusornis 90009 90030 90082 90081 90029 90008 Sinornis 90009 90030 90082 90081 90029 90008 Iberomesornis 90009 90030 90082 90081 90029 90008 Theriznosaurus 90009 90030 90082 90081 90029 90008 Nothronychus 90009 90030 90082 90081 90029 90008 Citipati 90009 90030 90082 90081 90029 90008 Falcarius 90009 90030 90082 90081 90029 90008 Alxasaurus 90009 90030 90082 90081 90029 90008 Chirostenotes 90009 90030 90082 90081 90029 90008 Avimimus 90009 90030 90082 90081 90029 90008 Khaan 90009 90030 90082 90081 90029 90008 Incisivosaurus 90009 90030 90082 90081 90029 90008 Caenagnathus 90009 90030 90082 90081 90029 90008 Troodon 90009 90030 90082 90081 90029 90008 Byronosaurus 90009 90030 90082 90081 90029 90008 Ingenia 90009 90030 90082 90081 90029 90008 Hesperonychus 90009 90030 90082 90081 90029 90008 Conchoraptor 90009 90030 90082 90081 90029 90008 Patagopteryx 90009 90030 90082 90081 90029 90008 Ambiortus 90009 90030 90082 90081 90029 90008 Hesperornis 90009 90030 — A diving seabird with prominent teeth.It’s also completely flightless. 90082 90081 90029 90008 Apsaravis 90009 90030 90082 90081 90029 90008 Ichthyornis 90009 90030 — A flying seabird with prominent teeth. 90082 90081 90029 90008 Columba 90009 90030 — One of many typical modern birds. 90082 90121 90026 Transitional mammalian fossils [edit] 90027 90078 Primates [edit] 90079 90078 Non-human primate to human [edit] 90079 90078 Cetaceans [edit] 90079 90080 90081 90029 90008 Indohyus 90009 90030 — a vaguely chevrotain-like or raccoon-like aquatic artiodactyl ungulate with an inner ear identical to that of whales.90082 90081 90029 90008 Ambulocetus 90100 90101 90009 90030 — an early whale that looks like a mammalian version of a crocodile 90082 90081 90029 90008 Pakicetus 90009 90030 — an early, semi-aquatic whale, a superficially wolf-like animal believed to be a direct ancestor of modern whales. 90082 90081 90029 Rhodocetus 90030 — An early whale with comparatively large hindlegs: not only represents a transition between semi-aquatic whales, like Ambulocetus, and obligately aquatic whales, like 90029 Basilosaurus 90030.90082 90081 90029 90008 Basilosaurus 90009 90030 — A large, elongated whale with vestigial hind flippers: transition from early marine whales (like 90029 Rhodocetus 90030) to modern whales 90082 90081 90029 90008 Dorudon 90100 90101 90009 90030 — A small whale with vestigial hind flippers, close relative of 90029 Basilosaurus 90030. 90082 90121 90078 Proboscideans [edit] 90079 90080 90081 90029 90008 Eritherium 90009 90030 90082 90081 90029 90008 Phosphatherium 90009 90030 90082 90081 90029 90008 Numidotherium 90009 90030 90082 90081 90029 90008 Barytherium 90009 90030 90082 90081 90029 90008 Phiomia 90009 90030 90082 90081 90029 90008 Prodeinotherium 90009 90030 90082 90081 90029 90008 Stegodon 90009 90030 90082 90121 90026 Transitional plant fossils [edit] 90027 90026 See also [edit] 90027 90026 External links [edit] 90027 90026 References [edit] 90027 .90000 Transitional form — CreationWiki, the encyclopedia of creation science 90001 90002 90003 A 90004 transitional form 90005 is a living or fossilized organism that is believed to be an evolutionary link between two distinct groups (also referred to as intermediates or 90006 missing links 90007). This is suggested when the organism possesses characteristics that uniquely belong to different taxa, or when fossils exhibit traits that are considered primitive in comparison to living varieties.90008 [1] 90009 For example, the Tiktaalik is thought by evolutionists to be an intermediate between fish and reptiles because it has fleshy fins, and thus one believes that the fins enabled it to evolve from shallow waters to life on land. 90010 90003 When we examine living populations, there is a clear distinction between various plants and animals, which has allowed us to classify them into taxonomic groups. Since evolution via random mutations must occur at an extremely slow and gradual rate, we should find a continuous series of intermediate fossils that illustrate the transition from one type of organism to another if this theory is valid.The sheer absence of these transitional forms is the most reiterated empirical evidence against evolution. In this arena quotes from honest evolutionists are numerous decrying the state of the fossil record. For example David B. Kitts of the School of Geology and Geophysics at the University of Oklahoma wrote: «90006 Evolution requires intermediate forms between species and paleontology does not provide them … 90007″. 90008 [2] 90009 90010 90003 The evolutionist Michael Denton also acknowledges this irrefutable problem.90010 90019 90020 90021 » 90022 90021 The fact that so many of the founders of modern biology, those who discovered all the basic facts of comparative morphology upon which modern evolutionary biology is based, held nature to be fundamentally a discontinuum of isolated and unique types unbridged by transitional varieties, a position absolutely at odds with evolutionary ideas, is obviously very difficult to reconcile with the popular notion that all the facts of biology irrefutably support an evolutionary interpretation.90008 [3] 90009 90022 90027 » 90022 90029 90030 90003 To nearly all of the great biologists and naturalists of the late eighteenth and early nineteenth centuries, Darwin’s theory failed to explain the discontinuity of species. Instead a typological model was adhered to, which recognized that organisms exist as distinct types. 90008 [4] 90009 To explain the absence of transitional forms, an uncharacterized macroevolution process was proposed as being responsible for the rapid saltation (evolutionary jumps) responsible for the gaps between taxa, with the traditional 90006 micro 90007 process operating to produce the slow and gradual change responsible for groups of similar species.90010 90037 Absence of fossil forms 90038 90003 There are over one hundred million identified and catalogued fossils currently in the world’s museums. 90008 [5] 90009 If all life on Earth has descended from a single common ancestor then there should be countless «transitional forms» in the fossil record that show the gradual transformation of organisms. Compared to the number of living and fossil organisms, transitional forms are conspicuously absent and often referred to simply as «missing links».90010 90003 Evolutionist Michael Denton stated: 90010 90019 90020 90021 » 90022 90021 It is still, as it was in Darwin’s day, overwhelmingly true that the first representatives of all the major classes of organisms known to biology are already highly characteristic of their class when they make their initial appearance in the fossil record. This phenomenon is particularly obvious in the case of the invertebrate fossil record. At its first appearance in the ancient paleozoic seas, invertebrate life was already divided into practically all the major groups with which we are familiar today.90008 [6] 90009 90022 90027 » 90022 90029 90030 90003 If Biblical young earth creationism is true then this precisely what should be found. The creationist view predicts the presence of distinct kinds of animal with no transitionals between; or at best a handful of disputed forms. Evolutionists have spent over 140 years trying to find transitional fossils and nothing approaching a conclusive form has ever been found — only a handful of doubtful examples exist. 90008 [7] 90009 90008 [8] 90009 90010 90003 Noted anthropologist Edmund Ronald Leach stated: 90010 90019 90020 90021 » 90022 90021 Missing links in the sequence of fossil evidence were a worry to Darwin.He felt sure they would eventually turn up, but they are still missing and seem likely to remain so. 90008 [9] 90009 90022 90027 » 90022 90029 90030 90003 David Raup, who was the curator of geology at the museum holding the world’s largest fossil collection (the Field Museum of Natural History in Chicago), observed: 90006 «[Darwin] was embarrassed by the fossil record because it did not look the way he predicted it would …. Well, we are now about 120 years after Darwin, and knowledge of the fossil record has been greatly expanded.We now have a quarter of a million fossil species but the situation has not changed much …. [W] e have even fewer examples of evolutionary transition than we had in Darwin’s time. «90007 (David M. Raup,» Conflicts Between Darwin and Paleontology, «90006 Field Museum of Natural History Bulletin 90007 50 (January 1979): 22-23, 24-25) 90010 90003 One of the most famous proponents of evolution was the late Harvard paleontologist Stephen Jay Gould. But Gould admitted, «The extreme rarity of transitional forms in the fossil record persists as the trade secret of paleontology.We fancy ourselves as the only true students of life’s history, yet to preserve our favored account of evolution by natural selection, we view our data as so bad that we never see the very process we profess to study. 90008 [10] 90009 In a 1977 paper titled «The Return of Hopeful Monsters», Gould wrote: «All paleontologists know that the fossil record contains precious little in the way of intermediate forms; transitions between major groups are characteristically abrupt.» 90008 [11] 90009 90010 90003 The senior paleontologist at the British Museum of Natural History, Dr.Colin Patterson, put it this way: «Gould and the American Museum people are hard to contradict when they say there are no transitional fossils.» 90008 [12] 90009 90010 90037 Evolutionist views 90038 90003 Evolutionists present several transition series. They are based on a comparison of selected parts, while often ignoring bigger differences. Sometimes the various types are represented only by fragmented fossil evidence. Often they will use what are called cousins ​​when they can not find a fossil in the right place to be able to call it an ancestor.The gaps recognized by evolutionists are often at critical parts in the fossil record. Jonathan Sarfati comments: 90010 90019 90020 90021 » 90022 90021 «90006 Many of the alleged transitional forms are based on fragmentary remains, which are therefore open to several interpretations, based on one’s axioms. Evolutionary bias means that such remains are often likely to be interpreted as transitional, as with Gingerich, and is also prevalent in ape-man claims. But when more bones are discovered, then the fossils nearly always fit one type or another, and are no longer plausible as transitional.It’s also notable that alleged intermediate forms are often trumpeted in the media, while retractions are usually muted or unpublicized. 90007 «90008 [13] 90009 90022 90027 » 90022 90029 90030 90003 Often the designation of a new genus or species is based on only one individual or at most only a few individuals. In such cases they may represent a small number of degenerative mutants, rather than an entire genus or species. This would greatly reduce the number of individuals implied by the fossil record.90010 90113 Darwin’s Concerns 90114 90003 Charles Darwin himself stated that evolutionary theory required the existence of «transitional forms.» Darwin wrote: «90006 So that the number of intermediate and transitional links, between all living and extinct species, must have been inconceivably great. But assuredly, if this theory be true, such have lived upon the earth 90007.» 90008 [14] 90009 90010 90003 The lack of transitional forms was one of the four prominent weaknesses to his theory that Darwin puzzled over in «90006 On the Origin of Species 90007,» and indeed devoted an entire chapter of the book (IV: On the Imperfection of the Fossil Record) 90008 [15] 90009 to what he acknowledged was a serious danger to his theory: 90010 90019 90020 90021 » 90022 90021 «In the four succeeding chapters, the most apparent and gravest difficulties on the theory will be given: namely, first, the difficulties of transitions, or in understanding how a simple being or a simple organ can be changed and perfected into a highly developed being or elaborately constructed organ; secondly, the subject of Instinct, or the mental powers of animals; thirdly, Hybridism, or the infertility of species and the fertility of varieties when intercrossed; and fourthly, the imperfection of the Geological Record.»(P. 6) 90008 [16] 90009 90022 90027 » 90022 90029 90030 90019 90020 90021 » 90022 90021 «Long before having arrived at this part of my work, a crowd of difficulties will have occurred to the reader. Some of them are so grave that to this day I can never reflect on them without being staggered; but, to the best of my judgment, the greater number are only apparent, and those that are real are not, I think, fatal to my theory. 90003 These difficulties and objections may be classed under the following heads: — 90010 90003 Firstly, why, if species have descended from other species by insensibly fine gradations, do we not everywhere see innumerable transitional forms? Why is not all nature in confusion instead of the species being, as we see them, well defined? 90010 90003 Secondly, is it possible that an animal having, for instance, the structure and habits of a bat, could have been formed by the modification of some animal with wholly different habits? Can we believe that natural selection could produce, on the one hand, organs of trifling importance, such as the tail of a giraffe, which serves as a fly-flapper, and, on the other hand, organs of such wonderful structure, as the eye, of which we hardly as yet fully understand the inimitable perfection? 90010 90003 Thirdly, can instincts be acquired and modified through natural selection? What shall we say to so marvellous an instinct as that which leads the bee to make cells, which have practically anticipated the discoveries of profound mathematicians? 90010 90003 Fourthly, how can we account for species, when crossed, being sterile and producing sterile offspring, whereas, when varieties are crossed, their fertility is unimpaired? «(Pp.171-172) 90008 [17] 90009 90010 90022 90027 » 90022 90029 90030 90003 Further, Darwin wrote: «90006 Why then is not every geological formation and every strata full of such intermediate links? Geology assuredly does not reveal any such finely-graduated organic chain; and this perhaps, is the most obvious and serious objection which can be urged against my theory 90007. » 90008 [18] 90009 Darwin thought the lack of transitional links in his time was because 90010 90019 90020 90021 » 90022 90021 only a small portion of the surface of the earth has been geologically explored and no part with sufficient care… «. 90008 [19] 90009 90022 90027 » 90022 90029 90030 90003 Darwin’s theory of evolution required that transitional forms exist. As Charles Darwin became older, however, he became increasingly concerned about this lack of evidence in regards to the fossil record. Darwin wrote, 90010 90019 90020 90021 » 90022 90021 When we descend to details, we can not prove that a single species has changed; nor can we prove that the supposed changes are beneficial, which is the groundwork of the theory.90008 [20] 90009 90022 90027 » 90022 90029 90030 90113 Disputed Existence 90114 90003 {{line-height | 1 | Some evolutionists aver that transitional forms exist, but that is certainly not the consensus. The eminent Pierre Grasse, editor of the 28 volume 90004 Traite de Zoologie 90005, one-time president of the 90006 Academie des Science 90007 attacked modern evolution theory on the grounds that it assumed to know much more than it did. Stephen Jay Gould in an often quoted statement calls the lack of transitional forms the «trade secret» in paleontology.It should be noted that this statement was made not in an effort to support creationism, but rather an explanation of why he felt the need to design a different theory to accommodate this absence. 90010 90003 The California University evolutionist biologist James W. Valentine makes this confession in 90006 What Darwin Began 90007: 90010 90019 90020 90021 » 90022 90021 The fossil record is of little use in providing direct evidence of the pathways of descent of the phyla or of invertebrate classes.Each phylum with a fossil record had already evolved its characteristic body plan when it first appeared, so far as we can tell from the fossil remains, and no phylum is connected to any other via intermediate fossil types. Indeed, none of the invertebrate classes can be connected with another class by series of intermediates. 90008 [21] 90009 90022 90027 » 90022 90029 90030 90003 Colin Patterson, senior paleontologist at the British Museum of Natural History, wrote to a reader in 1979 the following passage: 90010 90019 90020 90021 » 90022 90021 I fully agree with your commentary on the lack of evolutionary transitions in my book.If I knew of any, fossil or living, I would have certainly included them. I will lay it on the line, there is not one such fossil for which one could make a watertight argument. 90022 90027 » 90022 90029 90030 90003 It is claimed that Patterson asked the assemblage of paleontologists at a 1998 conference whether anyone knew of a genuine transitional form, receiving no answer but silence. This report is only partially substantiated. 90010 90113 Punctuated Equilibrium 90114 90233 90234 90006 Main Article: Punctuated equilibrium 90007 90237 90238 90003 The lack of support from the fossil record and the dearth of transitional forms composing a reasonably minimum continuum is something recognized by some evolutionists themselves.Evolutionists have de 90010.90000 biology | Definition, History, Concepts, Branches, & Facts 90001 90002 90003 Biology 90004, study of living things and their vital processes. The field deals with all the physicochemical aspects of life. The modern tendency toward cross-disciplinary research and the unification of scientific knowledge and investigation from different fields has resulted in significant overlap of the field of biology with other scientific disciplines. Modern principles of other fields-chemistry, medicine, and physics, for example-are integrated with those of biology in areas such as biochemistry, biomedicine, and biophysics.90005 90003 biology; microscope 90004 A researcher using a microscope to examine a specimen in the laboratory. 90008 © Radu Razvan / Fotolia 90009 90002 Top Questions 90005 90012 What is biology? 90013 90002 Biology is a branch of science that deals with living organisms and their vital processes. Biology encompasses diverse fields, including botany, conservation, ecology, evolution, genetics, marine biology, medicine, microbiology, molecular biology, physiology, and zoology. 90005 90016 Why is biology important? 90013 90018 Where do biology graduates work? 90013 90002 Biology graduates can hold a wide range of jobs, some of which may require additional education.A person with a degree in biology could work in agriculture, health care, biotechnology, education, environmental conservation, research, forensic science, policy, science communication, and many other areas. 90005 90002 Biology is subdivided into separate branches for convenience of study, though all the subdivisions are interrelated by basic principles. Thus, while it is custom to separate the study of plants (botany) from that of animals (zoology), and the study of the structure of organisms (morphology) from that of function (physiology), all living things share in common certain biological phenomena-for example, various means of reproduction, cell division, and the transmission of genetic material.90005 90002 Biology is often approached on the basis of levels that deal with fundamental units of life. At the level of molecular biology, for example, life is regarded as a manifestation of chemical and energy transformations that occur among the many chemical constituents that compose an organism. As a result of the development of increasingly powerful and precise laboratory instruments and techniques, it is possible to understand and define with high precision and accuracy not only the ultimate physiochemical organization (ultrastructure) of the molecules in living matter but also the way living matter reproduces at the molecular level.Especially crucial to those advances was the rise of genomics in the late 20th and early 21st centuries. 90005 90002 Cell biology is the study of cells-the fundamental units of structure and function in living organisms. Cells were first observed in the 17th century, when the compound microscope was invented. Before that time, the individual organism was studied as a whole in a field known as organismic biology; that area of ​​research remains an important component of the biological sciences. Population biology deals with groups or populations of organisms that inhabit a given area or region.Included at that level are studies of the roles that specific kinds of plants and animals play in the complex and self-perpetuating interrelationships that exist between the living and the nonliving world, as well as studies of the built-in controls that maintain those relationships naturally . Those broadly based levels-molecules, cells, whole organisms, and populations-may be further subdivided for study, giving rise to specializations such as morphology, taxonomy, biophysics, biochemistry, genetics, epigenetics, and ecology.A field of biology may be especially concerned with the investigation of one kind of living thing-for example, the study of birds in ornithology, the study of fishes in ichthyology, or the study of microorganisms in microbiology. 90005 Get exclusive access to content from our тисяча сімсот шістьдесят вісім First Edition with your subscription. Subscribe today 90028 Basic concepts of biology 90029 90028 Biological principles 90029 90002 The concept of homeostasis-that living things maintain a constant internal environment-was first suggested in the 19th century by French physiologist Claude Bernard, who stated that «all the vital mechanisms, varied as they are, have only one object: that of preserving constant the conditions of life.»90005 90002 As originally conceived by Bernard, homeostasis applied to the struggle of a single organism to survive. The concept was later extended to include any biological system from the cell to the entire biosphere, all the areas of Earth inhabited by living things. 90005 90028 Unity 90029 90002 All living organisms, regardless of their uniqueness, have certain biological, chemical, and physical characteristics in common. All, for example, are composed of basic units known as cells and of the same chemical substances, which, when analyzed, exhibit noteworthy similarities, even in such disparate organisms as bacteria and humans.Furthermore, since the action of any organism is determined by the manner in which its cells interact and since all cells interact in much the same way, the basic functioning of all organisms is also similar. 90005 90003 cells 90004 Animal cells and plant cells contain membrane-bound organelles, including a distinct nucleus. In contrast, bacterial cells do not contain organelles. 90008 Encyclopædia Britannica, Inc. 90009 90002 There is not only unity of basic living substance and functioning but also unity of origin of all living things.According to a theory proposed in 1855 by German pathologist Rudolf Virchow, «all living cells arise from pre-existing living cells.» That theory appears to be true for all living things at the present time under existing environmental conditions. If, however, life originated on Earth more than once in the past, the fact that all organisms have a sameness of basic structure, composition, and function would seem to indicate that only one original type succeeded. 90005 90002 A common origin of life would explain why in humans or bacteria-and in all forms of life in between-the same chemical substance, deoxyribonucleic acid (DNA), in the form of genes accounts for the ability of all living matter to replicate itself exactly and to transmit genetic information from parent to offspring.Furthermore, the mechanisms for that transmittal follow a pattern that is the same in all organisms. 90005 90002 Whenever a change in a gene (a mutation) occurs, there is a change of some kind in the organism that contains the gene. It is this universal phenomenon that gives rise to the differences (variations) in populations of organisms from which nature selects for survival those that are best able to cope with changing conditions in the environment. 90005 .90000 Selection | biology | Britannica 90001 90002 90003 Selection 90004, in biology, the preferential survival and reproduction or preferential elimination of individuals with certain genotypes (genetic compositions), by means of natural or artificial controlling factors. 90005 90003 artificial selection 90004 Great Dane (rear) stands with a Chihuahua mix to demonstrate the wide range of dog breed sizes created by artificial selection. 90008 Ellen Levy Finch 90009 90002 The theory of evolution by natural selection was proposed by Charles Darwin and Alfred Russel Wallace in тисячі вісімсот п’ятьдесят-вісім.They argued that species with useful adaptations to the environment are more likely to survive and produce progeny than are those with less useful adaptations, thereby increasing the frequency with which useful adaptations occur over the generations. The limited resources available in an environment promotes competition in which organisms of the same or different species struggle to survive. In the competition for food, space, and mates that occurs, the less well-adapted individuals must die or fail to reproduce, and those who are better adapted do survive and reproduce.In the absence of competition between organisms, natural selection may be due to purely environmental factors, such as inclement weather or seasonal variations. (90011 See 90012 natural selection.) 90005 90002 Artificial selection (or selective breeding) differs from natural selection in that heritable variations in a species are manipulated by humans through controlled breeding. The breeder attempts to isolate and propagate those genotypes that are responsible for a plant or animal’s desired qualities in a suitable environment.These qualities are economically or aesthetically desirable to humans, rather than useful to the organism in its natural environment. 90005 90002 In mass selection, a number of individuals chosen on the basis of appearance are mated; their progeny are further selected for the preferred characteristics, and the process is continued for as many generations as is desired. The choosing of breeding stock on the basis of ancestral reproductive ability and quality is known as pedigree selection. Progeny selection indicates choice of breeding stock on the basis of the performance or testing of their offspring or descendants.Family selection refers to mating of organisms from the same ancestral stock that are not directly related to each other. Pure-line selection involves selecting and breeding progeny from superior organisms for a number of generations until a pure line of organisms with only the desired characteristics has been established. 90005 Get exclusive access to content from our тисяча сімсот шістьдесят вісім First Edition with your subscription. Subscribe today 90002 Darwin also proposed a theory of sexual selection, in which females chose as mates the most attractive males; outstanding males thus helped generate more young than mediocre males.90005.