Introduction : Birds ar phylogenetically thought-about to be members of the bird-footed dinosaur dinosaurs; their nearest non-avian relatives ar the dromaeosaurid theropods. the primary better-known fossil bird is archeopteryx, from the late Jurassic of Bavaria, Germany, that is painted by seven skeletons and a feather. there's no fossil proof from before now that has been established to be of vertebrate origin. The fossil record of recent birds began within the early Tertiary (Padian et al., 1998).
There ar plenty of anatomical terms accustomed describe the evolution of birds, thus diagrams showing the skeletons of a saurischian, archeopteryx and a contemporary bird ar shown in figure one, so as to outline most of the terms used.
The archosaurian Hypothesis
The archosaurian hypothesis for the origin of birds is characterized by being a alternative. this is often as a result of it's undue to correlation of characters and taxa however thanks to the negative association with alternative taxa. it had been originally thought that theropods shared a lot of options with birds than the other cluster. However, at the time there was no proof for clavicles in theropods, that ar the equivalent of the os or furcula in birds. it had been thought that clavicles couldn't are lost and so re-evolved into os and then a a lot of ancient relation for birds was looked for.
The candidate advised was the thecodonts from that all alternative archosaurs ar thought to own evolved. a retardant is that the archosaurs ar a "wastebasket" cluster containing all archosaurian reptiles that don't match into dinosaurs, crocodiles or pterosaurs. Hence, they need no diagnostic characters of their own and aren't a "good" organic process cluster. This makes it tough to match them with alternative categorisation teams (Padian et al., 1998).
The archosaurian hypothesis for the origin of birds is characterized by being a alternative. this is often as a result of it's undue to correlation of characters and taxa however thanks to the negative association with alternative taxa. it had been originally thought that theropods shared a lot of options with birds than the other cluster. However, at the time there was no proof for clavicles in theropods, that ar the equivalent of the os or furcula in birds. it had been thought that clavicles couldn't are lost and so re-evolved into os and then a a lot of ancient relation for birds was looked for. The candidate advised was the thecodonts from that all alternative archosaurs ar thought to own evolved. a retardant is that the archosaurs ar a "wastebasket" cluster containing all archosaurian reptiles that don't match into dinosaurs, crocodiles or pterosaurs. Hence, they need no diagnostic characters of their own and aren't a "good" organic process cluster. This makes it tough to match them with alternative categorisation teams (Padian et al., 1998).
The Crocodylomorph Hypothesis
Crocodylomorphs embody crocodiles and a few Triassic-Jurassic forms that ar closely connected however not true crocodiles. This hypothesis has fewer issues than the archosaurian hypothesis, as crocodiles ar a monophyletic cluster, which may be compared to alternative taxa. The synapomorphies of birds and crocodiles are tested, however it's been found that almost all ar general to the archosaurs. notwithstanding these ar accepted, there ar solely 15-20 synapomorphies compared to over seventy with the theropods (Padian et al., 1998).
The bird-footed dinosaur Hypothesis
Cladistic analysis supports this hypothesis and shows that the foremost closely connected cluster (sister group) of theropods to the birds includes the dromaesaurids like maniraptor. There ar varied synapomorphies of the skeleton and os that link birds and theropods. There are found a series of skeletal changes in theropods that ar thought-about to be vertebrate. as an example, basal bird-footed dinosaur dinosaurs have gently designed bones and a foot reduced to 3 main toes, with the primary command off the bottom and also the fifth lost. Moving through the bird-footed dinosaur sequence towards birds, there's a discount and loss of manual digits four and 5, increasing lightness of the skeleton, and a discount within the range, and partial interlocking, of the tail vertebrae. In coelurosaurs, that embody birds, dromaeosaurids and alternative teams, the arms become longer and also the initial toe begins to rotate backwards behind the metatarsals. united clavicles (furcula) ar apparently basal to Tetanurae (carnosaurs and coelurosaurs) and bone plates ar better-known during a type of tetanurans. within the pelvis, the os and ischial bone begin to point out a bigger inequality long. Finally, within the dromaeosaurids and archeopteryx, the os begins to purpose backwards rather than forwards, the anterior projection on the foot of the os is lost, the tail becomes even shorter and also the hyperflexing radiocarpal joint is gift, that permits the action that's crucial to the flight stroke in birds. These options were passed to birds from their dinosaurian associatecestors and not specifically evolved for an vertebrate life-style (Padian et al., 1998).
The Evolution of Feathers
The first better-known feathers area unit from alittle coelurosaurian archosaur known as Sinosauropteryx, that features a row of tiny fringed structures on its skeletal structure. Therefore, feathers don't seem to be a synapomorphy of birds, they're shared by a broader cluster inside the theropod dinosaur dinosaurs. This shows that feathers failed to evolve specifically for flight (Padian et al., 1998).
Hypotheses for the initial perform of feathers embody insulation, show and camouflage. These area unit hypotheses area unit tough to check, and it looks possible that feathers were used for many completely different functions as they're currently (Padian et al., 1998).
The cursorial Theory
The cursorial theory is predicated on the proof that bird was a powerful, agile, biped, and from proof that birds evolved from tiny, active, running predators. it's been urged that feathers were used as nets to capture insects, however it's been shown that this action would cause the proto-bird to lose its balance by throwing off its momentum. As another, it absolutely was urged that insects were caught victimisation the teeth with the arms command out laterally. Associate in Nursing raised device surface would increase elevate and stability. this means that flight might have begun by running, saltation and sustaining short, leg-powered glides when prey or off from predators (Padian et al., 1998).
Criticisms of this embody the issues of drag and eager to work against gravity. it's biomechanically easier to evolve flight from sailing than from the bottom, though this doesn't indicate that it evolved this fashion. the opposite downside is that the ground speed that may be needed to succeed in a typical flight speed of 6-7 ms-1. fashionable lizards are reportable to succeed in theses speeds, however it's not better-known whether or not proto-birds might reach such speeds. There has been some doubt on whether or not choice might improve each the limb and hindlimb at an equivalent. However, decoupling of the fore- and hindlimbs was achieved within the coelurosaurs and therefore the tail and hindlimb were more and more decoupled within the earliest birds (Padian et al., 1998).
Any model conjointly must account for the evolution of the flight stroke. it's been shown that the immediate sister teams of birds, maniraptor and therefore the alternative dromaeosaurs, already had the sideways-flexing articulatio plana that in birds is important to the assembly of thrust. In dinosaurs, this feature was used as a prey-seizing stroke (Padian et al., 1998).
It would have solely taken a small adjustment of the angle of attack of this predatory stroke to form an acceptable vortex wake. By running, saltation and some such strokes, extension of the time within the air, and eventually flight from the bottom up might have evolved. this concept needs no options not already better-known from fossils. it's been urged that advantage could are taken of any ridge or incline and then the model will have a part of the arboreal theory (Padian et al., 1998).
Overall, the proof looks to purpose to a changed version of the cursorial theory of bird. Running leaps were power-assisted by wings extended for balance, the wings were dilated at the distal ends for raised stability. The leaps were bit by bit extended by short flap motions that elaborate the down and progress already gift within the sister teams of the primary birds. Running and saltation could are increased by "ridge-gliding" or jumping from tiny heights (Padian et al., 1998).
The Arboreal Theory
The arboreal theory is a lot of intuitive therein flight evolving from Associate in Nursing arboreal sailing stage would appear to be comparatively simple. However, this theory has very little support from comparative biology because it needs the power to climb trees and to glide. Neither capability looks to be gift in bird or in theropod dinosaur dinosaurs.
Archaeopteryx has none of the options of typical vertebrate gliders, neither is it aerodynamically designed to fly. it's been argued that the lateral grooves Associate in Nursingd curvature of the claws area unit an arboreal specialisation, however these have conjointly been compared to those of ground-dwelling birds. The proof for Associate in Nursing arboreal life style overall is weak. It ought to even be thought of that giant theropods like theropod and bird-footed dinosaur have hooklike claws with deep lateral grooves, and that they were clearly not arboreal. Also, palaeobotanical proof shows Associate in Nursing absence of huge trees anyplace close to the Solnhofen lagoons during which bird is preserved (Padian et al., 1998).
Flight Capabilities of bird
The general agreement is that bird was a weak flier. this is often supported by 2 arguments. the primary is that bird lacks proof of a supracoracoideus system, that in birds is that the connective tissue that powers the stroke. Experiments have shown that pigeons with a cut off supracoracoideus connective tissue cannot take-off from ground level, cannot maintain level flight and might not land safely. However, it's questionable whether or not bird is compared with fashionable birds during this manner (Padian et al., 1998).
The second argument is that the feathers of bird area unit asymmetrical, that suggests that it absolutely was capable of some flight. However, information shows that the spatial property is a smaller amount than that {of fashionable|of recent|of contemporary} fliers and gliders and this argument is simply valid if it's assumed that fossil animals should look precisely like modern ones once acting an equivalent perform (Padian et al., 1998).
Generally it's thought that bird might glide also as latest birds and fly by flap to some extent. The proof for this is; 1) its wing planform and size area unit like those of some fashionable feeble flying birds, 2) the flight feathers area unit well-developed, 3) the os was a powerful website of origin for flight muscles, and 4) its mechanics planform is in contrast to that of birds that solely glide (Padian et al., 1998).
Flight enhancements when bird
Phylogenetic analysis has shown that several of the characteristics related to the origin of flight were already gift in non-avian theropod dinosaur dinosaurs before birds evolved. Feathers evolved in non-avian coelurosaurs whose forelimbs were too short in reality purposeful wings. bird had a limb longer than the hindlimb, and flight feathers on the wings and long tail feathers. it's thought to own had a totally evolved flight stroke capable of generating thrust also as elevate. The presence of Associate in Nursing alula within the early Cretaceous Eoalulavis shows that the wing mechanism that allowed flying at lower speeds and to manoeuvre like living birds evolved early in bird history. Iberomesornis, of the first Cretaceous, has options diagnostic of a perching ability (Padian et al., 1998).
What types of isolation will cause the formation of a brand new species?
Introduction
According to the biological species conception, populations area unit completely different species if cistron flow between them is prevented by biological variations, called procreative barriers. If populations exchange genes they're conspecific, i.e. belong to the species, even though they dissent greatly in morphology. If they're reproductively isolated, they're completely different species even though they're indistinguishable phenotypically. so phylogeny arises from the evolution of biological barriers to cistron flow (Futuyma, 1998).
The factors resulting in procreative isolation is divided into 2 categories; prezygotic factors, that operate before fertilisation will occur; and postzygotic factors, that operate when fertilisation resulting in partial or complete failure of crosses between the 2 forms. These area unit summarised below.
A. Prezygotic factors
1. Geographical isolation: forms area unit separated by land or water barriers that they're unable to cross.
2. Ecological isolation: the forms fail to fulfill as a result of they board completely different places inside an equivalent geographical region.
3. Temporal isolation: the forms area unit active at completely different seasons or times of day.
4. behavioral isolation: the forms meet, however don't mate.
5. Mechanical isolation: congress happens, however no transfer of male gametes takes place.
6. Gametic incompatibility: reproductive cell transfer happens, however egg isn't impregnated.
B. Postzygotic factors
1. Cell dies: cell mortality presently when fertilisation.
2. F1 hybrids (first generation) have reduced viability (hybrid inviability).
3. F1 hybrids viable however have reduced fertility (hybrid sterility)
4. Hybrid breakdown: reduced viability or fertility in F2 (second generation) or copulate (F1 crossed with parents) generations.
Prezygotic Barriers
One or a lot of of those is also operative inside a given population at any time. The organic process functions of those mechanisms area unit an equivalent, to limit or stop cistron flow between species. they will occur solely partially; as an example, behavioral isolation is complete or females could solely show a small preference for males of their own species (Dobzhansky et al., 1977).