What are the units used for the ideal gas law? How does Charle's law relate to breathing? What is the ideal gas law constant? How do you calculate the ideal gas law constant? How do you find density in the ideal gas law? Does ideal gas law apply to liquids? Impact of this question views around the world. While it is true that Darwin assumed that rudimentary structures are useless, modern biologists do not make this assumption and therefore do not employ uselessness as a criterion for recognizing a vestigial structure.
Even so, vestigial structures can often be considered useless with respect to the usual function of their non-rudimentary counterparts. For example, python hindlimbs are useless as organs of locomotion, and cassowary wings are useless as organs of flight. This objection by creationists based on the functionality of these vestigial organs therefore arises from a misunderstanding of the concept of vestigiality.
Bergman argues that a definition of vestigiality based on reduction and not uselessness is meaningless, because biologists do not consider structures vestigial if they have been only slightly reduced.
It is correct that biologists do not consider slightly reduced structures vestigial, but Bergman is incorrect to assume that any degree of reduction is used to label a structure vestigial. Structures are labeled vestigial, based on reduction in size, only if that reduction is extreme. For example, the shortened limbs of a dachshund are not considered vestigial limbs, but the miniscule spurs of a python are.
This objection by Bergman is therefore based on a misunderstanding of the reduction criterion. Bergman correctly states that the evolutionary history of an organ must be known to determine whether it is vestigial. He then argues against the validity of determinations of vestigiality by claiming that evolutionary histories are not known for most such organs and that their identification as vestigial is based on direct comparison with modern and not fossil examples.
That claim shows two errors. First, evolutionary inference does not require direct observation of the history of all structures. Second, the evolutionary histories of vestigial skeletal structures are often well documented by fossil series. For example, in derived tyrannosauroid dinosaurs the third finger is reduced to a metacarpal splint with no phalanges Lambe , whereas early tyrannosauroids had a complete third finger; the fossil record therefore sufficiently documents the evolutionary history of the tyrannosauroid third finger to determine that in derived tyrannosauroids it is vestigial Xu and others This objection by Bergman is based on the incorrect assumption that fossil series are not used to determine vestigiality.
The above objection by Bergman is invalid for another reason. In evolutionary studies, a precursor to a rudimentary organ can be deduced by comparison with its non-rudimentary counterparts in close relatives.
By the same token, within the creationist paradigm a rudimentary structure in one species must be considered vestigial if the homologous structure is non-rudimentary in other species within the same baramin.
In such a case, even a creationist must concede that a rudimentary structure has evolved from a non-rudimentary homolog. Darwin and others explain that a biological structure may become vestigial if members of the evolutionary lineage in question stop using it. Some creationists claim that this explanation is Lamarckian and therefore false see Glover ; Bergman and Howe The term Lamarckian refers to the now-discredited hypothesis, named after the pre-Darwinian biologist Jean-Baptiste Lamarck, that traits acquired by an organism during its lifetime are heritable.
Lamarckian scenarios are unrealistic, as witness the fact that the children of amputees are born with their limbs intact. The misinterpretation is understandable, because the evolutionary scenario in question is often described with poor wording, as in the first sentence of this paragraph.
This scenario could be better worded thus: if members of an evolutionary lineage cease to use a given organ, then the survival of the lineage will not be compromised if one of its members is born with a heritable mutation that results in the reduction of that organ to a rudimentary state; the descendants of that individual will possess a vestigial organ. This is not a Lamarckian scenario, and the objection that it is one is therefore false.
Neither Darwin nor any modern evolutionary biologist makes the Lamarckian claim that atrophy of an organ due to disuse for example, withering of a muscle that an individual does not exercise is heritable.
Clearly, the creationist arguments against the existence of vestigial structures are based on misunderstandings and incorrect assumptions. Even so, these examples of vestigial structures do not necessarily demonstrate that vestigial structures exist within the creationist paradigm, because they relate to taxa that creationist authors have not identified as belonging to a single baramin. Baraminologists creationist researchers who seek to determine which extant taxa belong to which baramins have not placed pythons in the same baramin as any fully legged animal, and they have not yet studied cassowaries or tyrannosauroids.
However, examples of vestigial structures do exist within baramins that have been studied by and are recognized by baraminologists. The fossil horse series offers some examples. Creationists once considered fossil members of Equidae the horse family to have been created separately from modern horses Cousins ; Gish However, recent baraminological studies confirm that there is too much morphological continuity between the various fossil and extant members of Equidae to support that interpretation Garner ; Cavanaugh and others ; Wood The fossil record reveals that in the earliest equids each forelimb had four digits, each hindlimb had three digits, the shaft of the ulna extended the full length of the forearm, and the shaft of the fibula extended the full length of the shank Figure 1.
In each forelimb and hindlimb of later fossil equids all digits but number III were lost, and in modern horses thin splints of bone are all that remain of the metacarpal hand and metatarsal foot bones that supported digits II and IV in each limb. The metacarpal and metatarsal splints of modern equids are vestigial bones, and the ulnar and fibular splints are vestigial shafts of bones. Because these rudimentary skeletal structures are demonstrably derived from non-rudimentary structures in ancestral members of the same baramin, they must be considered vestigial within the creationist paradigm.
It should be noted that these vestigial skeletal structures perform useful functions in extant horses, and that they are nonetheless vestigial. The metacarpal and metatarsal splints serve as guides for ligaments, and remnants of the ulna and fibula function as muscle attachment sites Smythe Vestigial traits can still be considered adaptations because an adaptation is often defined as a trait that has been favored by natural selection.
Adaptations, therefore, need not be adaptive, as long as they were at some point. Learning Objectives Discuss the connection between evolution and the existence of vestigial structures. Key Points Structures that have no apparent function and appear to be residual parts from a past ancestor are called vestigial structures.
Examples of vestigial structures include the human appendix, the pelvic bone of a snake, and the wings of flightless birds. Vestigial structures can become detrimental, but in most cases these structures are harmless; however, these structures, like any other structure, require extra energy and are at risk for disease. Vestigial structures, especially non-harmful ones, take a long time to be phased out since eliminating them would require major alterations that could result in negative side effects.
Key Terms vestigial structure : Genetically determined structures or attributes that have lost most or all of their ancestral function in a given species.
0コメント