SUMMARY / RELATED TOPICS

Convergent evolution

Convergent evolution is the independent evolution of similar features in species of different periods or epochs in time. Convergent evolution creates analogous structures that have similar form or function but were not present in the last common ancestor of those groups; the cladistic term for the same phenomenon is homoplasy. The recurrent evolution of flight is a classic example, as flying insects, birds and bats have independently evolved the useful capacity of flight. Functionally similar features that have arisen through convergent evolution are analogous, whereas homologous structures or traits have a common origin but can have dissimilar functions. Bird and pterosaur wings are analogous structures, but their forelimbs are homologous, sharing an ancestral state despite serving different functions; the opposite of convergence is divergent evolution. Convergent evolution is similar to parallel evolution, which occurs when two independent species evolve in the same direction and thus independently acquire similar characteristics.

Many instances of convergent evolution are known in plants, including the repeated development of C4 photosynthesis, seed dispersal by fleshy fruits adapted to be eaten by animals, carnivory. In morphology, analogous traits arise when different species live in similar ways and/or a similar environment, so face the same environmental factors; when occupying similar ecological niches similar problems can lead to similar solutions. The British anatomist Richard Owen was the first to identify the fundamental difference between analogies and homologies. In biochemistry and chemical constraints on mechanisms have caused some active site arrangements such as the catalytic triad to evolve independently in separate enzyme superfamilies. In his 1989 book Wonderful Life, Stephen Jay Gould argued that if one could "rewind the tape of life the same conditions were encountered again, evolution could take a different course". Simon Conway Morris disputes this conclusion, arguing that convergence is a dominant force in evolution, given that the same environmental and physical constraints are at work, life will evolve toward an "optimum" body plan, at some point, evolution is bound to stumble upon intelligence, a trait presently identified with at least primates and cetaceans.

In cladistics, a homoplasy is a trait shared by two or more taxa for any reason other than that they share a common ancestry. Taxa which do share ancestry are part of the same clade. Homoplastic traits caused by convergence are therefore, from the point of view of cladistics, confounding factors which could lead to an incorrect analysis. In some cases, it is difficult to tell whether a trait has been lost and re-evolved convergently, or whether a gene has been switched off and re-enabled later; such a re-emerged trait is called an atavism. From a mathematical standpoint, an unused gene has a decreasing probability of retaining potential functionality over time; the time scale of this process varies in different phylogenies. When two species are similar in a particular character, evolution is defined as parallel if the ancestors were similar, convergent if they were not; some scientists have argued that there is a continuum between parallel and convergent evolution, while others maintain that despite some overlap, there are still important distinctions between the two.

When the ancestral forms are unspecified or unknown, or the range of traits considered is not specified, the distinction between parallel and convergent evolution becomes more subjective. For instance, the striking example of similar placental and marsupial forms is described by Richard Dawkins in The Blind Watchmaker as a case of convergent evolution, because mammals on each continent had a long evolutionary history prior to the extinction of the dinosaurs under which to accumulate relevant differences; the enzymology of proteases provides some of the clearest examples of convergent evolution. These examples reflect the intrinsic chemical constraints on enzymes, leading evolution to converge on equivalent solutions independently and repeatedly. Serine and cysteine proteases use different amino acid functional groups as a nucleophile. In order to activate that nucleophile, they orient an acidic and a basic residue in a catalytic triad; the chemical and physical constraints on enzyme catalysis have caused identical triad arrangements to evolve independently more than 20 times in different enzyme superfamilies.

Threonine proteases use the amino acid threonine as their catalytic nucleophile. Unlike cysteine and serine, threonine is a secondary alcohol; the methyl group of threonine restricts the possible orientations of triad and substrate, as the methyl clashes with either the enzyme backbone or the histidine base. Most threonine proteases use an N-terminal threonine in order to avoid such steric clashes. Several evolutionarily independent enzyme superfamilies with different protein folds use the N-terminal residue as a nucleophile; this commonality of active site but difference of protein fold indicates that the active site evolved convergently in those families. Convergence occurs at the level of DNA and the amino acid sequences produced by translating structural genes into proteins. Studies have found convergence in a

Ted Shackelford

Theodore Tillman Shackelford III is an American actor. He played Gary Ewing in the CBS television series Knots Landing. Shackelford was born in Oklahoma City and went to college at the University of Denver, he appeared in the role as Ray Gordon on the daytime soap opera Another World from 1975 to 1977. He has appeared in roles on Wonder Woman and most in The Division, he starred in the British science fiction series Space Precinct. Shackelford is best known for his role as Gary Ewing on the CBS television series Knots Landing, in which he starred from 1979 to 1993, he appeared as Gary Ewing on several episodes of CBS's Dallas. The role was played by actor David Ackroyd, unable to return for future appearances after his 1978 turn as the character. On February 2, 2006, Shackelford joined the cast of the daytime soap opera The Young and the Restless as Genoa City district attorney William Bardwell, his last air date on the popular soap as William Bardwell was on July 18, 2007, when his character succumbed to complications caused by a stroke.

However, on August 7, 2007, Shackelford returned to the show as Bardwell's identical twin brother, Jeffrey Bardwell. In 2013, Shackelford reprised the role of Gary Ewing in three episodes of the new Dallas series on TNT, in one of which he was reunited with Joan Van Ark as Valene. Ted Shackelford on IMDb Ted Shackelford at AllMovie

Prosector

A prosector is a person with the special task of preparing a dissection for demonstration in medical schools or hospitals. Many important anatomists began their careers as prosectors working for lecturers and demonstrators in anatomy and pathology; the act of prosecting differs from that of dissecting. A prosection is a professionally prepared dissection prepared by a prosector – a person, well versed in anatomy and who therefore prepares a specimen so that others may study and learn anatomy from it. A dissection is prepared by a student, dissecting the specimen for the purpose of learning more about the anatomical structures pertaining to that specimen; the term dissection may be used to describe the act of cutting. Therefore, a prosector dissects to prepare a prosection. Prosecting is intricate work. Scalpels and scissors allow for sharp dissection where tissue is cut, e.g. the biceps brachii muscle can be removed from the specimen by cutting the origin and insertion with a scalpel. Probes and the prosector's own fingers are examples of tools used for blunt dissection where tissue may be separated from surrounding structures without cutting, i.e. the bellies of biceps brachii and coracobrachialis muscle were made clearer by loosening the fascia between the two muscles with a blunt probe.

The risks to prosectors are low. Cadavers used for teaching purposes are embalmed before they are encountered by a prosector and students. Embalming fluid contains formaldehyde, phenol and glycerine which disinfect and kill pathogens within the cadaver. With exposure to embalming fluid and bodily fluids, such as blood, become fixed. Prosectors and students working with embalmed cadavers must always wear protective gloves, however this is more for protection against the harsh chemicals used in embalming such as formaldehyde and dettol which can cause moderate to severe skin irritation. Further to the protection that embalming allows against disease, educational institutions take great care in screening the cadavers accepted into their body donation programs. Cadavers are not accepted if they have a medical history of infectious disease such as tuberculosis and HIV/AIDS. Prosectors for autopsies of diseased cadavers may run a high risk of suffering from health problems when caution is not used as cadavers are not fixed when being dissected for autopsy.

At least two diseases are named after prosectors: Prosector's paronychia: a primary inoculation of tuberculosis of the skin and nails. Prosector's wart, a skin lesion caused by contamination with tuberculous materialContracting infections caused by contaminated cadavers is a constant danger among prosectors if a skin puncture accident results from the sharp surgical instruments used in this kind of work. In this case, thin surgical gloves are not enough to protect. There are many cases of pathologists. Another example, a famous historical case, is that of Ernst von Fleischl-Marxow, an Austrian physician and physiologist, who infected his finger during an autopsy and became dependent on morphine, due to the pain. Presently, AIDS presents a problem. Although it is difficult to contract it by a single puncture incident, at least one case has been reported among pathologists; the continuous respiratory exposure to formaldehyde, used to preserve cadavers, is an occupational risk of prosectors as well as medical students and pathologists.

Inhaled formaldehyde can irritate the eyes and mucous membranes, resulting in watery eyes, headache, a burning sensation in the throat, difficulty breathing. Formaldehyde is listed as a potential human carcinogen