 Wysłany: Śro 10:50, 23 Mar 2011 Temat postu: organisms as conspecific nike air max chaussures |
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| Richard Dawkins defines two organisms as conspecific if and only if they have the same number of chromosomes and,nike air max chaussures for each chromosome, both organisms have the same number of nucleotides (The Blind Watchmaker, p. 118). However, most if not all taxonomists would strongly disagree[citation needed]. For example, in many amphibians, most notably in New Zealand's Leiopelma frogs, the genome consists of "core" chromosomes that are mostly invariable and accessory chromosomes, of which exist a number of possible combinations. Even though the chromosome numbers are highly variable between populations, these can interbreed successfully and form a single evolutionary unit. In plants, polyploidy is extremely commonplace with few restrictions on interbreeding; as individuals with an odd number of chromosome sets are usually sterile, depending on the actual number of chromosome sets present, this results in the odd situation where some individuals of the same evolutionary unit can interbreed with certain others and some cannot, with all populations being eventually linked as to form a common gene pool.
The classification of species has been profoundly affected by technological advances that have allowed researchers to determine relatedness based on molecular markers, starting with the comparatively crude blood plasma precipitation assays in the mid-20th century to Charles Sibley's ground-breaking DNA-DNA hybridizationstudies in the 1970s leading to DNA sequencing techniques.rolex submariner The results of these techniques caused revolutionary changes in the higher taxonomic categories (such asphyla and classes), resulting in the reordering of many branches of the phylogenetic tree (see also: molecular phylogeny). For taxonomic categories below genera, the results have been mixed so far; the pace of evolutionary change on the molecular level is rather slow,nike air max, yielding clear differences only after considerable periods of reproductive separation. DNA-DNA hybridization results have led to misleading conclusions, the Pomarine Skua – Great Skua phenomenon being a famous example. Turtleshave been determined to evolve with just one-eighth of the speed of other reptiles on the molecular level, and the rate of molecular evolution in albatrosses is half of what is found in the rather closely related storm-petrels. The hybridization technique is now obsolete and is replaced by more reliable computational approaches for sequence comparison. Molecular taxonomy is not directly based on the evolutionary processes, but rather on the overall change brought upon by these processes. The processes that lead to the generation and maintenance of variation such as mutation,breitling watches crossover and selection are not uniform (see also molecular clock). DNA is only extremely rarely a direct target of natural selection rather than changes in the DNA sequence enduring over generations being a result of the latter; for example, silent transition-transversion combinations would alter the melting point of the DNA sequence, but not the sequence of the encoded proteins and thus are a possible example where, for example in microorganisms, a mutation confers a change in fitness all by itself. |
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