Essay on Evolution: Natural Selection

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Essay on EvolutionThere are many mechanisms that lead to evolutionary change. One of themost important mechanism in evolution is natural selection which is thedifferential success in the reproduction of different phenotypes resulting fromthe interaction of organisms with their environment. Natural selection occurswhen a environment makes a individual adapt to that certain environment byvariations that arise by mutation and genetic recombination. Also it favorscertain traits in a individual than other traits so that these favored traitswill be presented in the next generation. Another mechanism of evolution isgenetic drift. Genetic drift is a random change in a small gene pool due tosampling errors in propagation of alleles or chance. Genetic drift dependsgreatly on the size of the gene pool. If the gene pool is large, the better itwill represent the gene pool of the previous generation. If it is small, itsgene pool may not be accurately represented in the next generation due tosampling error. Genetic drift usually occurs in small populations that containless than 100 individuals, but in large populations drift may have nosignificant effect on the population. Another mechanism is gene flow which iswhen a population may gain or lose alleles by the migration of fertileindividuals between populations. This may cause the allele frequencies in a genepool to change and allow the organism to evolve. The most obvious mechanismwould have to be mutation that arises in the gene pool of a population orindividual. It is also the original source of the genetic variation that servesas raw material for natural selection.

Not only are there mechanisms of evolution, but there is also evidenceto prove that these mechanisms are valid and have helped create the geneticvariety of species that exists today. Antibiotic resistance in bacteria is oneexample of evolutionary evidence. In the 1950’s, Japanese physicians realizedthat a antibiotic given to patients who had a infection that caused severediarrhea was not responding. Many years later, scientists found out that acertain strain of bacteria called Shigella contained the specific gene thatconferred antibiotic resistance. Some bacteria had genes that coded for enzymesthat specifically destroyed certain antibiotics such as ampicillin. From thisincident, scientists were able to deduce that natural selection helped thebacteria to inherit the genes for antibiotic resistance.

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Scientists have also been able to use biochemistry as a source ofevidence. The comparison of genes of two species is the most direct measure ofcommon inheritance from shared ancestors. Using DNA-DNA hybridization, wholegenomes can be compared by measuring the extent of hydrogen bonding betweensingle-stranded DNA obtained from two sources. The similarity of the two genescan be seen by how tightly the DNA of one specie bonds to the DNA of the otherspecie. Many taxonomic debates have been answered using this method such aswhether flamingos are more closely related to storks or geese. This methodcompared the DNA of the flamingo to be more closely related to the DNA of thestork than the geese. The only disadvantage of this method is that it does notgive precise information about the matchup in specific nucleotide sequences ofthe DNA which restriction mapping does. This technique uses restriction enzymesthat recognizes a specific sequence of a few nucleotides and cleaves DNAwherever such sequences are found in the genome. Then the DNA fragments areseparated by electrophoresis and compared to the other DNA fragments of theother species. This technique has been used to compare mtDNA from people ofseveral different ethnicity’s to find out that the human species originated fromAfrica. The most precise and powerful method for comparing DNA from two speciesis DNA sequencing which determines the nucleotide sequences of entire DNAsegments that have been cloned by recombinant DNA techniques. This type ofcomparison tells us exactly how much divergence there has been in the evolutionof two genes derived from the same ancestral gene. In 1990, a team ofresearchers used PCR(polymerase chain reaction) a new technique to compare ashort piece of ancient DNA to homologous DNA from a certain plant. Scientistshave also compared the proteins between different species such as in bats anddolphins.

The oldest type of evidence has been the fossil record which are thehistorical documents of biology. They are preserved remnants found insedimentary rocks and are preserved by a process called pretrification. Tocompare fossils the ages must be determined first by relative dating. Fossilsare preserved in strata, rock forms in layers that have different periods ofsedimentation which occurs in intervals when the sea level changes. Since eachfossils has a different period of sedimentation it is possible to find the ageof the fossil. Geologists have also established a time scale with a consistentsequence of geological periods. These periods are: the Precambrian, Paleozoic,Mesozoic and the Cenozoic eras. With this time scale, geologists have been ableto deduce which fossils belong in what time scale and determine if a certainspecie evolved from another specie. Radioactive dating is the best method fordetermining the age of rocks and fossils on a scale of absolute time. Allfossils contain isotopes of elements that accumulated in the organisms when theywere alive. By determining an isotope’s half-life which is the number of yearsit takes for 50% of the original sample to decay, it is possible to determinethe fossil’s age.

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