Реферат: Evolution Essay Research Paper Essay on Evolution

Evolution Essay, Research Paper

Essay on Evolution

There are many mechanisms that lead to evolutionary change. One of the

most important mechanism in evolution is natural selection which is the

differential success in the reproduction of different phenotypes resulting from the

interaction of organisms with their environment. Natural selection occurs when a

environment makes a individual adapt to that certain environment by variations

that arise by mutation and genetic recombination. Also it favors certain traits in a

individual than other traits so that these favored traits will be presented in the

next generation. Another mechanism of evolution is genetic drift. Genetic drift is

a random change in a small gene pool due to sampling errors in propagation of

alleles or chance. Genetic drift depends greatly on the size of the gene pool. If

the gene pool is large, the better it will represent the gene pool of the previous

generation. If it is small, its gene pool may not be accurately represented in the

next generation due to sampling error. Genetic drift usually occurs in small

populations that contain less than 100 individuals, but in large populations drift

may have no significant effect on the population. Another mechanism is gene

flow which is when a population may gain or lose alleles by the migration of

fertile individuals between populations. This may cause the allele frequencies in

a gene pool to change and allow the organism to evolve. The most obvious

mechanism would have to be mutation that arises in the gene pool of a

population or individual. It is also the original source of the genetic variation that

serves as raw material for natural selection.

Not only are there mechanisms of evolution, but there is also evidence to

prove that these mechanisms are valid and have helped create the genetic

variety of species that exists today. Antibiotic resistance in bacteria is one

example of evolutionary evidence. In the 1950’s, Japanese physicians realized

that a antibiotic given to patients who had a infection that caused severe

diarrhea was not responding. Many years later, scientists found out that a

certain strain of bacteria called Shigella contained the specific gene that

conferred antibiotic resistance. Some bacteria had genes that coded for

enzymes that specifically destroyed certain antibiotics such as ampicillin. From

this incident, scientists were able to deduce that natural selection helped the

bacteria to inherit the genes for antibiotic resistance.

Scientists have also been able to use biochemistry as a source of

evidence. The comparison of genes of two species is the most direct measure of

common inheritance from shared ancestors. Using DNA-DNA hybridization,

whole genomes can be compared by measuring the extent of hydrogen bonding

between single-stranded DNA obtained from two sources. The similarity of the

two genes can be seen by how tightly the DNA of one specie bonds to the DNA

of the other specie. Many taxonomic debates have been answered using this

method such as whether flamingos are more closely related to storks or geese.

This method compared the DNA of the flamingo to be more closely related to the

DNA of the stork than the geese. The only disadvantage of this method is that it

does not give precise information about the matchup in specific nucleotide

sequences of the DNA which restriction mapping does. This technique uses

restriction enzymes that recognizes a specific sequence of a few nucleotides and

cleaves DNA wherever such sequences are found in the genome. Then the DNA

fragments are separated by electrophoresis and compared to the other DNA

fragments of the other species. This technique has been used to compare

mtDNA from people of several different ethnicity’s to find out that the human

species originated from Africa. The most precise and powerful method for

comparing DNA from two species is DNA sequencing which determines the

nucleotide sequences of entire DNA segments that have been cloned by

recombinant DNA techniques. This type of comparison tells us exactly how much

divergence there has been in the evolution of two genes derived from the same

ancestral gene. In 1990, a team of researchers used PCR(polymerase chain

reaction) a new technique to compare a short piece of ancient DNA to

homologous DNA from a certain plant. Scientists have also compared the

proteins between different species such as in bats and dolphins.

The oldest type of evidence has been the fossil record which are the

historical documents of biology. They are preserved remnants found in

sedimentary rocks and are preserved by a process called pretrification. To

compare fossils the ages must be determined first by relative dating. Fossils are

preserved in strata, rock forms in layers that have different periods of

sedimentation which occurs in intervals when the sea level changes. Since each

fossils has a different period of sedimentation it is possible to find the age of the

fossil. Geologists have also established a time scale with a consistent sequence

of geological periods. These periods are: the Precambrian, Paleozoic, Mesozoic

and the Cenozoic eras. With this time scale, geologists have been able to

deduce which fossils belong in what time scale and determine if a certain specie

evolved from another specie. Radioactive dating is the best method for

determining the age of rocks and fossils on a scale of absolute time. All fossils

contain isotopes of elements that accumulated in the organisms when they were

alive. By determining an isotope’s half-life which is the number of years it takes

for 50% of the original sample to decay, it is possible to determine the fossil’s

age.