Essay On 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.

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