Essay/Term paper: Essay on evolution

Essay, term paper, research paper:  Science Reports

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.