Genteic Variation Essay, Research Paper
Genetic variation and environmental variation play a key role in the process of evolution.
Genetic variation refers to the differences between individuals that are caused by the genes
they inherited from their parents. Genetic variation takes place on three different levels.
First it takes place in the DNA through random mutations. Secondly, genetic variation
occurs at the level of the organism by meiosis and fertilization. Thirdly, genetic variation
takes place at the level of a whole population through genetic drift. Environmental
variation pertains to the organism?s phenotypes that are caused by the environment that
surrounds them.
Mutations take place at the level of DNA. Mutations are believed to be the
ultimate source of genetic variation.(McHenry 2000) When genes are replicated, they do
so with amazing accuracy. This is accomplished because of a number of repair
mechanisms that go through and double check the DNA, and fixes any mistakes that are
found. The mistakes are caused by damage done to the DNA that is caused by ionizing
radiation and certain types of chemicals. Every so often a mistake goes unnoticed, and
this is called a mutation. Mutations take place at a very low rate. Rates of mutation are
very low, from anywhere of one in one hundred thousand. (Boyd & Silk 2000) Mutations
are random with respect to the direction of adaptation. Most of the mutations are either
deleterious or neutral. Part of An example of a mutation that aided a organism is the
sickle-cell trait in human beings, a common trait of people in West Africa. The condition
is caused by a single alteration in the primary sequence of amino acids in the hemoglobin
molecule. Valine is inserted in the place of Glutamic acid, and the alteration causes the
way that the molecule folds to change. This reducing the molecules ability to bind
oxygen.(Boyd & Silk 2000) The mistake in the transcription of DNA to RNA caused a
single protein to be translated differently. Although most people who have the
homogeneous sickle-cell trait die, people who possess the heterogeneous genotype are
resistant to malaria. These heterogeneous people were more fit to live in an environment
in which malaria was present, and consequentially they survived and passed on their genes
to the next generation.(Boyd Silk 2000)
Most Genetic variation also arises through meiosis and the fertilization processes.
In meiosis the important part is that the resulting cells are haploid. During the processes
of meiosis in humans there are four cells created with only one pair of homologous
chromosomes. ?When a new individual is conceived , a haploid sperm from the father
unites with a haploid egg from the mother to produce a diploid zygote.? (Boyd & Silk
2000) The new organism contains the genetic material of the father and the mother, both
of which are fit for their environment. With that new genetic material, the cell divides by
mitosis to make up the millions of cells that form the body.
Genetic variation also occurs in a population by means of genetic flow. Gene flow
is the movement of genetic material within parts of a population of from one population to
another. Gene flow tends to help preserve the homogeneity within a species. An example
of this would be Darwin?s finches. One species of finches that were studied lived on a
island that had both dry and moist climates. Natural selection favored large beaks in the
dry climate and small beaks in the moist climate. Because there was free gene flow on the
island, a majority of the birds had a average sized beak. This is because unless natural
selection is hard at work against either the large bare phenotype or small beak phenotype,
then the finches will have a compromise of the two phenotypes.(Boyd & Silk 2000)
Natural selection plays a huge role in the process of evolution. The idea was first
introduced by Charles Darwin in his book Origin of the Species (1859). There are three
main observations that Darwin made of populations. First, they have the tendency to
over-populate. Secondly there is a huge amount of variation within a single species.
Finally, the organisms that posses the best fit variants tend to leave more offspring. With
this he noted that natural selection acted upon the phenotypic trait and no the genotype
variation. He also noted that evolution follows a set direction. An example of this would
be the industrial melanism in the moths of England and their process of industrial
melanism. Before the industrial revolution the moths of England were light in color.
After the industrial revolution pollution become more and more prevalent. In heavy
industrial areas, the color of the pigment of the moths began to darken. This is because
the moths of darker color were better to fit in an environment that was darker.
Consequently less moths of the darker color were killed by predators , and they were able
to be more successful in reproducing than their lighter counter-parts. Mostly By using
these natural selection acts upon the phenotypes of organisms in a population. The
organisms that are most fit for their surrounding environment leave more offspring, and
thus more of their genes are passed on to following generations. Natural selection also
selected for people who carried one ?S? allele for sickle-cell. Although most people who
have the homogeneous sickle-cell trait die, people who possess the heterogeneous
genotype are resistant to the most deadly form of malaria. These heterogeneous people
were more fit to live in an environment in which malaria was present. Consequentially
they survived and passed on their genes to the next generation, while people with two ?S?
alleles and people with the normal ?A? alletes were selected against.(Boyd Silk 2000)
Enviromental variation can also cause differences in individuals. It does not matter
whether a phenotype is affected by one or more loci, enviroment will have some effect on
the outcome of the character. An example of this is the size of people relative to their
standard of living. An example of this is on the attached graph. The 1958 British average
is at 175 cm for boys. 1955 upper social classes in Britian are slightly taller at about 177
cm, while 1833 factory workers are considerably smaller at 161 cm. Even though there is
a time difference between the two measurements, they are much to close for evolution to
have selected one way or the other. This is also noted with immigrants to the United
States. The children of the
Natural selection and genetic variability both play crucial roles in the process of evolution.
Without either one of them, the theory would unravel.
Bibliography
Boyd, Robert, and Joan B. Silk. How Humans Evolved. New York, NY. W.W.
Norton & Company.2000
McHenry, Henry. Antropology 1, Human Evolutionary Biology, Fall 2000