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Natural Selection
Evolution refers to change over time, or transformation over time. Evolution assumes that all natural forms arose from their ancestors and adapted over time to their environments, thus leading to variation. In evolution, there are many rules the environment places upon the survival of a species. There are also numerous ways in which evolution occurs, the most noted are Natural Selection and Adaptation.
The earth is rich in diverse environments and eco-systems. At the core of evolution is the way a specific species adapts to its environment. An example in humans is sickle cell anemia. In North America, sickle cell anemia is uncommon and a disadvantage. However, in other parts of the world, specifically areas where malaria is common, the occurrence of sickle cell anemia is greater. Why? Because the heterozygous Sickle Cell Anemia genotype gives a higher resistance to malaria, however a homozygous genotype is still a disadvantage. Adaptation occurs on many levels. Another adaptive trait of humans is body size. While body size is generally very subjective, it is an adaptive trait that people in southern, tropical, or warm climates are taller than those in colder, northern, or arctic climates. The reason behind the adaptation is the distribution of heat throughout the body. A taller person in a hot climate distributes heat throughout a larger percent of their body to disperse it easier, whereas a shorter person in a northern climate has a smaller body in which to distribute heat which helps to retain it. Other adaptations with other organisms follow the same general premise. All adaptations are a consequence of the environment, and are meant to help an organism survive within that environment. This is known as the Adaptive Strategy of an organism.
Natural Selection
The Adaptive Strategy of an organism is not the only thing which creates biological changes. A species is always within a constant state of flux, as is the environment where they live. The obvious question is, is an organism able to adapt completely to its environment, and the honest answer is no. The environment of earth is in a constant state of flux, but changes in an environment generally occur within time spans of tens of thousands of years, save for a few examples where changes occurred in an interval of a 100 or 200 year time span (research into environmental change and evolution is on going, some researchers challenge the length of adaptive time to be more or less than the figures above).
The best example of a quick change in the environment and a species
ability to adapt concerns the the color of the Gypsy Moths in England. When the
industrial revolution occurred, coal and other industrial factories spewed out
massive amounts of air pollutants, so much so that even during the day the
skies were as dark as night. The original color of the gypsy moths was a light
gray; such a color blended in with the trees in their environment, and acted as
camouflage against predators. With the change in the environment the camouflage
adaptation no longer functioned because the tree trunks were darker colored
from the air pollution. The dark gray gypsy, once at a disadvantage and quickly
eaten by predators, now survived and bred, while their lighter counterparts
were eaten. As a result the gypsy moth, through adaptation and natural
selection, was able to gradually change it's coloring to a dark gray-black, to
match the surface of the trees covered in pollution. The gypsy moths didn't
just decide one day to change their color, at the basis of such a change was
the concept of Natural Selection. 
Natural selection plays upon variation and adaptation, all of which occur simultaneously. There is biological variation within every species. This is best illustrated by the bell curve model of species variation. In the example of the gypsy moth, the majority of the species will have the same color trait and will be clumped into the orange and green sections of the bell curve. The lower end of the bell curve are different from the average in some way shape or form. In the case of the gypsy moth and it's adaptation for the gray color, the darker moths were at first the exception rather than the rule. When the environment began to be polluted the lighter moths which occupied the middle section of the bell curve began to be killed by predators, while the darker moths at the other end of the bell curve were not. When such a change occurs, it is said that a trait has been selected for, or a trait has been selected out of the gene pool.
Sexual Selection
Another way that change occurs is through sexual selection. An example of sexual selection is found in peacocks. Male peacocks have large tails that were sexually selected for by female peacocks. Male peacocks fan out their feathers and strut as a mating show for the female peacocks who select between them based on color and physical prowess.
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