DNA is a special chemical structure that contains a code for the construction of living creatures, plants and animals alike. Genes are chemical sequences in the DNA that define a certain trait or set of traits. An allele is a part of a gene, a variation that is responsible for a particular individual trait. "Given the diversity of the human species, there is no 'normal' human genome sequence," geneticist Greg Feero has written. "We are all mutants" [source: Feero et al.].
Allele frequency is the tendency of a trait to show up in a population of organisms. Evolution is a change in allele frequencies spanning several generations. Population pressure occurs when some environmental factor makes the survival of organisms more difficult. Under high pressure, only the individuals with traits most suited to surviving that pressure will be likely to reproduce well. In this scenario, allele frequencies can see drastic changes within one to two generations.
A population bottleneck can drastically affect allele frequency. If a chance event takes out a large percentage of a species' population, those remaining may simply happen to have a high concentration of alleles that were previously rare. These alleles become standard as the remaining population reproduces. An example of this phenomenon is the northern elephant seal. Hunters decimated the population in the 19th century, so that only about 20 remained. Today there are more than 30,000 of these animals, but they show much less genetic diversity than a similar, less-hunted population in the south [source: Understanding Evolution].
The "founder effect" occurs when a group migrates to a new area and doesn't reproduce with anyone from the previous area. This tendency can be seen in such population as the Amish in Pennsylvania [source: Evolution]. The Amish manifest an above-average frequency of Ellis-van Creveld syndrome, a type of dwarfism accompanied by extra fingers and toes, as well as problems with the nails, teeth and heart. The disorder has been traced back to a single couple who arrived in Pennsylvania in 1744.
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