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==Population Genetics==
In terms of Population Genetics, the definition of Evolution may be further refined to mean: ''A generation to generation change in the frequencies of alleles within a population that share a common gene pool.'' A [[population]] is defined as: ''A localized group of individuals belonging to the same species.'' An example would be all the bass sharing a single pond. A [[gene pool]] is described as: ''The sum of all the alleles (variations in genes) shared by members of a single population.'' Thus, a population is united by its gene pool. Small scale changes in the frequencies of genes in a gene pool is referred to as [[microevolution]].
To understand the mechanisms that allow a population to evolve, it may be beneficial to first describe the circumstances that would prevent microevolution. Such a situation is described by the Hardy-Weinberg theorem.
The Hardy-Weinberg Theorem states that: The frequencies of alleles in a population’s gene pool will not change over time unless acted upon by forces other than both random reshuffling of alleles during sex cell formation and random combination of sex cells during fertilization.
For example: In a population of mice that inhabit a barn, there are only two variations in the gene that controls fur color. One allele codes for black and accounts for 75% of the gene pool, the other for white, which accounts for the remaining 25%. According to the Hardy-Weinberg Theorem, if only the only factors determining an allele’s potential to be passed on to the next generation are random formation of sex cells and fertilization; the frequencies will remain 75% black and 25% white. In this case, no changes in the frequencies of alleles in the mouse population, thus no microevolution, at least in regards to fur color. Such a population is said to be in Hardy-Weinberg Equilibrium
For a population to exist in Hardy-Weinberg Equilibrium the following conditions must be met.
# The population must be large. Small populations are subjected to chance fluctuations in the gene pool, a condition known as genetic drift.
# There can be no exchange of members between populations. Such migration between populations is called gene flow.
# There can be no mutations. Mutations introduce new variations in a trait will result in changes in frequencies; although the initial effect would be minimal.
# Mate selection must be random. There can be no preference for any particular allele during mate selection.
# Natural selection can not be a factor. The probability of the survival and reproduction of all the alleles must be equal.
Such criterion rarely if ever, exists in a natural population. Therefore, frequencies of alleles in a gene pool are always changing, resulting in the microevolution of populations over successive generations.
==Evidence For Evolution==
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