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Definition of "Species"

Alleles  -  Variations of a given gene.
Allelic frequency  -  The frequency with which a particular allele appears among the possible alleles in a population.
Allopatric speciation  -  The creation of a new species due to the geographic isolation of a population. Compare with sympatric speciation.
Anagenesis  -  The creation of a new species from a single lineage. Compare with cladogenesis.
Cladogenesis  -  The splitting of a single lineage into two new species.
Founder effect  -  When separated, two populations may have contained different allelic frequencies than the original population. Selection and genetic drift will act differently on these two different genetic backgrounds, creating genetic differences between the two new species.
Gene  -  Genes are the hereditary factors that produce traits.
Gene flow  -  The movement of genes through a population or between two populations through mating
Genetic drift  -  Random changes in allelic frequencies due to chance rather than selection.
Natural Selection  -  The theory holding that competition exists within species, determining which species live to have offspring, and pass their traits on to those offspring.
Niche rule  -  The principle that two closely related species cannot fill the same role in the same habitat
Parapatric Speciation  -  An extremely rare type of speciation that occurs when populations are separated not by a geographical barrier, such as a body of water, but by an extreme change in habitat.
Polyploidy  -  Having more than two copies of each chromosome.
Reproductive Isolation  -  The inability of the individuals of a species to create offspring with members of any other species. Read the SparkNote on Reproductive Isolation.
Species  -  The most specific categorization for organisms. The term "species" refers to a group of organisms that shares the same gene pool and can successfully mate. A bulldog and a greyhound are of the same species because they can produce young. A cat and a bulldog, which experience reproductive isolation from each other and therefore cannot produce young, are separate species.
Sympatric speciation  -  The creation of a new species from populations that are not geographically isolated. Compare with allopatric speciation.
Trait  -  A particular expressed characteristic of an individual plant or animal; natural selection selects for or against some traits.

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