Be fore I talk about what speciation is I must first talk about what a species is.
A species is a group of organisms that can interbreed and producing an fertile offspring's. For example: Lions and Tigers can mate and have an offspring (Liger) however the offspring will not be infertile.[1][2]

Speciation the splitting of a population into two separate species. This happens due to the a population becoming genetically isolated from the parent population (breeding between the (now) two population limited to none) then through the processes of mutation natural selection and genetic drift the populations evolve separately until it becomes imposable for the two populations to interbreed and produce a fertile offspring.[2][3]

This can from a verity of reasons from geologic changes that can split a population or isolate a portion of said population to species migration that spreads a species out leading to little to no interbreeding between populations in one area and populations in another.[2][4]

Contrary to some creationist claims there is evidence for specieation.[5][6][7]

Sources
1. Understanding Evolution -Evolution 101 - Defining a species
2. Wikipedia - Speciation
3. Understanding Evolution -Evolution 101 - Defining speciation
4. Understanding Evolution -Evolution 101 - Causes of speciation
5. Talk Origins - Observed Instances of Speciation
6. Understanding Evolution -Evolution 101 - Evidence for speciation
7. Talk Origins - Creationist Claims - Claim CB910: No new species have been observed.

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