What do we Mean by a Species?

Thoughts

1. Gog and Magog, are they different species from humans?
2. Therefore, by definition, humans all belong to the same species.
3. Have Gog and Magog specialized and becomes a different species than humans due to isolation?
4. What about isolated communities like in Siwa, Nubia and other more isolated ones such as in Islands (belonging to India for instance where contact has been attempted yet unsuccessfully).
5. Hybridization in American population, what does it lead to.
6. Inadvertent hybridization of F1 hybrids with other forms of the same species or hybridization among different cultivars of a plant may result in unfavorable results in offspring.
7. What is a cultivar? (A form of ecotype?)
8. Isolated communities of humans, like Siwa and Nubia, have imposed cultural rules of not marrying from outside their communities.
9. Donkeys and horses produce infertile mules when they mate. This fits them perfectly within the definition of being two distinct species (two distinct biospecies).

Hybridization against forces of natural selection

Digest

• Speciation, the forming of distinct species, is a process not a sudden event
• Hybridization and natural selection are two opposing forces
• According to the Mayr-Dobzhansky test, two organisms belong to the same biospecies if they could breed together in nature to produce fertile offspring
• Allopatric speciation vs sympatric speciation
• Ecological speciation is speciation driven by divergent natural selection in distinct subpopulations according to Dolph Schluter (2001)
• Stages of speciation (in the most orthodox view of ecological speciation):
1. Two subpopulations become geographically isolated and natural selection drives genetic adaptation to their local environments
2. Reproductive isolation (pre-zygotic or post-zygotic) builds up between the two subpopulations
3. The two subpopulations re-meet in a phase of secondary contact
4. The hybrids between individuals from the different subpopulations are now of low fitness. Natural selection will then favor any feature in either subpopulation that reinforces reproductive isolation preventing the production of low fitness hybrid offspring. These breeding barriers then cement the distinction between what have now become separate species.
• In northern Europe, the lesser black-backed gull and the herring gull are now two distinct species. They do not hybridize and are therefore true biospecies.  Their 'ancestors' still hybridize freely after spreading east and west from Siberia.

European herring gull (top) and lesser black-backed gull (bottom)

Questions

1. So, what exactly is a species?
2. So, ecotypes are different forms of the same species? Also different forms, like peppered moths (melanic and nonmelanic) belong to the same species?
3. How come species are classified by scientists then reclassified later on in a different way?