Species Definition - What Defines a Species?

The Species is the most detailed level of the standard category breakdown of living organisms. Just what is a species - how is a species defined?

The top level of categorizing life is "Kingdom". There are four main kingdoms, in essence breaking up life into plants & animals, virusses, etc. (this is a very general description). From there the breakdown goes Kingdom - Phylum - Class - Order - Family - Group - Species. So while a Kingdom might be "things with cells", a species might be "blue jay".

Just what IS a species? Why is a blue jay a separate species from, say, a robin? Just as the classifications have evolved over the year, so have the definitions of WHY the lines are drawn in certain places. At one time "kingdom" meant plant vs animal. All bluebirds might be considered a "species".

By Darwin's time, a species was a group of animals that looked similar down to a more specific level. So a group of bluebirds in the eastern US might be considered a different species if they all looked slightly different from a group of bluebirds in the western US.

In the 1940s and 50s, as people learned more and more about genetics and breeding, a species became a group of animals that could breed with each other. Cats were one species, and dogs were another. They both have fur, they both have tails and ears, but they can't breed with another. Note that all dogs are ONE SPECIES even though they can look vastly different from each other. That's because - even while a great dane looks quite different from a dalmation - the two are genetically able to breed and have puppies.

Nowadays, the breeding line is the one most biologists draw. This line can be drawn for two reasons. One, the creatures simply cannot create a viable new baby organism. That would be the case between for examples cats and dogs. Even if you did convince them to mate, their sperm and egg could not join properly. The second situation is *situational* non-viability - where the two groups of creatures never interact properly to mate. This would be the case where one group of black monkeys lives in Africa and another brouwn group lives in Asia. Maybe they COULD mate and make babies if they ever met, but they simply don't.

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