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The more genes two animals have in common, the
closer they are "related" (not by blood, but by
evolution) to one another. Therefore, different
levels of classification exist to illustrate this
relationship. There are seven levels of classification,
the highest group being a kingdom, and the lowest
group a species (the others between the two extremes
are, in order, phylum, class, order, family, and
genus). Many thousands of different organisms may
belong to a kingdom, but only one to a species.
For example: humans, dogs, porcupines, rats, eagles,
and salmon all belong to the same kingdom (Kingdom
Anamalia), but only humans belong to the species
sapiens and only dogs to Canis domesticus.
Two organism are more closely "related" as they
approach the level of species. This also means they
have more genes in common. Taxonomists (scientists
who classify living organisms into the seven groups)
define a species as any group of closely related
organisms that can produce fertile offspring. Sometimes,
members of the same species will look alike (all
brown bears look alike and all humpbacked whales
look alike) and other times they won't (such as
German Shepherds and Chihuahuas, which are both
members of the same species).
The level of species can be further divided into
smaller segments. A population is the smallest unit
of a species and is made up of organisms of the
same species. All the hemlock trees in a forest
make up a population of hemlock trees, and all the
rattlesnakes on an island make up a population of
rattlesnakes, and so forth. Sometimes, a population
will physically alter over time to suit the needs
of its environment. This is called a cline, and
can make members of the same species look different.
A subspecies is a distinct population of a species.
A good example of a subspecies is the western flicker
which has red wings and looks different than the
eastern flicker which has yellow wings. The two
types of birds belong to the same species, though,
because they can produce fertile offspring (sometimes
with orange wings!).
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