A population, however large or small, in which all individuals
share basic genetic characteristics, and therefore produce
fertile offspring, constitutes a species.
If a species occurs over a wide geographical area, as
for example, the Plains Zebra, (north-east Africa to South
Africa), populations in different parts of the distribution
area, especially at the opposite ends, may look quite
different from each other. Yet, when members of those
populations are mated, they produce fertile offspring;
that is, these offspring are able to reproduce. If various
populations within a huge distribution area do differ
from each other in appearance, they are considered different
subspecies.
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If
there are no geographical barriers which separate such populations
or subspecies, the change in appearance
is gradual and is referred to as "cline". If however,
there are geographical barriers which separate populations
that were formerly part of a unified distribution, such
isolated populations or subspecies could differ from others
more markedly. Depending on how long they have been isolated,
they may be on the verge of becoming separate species, as
there is no more exchange of genes between these and other
subspecies.
A variety of zebra, known
as the "Quagga", inhabited the Karoo and southern
Free State of South Africa well into the second half of
the 19th century, when it became extinct. It differed from
other zebras mainly in having been striped on the head,
neck and front portion of it’s body only, and in having
been brownish, rather than white, in it’s upper parts.
The belly and legs were unstriped and whitish. |
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