Extinction is when a species dies out completely. It could be
argued that it can occur before that, say, if there are only
males or females left in a sexually reproducing species, or if
the remaining pairs are geographically isolated preventing
mating. Evolutionists believe that over the history of the
earth, many more species have become extinct than have existed
at any one time. Sometimes a species is replaced by descendants
that have altered their form enough not to be considered the
same species anymore. An example of this is the history of the
modern horse, which has descended through a series of distinct,
and now extinct, precursor species.