Invisible <voi### [at] devnull> wrote:
> Evolution asserts that species are not fixed; they /change/. And so the 
> species we see today were not always there. And, in particular, not so 
> much evolution itself, but molecular evidence says that all life on 
> Earth has a single common ancestor.

  Actually the definition of "species" is a hard one.

  By definition two animals are of different species if they cannot produce
fertile offspring. However, this definition is lacking.

  It's lacking because the definition implies transitivity: If A can reproduce
with B (thus A and B are of the same species), and B can reproduce with C
(thus B and C are of the same species), by definition A and C should be of
the same species (and hence able to reproduce). However, there are eg.
so-called ring species (look it up) where this doesn't happen: It may be
that A and C cannot reproduce, making them by definition members of
different species, even though B can reproduce with either A or C. This
makes a clear definition of B's species rather difficult.

  (And with ring species we are not necessarily talking here about some
weird obscure unicellular organisms living in the depths of the ocean
which defy all classification. We are talking about regular everyday
animals, such as certain species of seagull.)

  A ring species is actually an excellent demonstration of how speciation
can happen gradually, unlike the straw man that some creationists present
of "a species suddenly transforming into another": Yes, it *is* possible
for a subgroup of a species to *slowly* transform into a different species
over time, and during this period the ability of procreation between the
two groups gradually becomes less and less possible (the probability of
a successful fertilization becomes smaller and smaller because of the
constantly increasing differences in genes). If this subgroup further
spawns an even more isolated sub-subgroup, we may end up precisely in the
situation where a member of group A can procreate with a member of group
B, and a member of B with a member of C, but a member of A cannot procreate
with a member of C.

  The difficulty of classifying B in the example also demonstrates the
completely fuzzy line between when a species becomes another species.
If you trace the ancestry of a modern species back to an ancestral species
which spawned one or more other modern species, it's hard to define when
exactly the modern species became to exist exactly. By definition every
offspring is of the same species has its parents, but if you continue
this back long enough, you end up with animals that are so different from
the modern equivalents that they cannot be considered of the same species
anymore (if that ancient species was brought back to life today, it would
be unable to reproduce with its modern descendant). However, drawing the
dividing line where the ancient species became the modern one is impossible.
It's a fuzzy line.

-- 
                                                          - Warp

Post a reply to this message