Re: "Species"? Feces!

Alan Meyer wrote:
<msadkins04@xxxxxxxxx> wrote in message news:efrgi0$5fp$1@xxxxxxxxxxxxxxxxxxxxxx
Let me see if I have this straight:

One of the defining characteristics of a "species" is that members of
one species cannot reproduce with members of another species.

Evolutionary theory states that random genetic mutations are
responsible for the appearance of new species.

Therefore: only random genetic mutations which result in the inability
of the mutant to reproduce with others of its kind (i.e., with
non-mutants vis a vis this characteristic) can be described as
"species-producing mutations". A funny nose, for example, does not
make a new species, if its bearer can still reproduce with its
non-funny-nosed brethren.

Each species-producing random genetic mutation is produced in one
member (new offspring) of an otherwise reproductively compatible group
of animals.

However, being reproductively incompatible with its fellows, this
mutation is not passed on.

Therefore, no new species can occur.

Right?

That's clever, but the evolutionary process doesn't work that way.

One way evolution happens is by genetic drift. Two populations
may be isolated from each other for a variety of reasons. They
may be on each side of a mountain. Some individuals may have
travelled to an island. Animals or plants in one area may have
different local conditions to adapt to - different food, temperature,
rainfall, or presence of predators or prey.

Adaptations in one group are kept within that group and not
spread to the other, not because they couldn't be, but because
the populations just don't mix that much.

The longer they are apart, the more they drift apart genetically
until, for one reason or another, interbreeding becomes difficult
and eventually, impossible.

It's not that a sparrow suddenly gives birth to a robin, it's that
populations of birds become gradually more and more specialized
until, many many generations later, they are no longer the same
bird.

Alan


I don't buy this, because when the single group is divided into two
geographically separated groups, the same argument that I gave
originally now applies to each group with respect to itself. Let's say
that a population, all of whose members belong to species X, undergoes
geographic separation into groups A and B. At the time of separation,
groups A and B are both classified as species X. So, the question
isn't how A and B become two different species due to genetic drift
between them, but how a new species can arise *within* either group.
That is, members of one group (e.g., A) must give birth to a new
species Y as a subgroup of its own population, before any members of A
can be reproductively incompatible with members of B (since until then,
all members of A are of species X, as are all members of B); only if an
offspring of Group A bears a random genetic mutation rendering it
reproductively incompatible with the other members of Group A, can a
"new species" Y be said to exist (using the definition of "species" we
have accepted); but this very reproductive incompatibility then makes
it impossible for the new species to continue to exist, much less to
become totally dominant so that members of A in general become
reproductively incompatible with members of B.

Within any single group of animals, either the offspring are
reproductively compatible with their brethren, in which case they have
at least a theoretical chance of passing on their genes; or else they
are reproductively incompatible with their brethren, and they cannot
pass on their genes. Unless and until an offspring is genetically
incompatible with its brethren, it cannot be regarded as a member of a
different species from them, under the definition we have accepted; and
until a new species arises within one group, none of its members can be
of a different species when considered with respect to the members of
another group who were originally genetically homogeneous with them,
but who merely became geographically separated from them.

If, within a group, a genetic mutation merely reduces the chances of
successful reproduction by that member with others of the same group,
it cannot be regarded as the first member of a new species. And there
*must* be a first member, unless a new species comes into existence
wholesale, and I take it that we are rejecting that. (And in any
case, why should less successful reproductive chances help those new
genes to spread and eventually completely dominate the group genome?)

Mark Adkins
msadkins04@xxxxxxxxx


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