On Tue, 31 Aug 2004 04:34:37 +0000 (UTC), johngrahamiv{at}netscape.net
(johng) wrote:


>r norman  wrote in message
news:...
>> On Sun, 29 Aug 2004 04:34:32 +0000 (UTC), johngrahamiv{at}netscape.net
>> (johng) wrote:
>> 
>> >Being asexual, how did the Hydra develop as a species?
>> 
>> First, there are very large numbers of species that reproduce
>> completely by asexual methods.
>> 
>> Second, Hydra also reproduce sexually.

>so my question is, how do species that reproduce asexually evolve?  is
>it through mutatation alone?
>

In general, most of the living organisms on earth -- that is, the
bacteria and the single-celled protists -- reproduce asexually.   And
yes, they evolve quite nicely. (Technical detail: Yes, there often is
some mechanism for exchange of genetic material and there are
unicellular Protista with true sex: meiosis and fertilization, but
this is an answer at a more elementary level.) 

The mechanisms of evolution include mutation as the source of new
variation in the genetic material and selection as a way of increasing
or decreasing the number of copies of particular variants.  There is
also drift, where pure chance or probabilistic factors determine which
variants increase or decrease in number.  These mechanisms do not
require sexual reproduction.  If one bacterium produces a large line
of descendants (a clone), but one of them develops a mutation, then
there will be two separate lines: one with the mutation, one without.
If one line has greater fitness, it will dominate the population and
the genetic composition of that bacterial species will have changed.
That is, the line has evolved.

Sexual reproduction is a way of combining the genetic material of two
parents to produce each offspring.  As a result it produces new
combinations of existing genetic material.  These new combinations can
produce effects on the resulting organism (the phenotype) that are
new, hence increase variability in the population.  The process by
which traits are passed on from parent (or parents) to offspring
differs in sexual and asexual reproduction.  But the basic mechanism
of evolution, mutation plus selection and drift,  remains the same.

You may also be thinking also of the biological definition of species,
which depends on two populations being able to interbreed or not.
That definition obviously only applies to species that reproduce
sexually.  For asexually organisms, species are defined differently:
by their appearance, behavior, and genetic composition.  Two
populations that are different in these aspects are considered
different species.
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