>>Clearly from this, there has been an increase in complexity throughout
>>geologic time.
MikeBGene replied:
>That depends entirely on the chosen perspective. For example, *among*
>porifera, has there been an increase in complexity in the last 570 million
>years? Among arthropods, has there been an increase in complexity in
>the last 530 million years? Etc. [Also, don't forget to change the date of
>agnatha in light of the recent fossil finds].
>
>>The authors then constructed a computer model with 2000
>>beings in which each generation of the model the beings could go up or down
>>in the number of cell types. This was done by pure chance. The only limit
>>was that they couldn't have negative cell types. over a number of
>>generations, the maximum number of cell types increased. >>
>
>So why haven't sponges and arthropods increased their number of cell types?
>Are we to think they stopped evolving since they appeared half a billion years
>ago?
Judging from the evidence provided I think the answer to your first question
is" they *did* evolve into something else. The fact that a branch of the
family still exists which resemble the common ancestor isn't particularly
meaningful. They are suited to the nitch which they inhabit. Other branches
of the family moved into other nitches and became something else. The answer
to your second question is that they *do* continue to evolve. Otherwise
there would only be a single species of sponge (or shark or whatever). They
evolve very slowly because they are suited to a pretty stable environment.
Susan
--------
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--Martin Luther King, Jr.
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