Re: Speciation and Macroevolution

R. Joel Duff (
Mon, 15 Dec 1997 11:59:59 -0600 (CST)

Paul Nelson,
>Speciation -- i.e., the formation of reproductively isolated groups --
>may arguably be necessary for macroevolution to occur, but it is
>difficult to see how it could possibly be sufficient. If speciation
>were the whole of the macroevolutionary story, the Earth should be
>swarming with millions of species of cyanobacteria (or whatever), but
>little more than that.
chop chop
>How one gets dozens (or thousands) of species of butterflies, or nematodes,
>or langurs, is an interesting problem -- but that is a very different matter
>from, and does not explain, how to build a butterfly, or a nematode, or a
>langur in the first place.

Paul and others,

Let me clarify that I have not been specifically attempting to provide
examples of macroevolution but rather have been addressing a more limited
question. At the same time my aphid example did suggest that at least 11
genera of aphids covering a large range of morphological variation were
derived from a single common ancestor. I might consider this
macroevolution since it does entail changes that have resulted significant
morphogical diversity.

Let me just throw out one little appreciated method of diversification. In
just the last few years many new examples of combination organisms have
been characterized. Much is this is due to advances in techniques used to
investigate these organisms. In the examples of aphids and their
endosymbiotic bacteria it was nearly impossible to investigate the bacteria
because they can not be cultured independently of the aphid. Only through
the use of PCR and recombinant DNA techniques have we been able to
understand anything about these bacteria. Other examples of endosymbionts
and other "odd" relationships among organisms have been popping up by the
dozens in just the last 5 years. I would suggest that these cases will not
be considered "odd" at all 10 years from now.

Examples of endosymbiosis and subsequest genetic less by the symbiont is
often viewed as simply evolution/speciation by degredation or loss. It is
observed that these are not cases of an increase in complexity but I would
argue that this is not correct. These cases are actually examples of
increases in complexity and can explain how one organims can obtain
completely new genetic information including new metabolic pathways. In
the case of the aphids (see previous post) the presence of bacteria with
highly reduced, but specialized, genomes is actually less about the
evolution of the bacteria and more about the aphid obtaining a new
"organelle." This new organelle can synthesize amino acids that the aphid
(or deeper level taxa) formerly were incapable of making. Ultimately we
are witnessing, in progress, the integration of new genetic information
into the aphid.

Another example of this may be the arbuscular-cycorrhizal fungus (and
obligate endosymbiont that colonized the roots of almost 80% of all land
plants) which ITSELF harbors an obligately intracellular bacteria. Land
plant roots then can be considered to be a combination organisms of
plant/fungi/bacteia all partitioning responsibilities. The bacteria again
appears to be playing a role in metabolism. Again the bacteria in several
different fungal species are similar but not the same, they appear to be
coevolving with their respective fungal hosts. At what point do we stop
considering the fungus to be the host and just consider the bacteria a
functional component of the fungus. It appears that "gain" of this new
"organelle" is beneficial and is an important source of evolutionary
innovation. This is still new stuff, my prediction is that when these
bacterial genomes are examined we will find that some species will be
highly reduced in total size and gene content, some horizontal transfer of
genes has taken place to the fungal genome and that the patterns of genome
loss and transfer will be different in different species of fungi.


Joel and Dawn Duff / | ' \ Spell Check?
Carbondale IL 62901 ( ) 0
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