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Re: Quo vadis AAAARG metaphor limits



I'm certain that others have gotten to this one by now, but here's my $.02

>        The biggest single problem with the tree metaphor is that it 
>includes _only divergence_ and the phylogenetic tree has a hell of a lot 
>of very interesting examples of branches coming back together- eukaryotic 
>cells, for example, which are comprised of a host cell, aerobic bacteria 
>(mitochondria) b-g algae in the case of plants (chloroplasts) and perhaps 
>something else that made up the centrioles. Or look at lichens, for that 
>matter- branches of eukaryotes coming together. Somewhat less compelling 
>examples come from things like corals and giant clams that keep algae in 
>their tissues, or animals and digestive bacteria. But if you bring in 
>sexual reproduction, what you have is a recombination of branches, 
>branches coming back together,if you look close enough. At the 
>individual level, each of us would be a branch, every time we have a kid 
>with somebody, that's a joining of branches- you know, how the Family 
>Tree of a person branches out rather than narrowing down as you go back 
>in time, as it generally does with species.

What you're describing is symbiosis.  I'm not sure if this would really classify
as a "joining of branches" though.  Since mitochondria maintain their own DNA,
it becomes more likely that they could be classified as a separate organism;
just one that has no life of it's own outside the confines of a cellular
membrane

> But even at the species level, what about hybrids, 
>subspecies, chain-species, etc.? There's even been some talk about species 
>arising through hybridization, based on the finch studies in the 
>Galapagos, so there goes our branching analogy again. Life doesn't just 
>branch, it rejoins branches back together. I don't know what an 
>appropriate metaphor would be. I know corals can branch and then the 
>branches can merge again
Most hybrid species are sterile; in fact, this aspect helps keep species in
"reproductive isolation" (a key part of the definition of a species).

The problem with your coral analogy, is that most coral families evolve in an
"iterative evolution" pattern, where the same form evolves over and over again
(since there are only so many things you can do to the bowl that holds the
polyp).  The branches only appear to merge again.

Rob

***
The man who has everything ... should be quarantined!