Monday, May 15, 2017

Back to the future

One of the most pervasive carryovers from ladder-of-life thinking is that some living species are older than others.  These are often thought of as sitting on the lower rungs of the ladder.  Of course, tree-thinking rejects this notion as all living species have evolved the same amount of time from their shared common ancestor at the base.  Nonetheless I hear so many phrases that reflect the continuing influence of ladder-thinking, even among evolutionary biologists.  So here's a good one - 'That trait goes all the way back to echinoderms' [or insert any taxon perceived to be old]. The speaker almost certainly wanted to communicate that the trait was present in the common ancestor of echinoderms and vertebrates.  So what's the problem with using the shorthand 'back to echinoderms'?  Well, it suggests that an echinoderm was the common ancestor of echinoderms and vertebrates (not true even if that ancestor *looked* more like an echinoderm) and/or that echinoderms haven't been evolving since their split with other deuterostomes.  This is particularly dangerous as it often leads to poor experimental design --  using living species perceived to be old or primitive as stand-ins for ancestors.  Certainly studying echinoderms can help us make inferences about that common ancestor, but so would studying any other descendant.

So in summary:

p.s.  I think it would be a little better to say 'out to echinoderms' -- if your listener assumes humans are starting point, then going 'out' to echinoderms spans the clade that captures both humans and echinoderms, which maps to a particular ancestral node.  But why not just use a couple more words and say the trait was present in the common ancestor of deuterostomes.

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