Thursday, September 13, 2012 stay the same?

Did you see the report (e.g., here on the BBC) of a 150 million year old fossil crab's last death walk?  The trail it followed until it finally expired, where it (and the trail) were fossilized have been discovered and are fascinating.  Here is the spoor it left in the sand (top is a photo of the fossil track, bottom the path of the track), poor thing:

This picture here shows the crab itself:

Now this is interesting for its own sake.  but look at this image:

This one is contemporary, and you can see them along the tidal beaches in many places in our country and elsewhere.

What's interesting is how the horesehoe crab hasn't changed over so long a time.  We are trained by textbooks, by Darwin himself (for those who actually read him), and of course by the media, to think of evolution as constantly happening in a relentless rat-race of competition, always needing to adapt to the changing environment or improved competitors.

But here is an organism that has been running the race by staying in place.  In fact, at the DNA level the currently available data are scant, but this group of arthropods has continued to evolve, so that its DNA sequences reflect very distant common ancestry--for some groups that look like horsehoe crabs and the horseshoes themselves, hundreds of millions of years.  That means that the accumulation of mutations in their DNA has continued to evolve more or less as would be expected.

Now much of the DNA that is used as a molecular clock to estimate species ancestral split times has little if any function and is chosen for study specifically for that reason, since it is more reliably clock-like than functional DNA that can be affected by quirky natural selection.

But if adaptation is so intense a pressure to change in response to one's surroundings, how is it that there has been essentially no evolution of functional aspects of the genome in 150 million years?  The simple answer is  "Well, no problem!  Their environment hasn't changed."  There's no easy way to refute that, though geological and paleoecological data might suggest otherwise.  Environment is not just the temperature and salinity of the ocean, it includes networks of plants and animals, each preying on the other and so on.  So it is perplexing that so little change has happened.

Perhaps horseshoe crab physiology is different enough to show selective pressures, and it's just the shell that hasn't changed. But the fossil anatomy is not just 'skin' (shell) deep, but is in so many of the details. Even just by chance one would expect that minor variations in the shape and shell structures etc. of these creatures would have arisen.  Really, one should expect more than trivial changes.

There should be many ways to modify shape and stay 'fit'.  Basic body plans are rather stable, but they do change, as we see by the variation among, say, fish and humans or wasps and butterflies.  One might say that the crabs' shape is determined by just one or two genes that controls so many of the conserved parts.  Then, any mutation might be harmful and removed.

But developmentally, such traits and complexity rarely, if ever, is so simple and usually involves many different genes' contributions.  So how such conservation has happened--and there are other examples such as flies or ants entombed in millions-years old amber etc., who look just like our friends and pests today--is rather mysterious, and very interesting to try to explain in terms of our current knowledge of developmental and population genetics.

So, eons later, we have the last walk of this poor doomed crab.  But although it was going somewhere as its spirit left its shell for the Other World, its lineage wasn't going anywhere at all.


rich lawler said...

Stabilizing selection...? Seems to be a good candidate for explaining the observed stasis, at least according to the study by Estes and Arnold (2007) "Resolving the paradox of stasis: models with stabilizing selection explain evolutionary divergence on all timescales" Am Nat.

Ken Weiss said...

That is clearly the most plausible explanation. But this is very 'stabilized' and one has to ask why morphological drift cannot have taken place. After all, other arthropods and sea life have very varied bodies, so why would the environment be so constraining for this group (as it is for others I'm aware of, such as various insects)?

Doug Futuyma has suggested various kinds of chromosome incompatibilities as being involved, or one might suggest some network genes affecting shape and some other unmoveable trait. But these are to me rather forced arguments.

I have not read Estes and Arnold, or (it does seem vaguely familiar) at least not for long enough that I remember anything.

It's an interesting question.

Holly Dunsworth said...

If we're talking about body plan staying that same shape... it really could be one or a couple genes. I'm thinking of the tiniest tweaks that turn a chicken shaped beak into a duck shaped one (which looks like this guy's body).

Ken Weiss said...

Sure enough that is an experimental finding. Whether it's what would happen in wild rather than experimental models is not totally obvious. But there are lots of beak shapes in birds (as your example shows), so why would chicken beaks (say) stay unchanged for 150 million years? Why isn't there some drift over such a time period?

Obviously, in the case of these deeply conserved traits, they _are_ conserved so the question for me is not whether a gene related to that exists but why no real modification even in its regulation?

Bill said...

Check out Briggs et al:

and Jerry Coyne's comments on same in WEIT.

Ken Weiss said...

I did not know of this paper, but on a quick glance I'm not sure of your point. It seems to deal with the evolution of arthropod limbs as exemplified by the horseshoe crab, but how is it relevant to why their morphology hasn't changed for so many million years (many more than I had been aware of!)?

I had also not been aware of Coin's post on this paper, and thanks for pointing it out.

Again, on a quick glance, it shows that the 'crab's morphology hasn't been as static as was widely thought (including by me), but still I'd say that there is quite a lot of morphological conservation and the question remains as to how any trait can seem so conserved.

There are beetles, ants, flies and so on (and undoubtedly crustaceans and other groups--plants, too perhaps) that seem very highly conserved.

My interest is just in understanding how traits can avoid more than a tiny amount of change--even if just by drift--for so long given the widely (if perhaps uncritically) accepted Red Queen view of life as relentlessly competitive, etc.

Again on quick glance, which is all I've had time for, Coyne seems to say we don't know what the fossil creatures' DNA was like, but I think we do, in the indirect sense that current species don't show any anomalous lack of DNA change--they reflect clock-like change--so far as I'm aware. The PNAS paper points out that there have been morphological changes as well.

So how do major traits stay so similar.....or is our evolutionary model of the pace of change in complex traits missing something?