E&G begin by noting that research is not conducted in a vacuum and that we don't observe data with a "viewpoint from nowhere": we use theory to organize and interpret data. they call this a "picture" rather than a paradigm/research programme/etc, explicitly trying to avoid the longstanding debate over those terms.
Stepping off from this, they claim that most paleontologists hold a conceptual picture of evolution walking along with slow and small steps; they term this "phyletic gradualism" and link it with sympatric speciation. Importantly they feel that paleontologists haven't been keeping up with the mainstream of population biology, where allopatric speciation is (or at least was in the '70s when the paper was written) was all the rage.
So with that hypothetical apparatus in mind (that "picture" influences theory and paleos currently work under a gradualist picture), they consider how allopatric speciation would look in the fossil record. E&G look at two fossil groups and attempt to establish that the data can be explained, and can possibly be more "interestingly" explained, under the allopatric model.
First they consider Poecilozonites, a genus of pulmonate snails from Bermuda. Using different pictures, they can argue in support of allo- or sympatric speciation. The species under consideration are all subspecies of P. bermudensis that are marked in being paedomorphic; the adults retain juvenile features. A story of gradual cumulative change can be laid out, but when you start including geographic information, more support is seen for allopatry.
Second they review Phacops rana and related trilobite species from Devonian New York--Ohio strata. In particular the discussion focuses on changes in one 'character' (although it's a complex character with many related components, as the
authors point out), the number of "dorso-ventral files" in the eye. They find that the mainline species has 18 of these eye-files, and argue that marginal populations arise with variable number of eye-files. These marginal peripheral populations then expand/migrate, overtaking the mainline. This is the allopatric model in essence. In each case of these triblobites there's a reduction of the number of files in the eye (see Figure 1).
 |
| Figure 1 - Hypothetical Phylogeny |
I don't really know anything about the eyes of trilobites, other than that they're complex/compound and insect-like, but are not related to insect eyes. Eyes are fascinating structures, famously Darwin seemed to waiver that natural selection could produce something so complex, and who's function seems so reliant on the interdependence of parts. But of course Darwin immediately recognized that the eye could evolve in stages, and he even cited some fossil examples of probable stage. In fact, one would think that that was a lucky accident and that eyes turned up once in a primitive ancestor and have been inherited by all eyed organisms today-- or maybe they evolved twice, one for organisms with eyes 'like ours' and one for compound eye type organisms). But that's not the case, eyes of various sorts have independently evolved many times amoung animals, up to 100 times.
The only other thing I know about trilobite eyes is that their lenses are made of calcite, a mineral. Our eyes lenses are nothing like this, they contain crystallin, which is a protein, not a mineral (despite what its name might suggest to some) and our lenses are metabolically active.
So this business of "dorsoventral" whatever seemed like it was worth looking into. Trilobite eyes are compound, similar to an insect's, but independently evolved (they're possibly the oldest eyes we have on record). Each facet is made up of a small calcite lens (and other tissues), and a string of lenses is what E&G is referring to as a "dorso-ventral file". There's more to the eye, with the visual unit, capped by a lens, being called an ommatidia. Lines of lenses that run between the dorsal and ventral surface of an eye are called d-v files, and lines of lenses that run horizontally across an eye are simply called rows. The number of files is used in the determination of species within trilobites.
 |
| Figure 2 - Trilobite eye structure |
Interestingly, the some of the specimens referred to by E&G are from the Marcellus Shale in NY (a source of hydrofraked natural gas).
E&G go on to state that there's an expectation amoung paleontologists that successively higher taxonomic ranks should have progressively more and more taxa within it, they believe this incorrect assumption is a result of the "picture" of phyletic gradualism; as time goes on more and more species are produced. The reality is that there are, infamously, lots of higher ranks that are species poor, so we in some families there are hundreds of genera each containing dozens of species and good sub-species, but often enough we can have Orders with a few monotypic genera. Allopatric speciation can explain this as repeated splitting with either 1) the parent species going extinct and only marginal ones surviving; 2) when geographic isolates adapt through new modes of feeding/motion/protection/etc; and 3) when it involves a small isolated lineage.
Finally E&G address that exemplar of phyletic gradualism, the evolution of long-term (and especially adaptive) trends in a lineage. They feel that allopatry can result in the appearance of a trend by way of analogy to how random mutation in a population can still result in the overall production of a trend within that population. Selection pressure moves the population in one direction, and something that would eventually become called "species sorting", IIRC, similarly produces the trend at a higher level. They point out a mechanism in a little more detail, relying on something like the genetic and historical constraints of the mainline species tending to result in marginal species reacting strongly and in the same way to particular to similar marginal environments--say, developing thick skin in desert environments; the net effect is an overall trend for the group of species.
You can see a lot of anticipations of Gould's later work on hierarchical levels of evolution in this work, along with some material that, probably through uncharitable readings, was used to charge Eldridge and Gould with being monstrous saltationists.
I have to wonder at some of their examples though and if they really show a signal of allopatry. With Phacops, we see a few marginal populations developing, in these cases through paedomorphosis, in different locations and then expanding over the ancestral range. E&G note that the mainline population is invariant, with 18 d-v files, while the marginals are at first more variable, and then later less variable with a reduced number of d-v files. But why isn't this just a large, general population with variability in the number of d-v files, why consider the variable population to be an isolate? If you look at any one slice through time, you find a wide-spanning population with variable d-v file numbers, some living in epeiric seas, other in marginal seas, which aren't terribly different environments either.
 |
| Figure 3 - Some notes on the hypothetical Trilobite phylogeny. Red lines 1--3 are samples at a particular time, the green arrow is a possible trendline. |
Figure 3 show populations (marked with red lines) with 1) 18--17 d-v files; 2) only 17 d-v files; and 3) 15--17 d-v files. Further, the green line in Figure 3 shows that in epeiric seas as you move through time the number of d-v files changes 18 to 17 to 15, a trend of reduction in this group at this location. The authors posit that migration has occurred not evolution in place over a long period of time.
Obviously the justification for allopatry must be in Eldrige's (and others) stratigraphic and geographic work on the group, but it'd be more helpful to have some discussion of that.
One other thing that really sticks out in this work is that E&G are heavily operating within the adaptationist programme, ironic given that Gould is such a critic of that. Whether they're considering allopatry or sympatry, they can find adaptationist explanations for all the features. Perhaps shells became thinner as an adaptation to living in limey soil, or perhaps that was just the result of drift, a meaningless fixation. It's hard to believe that you can have a wide ranging population of Phacops trilobites with something like the structure of the eye varying so much, and that this is the result of selection pressure for the number of d-v files, rather than just meaningless variability in their number. Eldridge, and others, promoted the idea of identifying species within the trilobites by counting (presumably amoung other things) the number of dorso-ventral lines. Perhaps that 'picture' of trilobite evolution coloured his ideas here.
No comments:
Post a Comment