Tuesday, August 5, 2008

Wonderful Life Part 2

Sandwalk: Science and Philosophy Book Club: <i>Wonderful Life</i>

Over at Sandwalk, Larry Moran announced a discussion forum on Gould's Wonderful Life. In part 1 of this post, I briefly presented my opinion of Wonderful Life - a deliciously audacious attempt to tell the story of how paleontology, and the Burgess Shale, changed the world. How? By showing that humans are not a pre-destined outcome of evolution, but rather lucky survivors in a world that stochastically decimates most species.

Here, I'd like to discuss a particluar comment, originally written by Dawkins and posted on Sandwalk. If true, this comment does indeed nullify most of the point of Wonderful Life.

"Gould expects us to be surprised. Why? The view that he is attacking—that evolution marches inexorably towards a pinnacle such as man—has not been believed for years. But his quixotic strawmandering, his shameless windmill-tilting, seem almost designed to encourage misunderstanding."

Is it true that it's not believed that evolution marches inexorably towards a pinnacle such as man? My take is that there is a tendancy to think of evolution as marching toward a pinnacle, especially among Gould's target audience of laypersons. First, I'll address my "target audience" argument, then I'll paste in some text from a recently submitted article on the molecular evolution of phototransduction that argues for this linear thinking (marching toward a pinnacle) even among profession biologists, a common theme of other posts here.

First, I agree in some ways with Dawkins' comment that many do not believe evolution marches toward a pinnacle such as man. These kinds of views, such as orthogenesis in its most elaborated form (the first definitions were much more modest) argued that evolution is a fully predictable developmental process, akin to ontogeny. And these view points mostly were dismissed decades ago. Evolutionists understand that there is no end goal, that evolution is a response to the here and now, influenced by past heres and nows.

But by stating this agreement, I am talking mostly about the field of evolution, the scientists doing the work. This is different from the perceptions of the general public, which is the target audience of Wonderful Life. I think chapter one makes a rather convincing case, with figure after figure, that evolution is viewed by the public as a march of progress.

Still, even among professional biologists, there is a tendency to present evolution as linear march of progress. Even though we/they know evolution doesn't proceed this way, it is difficult to stay away from such tendancies. I've collected examples, posted on this blog, from ostracods, textbooks, and even cellular phone 'evolution'. I hasten to note that I am here discussing a march of progress of traits, not species. By focusing on traits, the mis-understanding is not quite as egregious as a species march of progress, because we have a reason to choose one trait over another. We want to know how evolution could build the most complex thing we see, and so we pick that as an end point. When we are dealing with species, only ego would lead us to define humans as the most complex species, and therefore as an artificial endpoint. Still, as I recount below, in some cases this human-centric ego has led some to define the most complex eye as most human like. Furthermore, in the example below, a deep-seated assumption of a marching line of progress has led to rank eyes of living species as more or less evolved, along a scale toward human eye-ness. I think this is the march of progress that Gould points out, and Dawkins argues that no one believes.

I paste below the excerpt from a draft of "Opening the Black box: The genetic and biochemical basis of eye evolution", submitted to Evolution Education and outreach, and co-authored with MS Pankey. By way of set up, we earlier in the article define the "gradual-morphological" model of eye evolution as the idea researchers portray of eyes gradually and linearly evolving from spot to complex eye. Here is a nice video illustrating the idea:

This model is not wrong - it does help us understand how something so complex could evolve. But the model is incomplete in that it 1) makes no mention of the origins of variation (see this for more details on that idea) - and it 2) decompresses complexity into a single variable, and also ignores the branching history of all biological entities. There is no need to invoke intelligent design here, but there is a cause to do more science, and more scientific thinking.

On to the promised excerpt:

"Students of biology at all levels, but especially those less experienced, commonly have a strong tendency to view evolution as a linear series of events. What’s more, their perceived series often proceeds from simple to complex, and often equates highest perceived complexity with most human-like. This way of thinking has conceptual antecedents at least to Aristotle’s “Great Chain of Being” and to prominent biologists like Linnaeus, Lamark, and even Haeckel (Dayrat 2003), who was post-Darwinian. The biases are further re-enforced by commonplace graphics portraying evolution itself as a parade of primates, from the knuckle-walker (bringing up the rear) to the modern human (leading the way), representing an “Iconography of Expectation” of increasing complexity (Gould 1989). Such tendencies may be mired in human bias, by a deep-seated need to view humans as special, as a pinnacle of evolutionary progress (Gould 1989). Human vanity certainly seems logical, but an additional, perhaps even more fundamental impediment to tree-thinking, may be that it requires conceptions of time that are largely outside of human experience. Humans experience, imagine, understand, and communicate time as a linear phenomenon. We understand time unfolding as a process in one-dimension. Phylogenetic “time” is different because it branches, leading to multiple parallel trajectories of evolutionary history. It takes practice to learn “tree-thinking” and it is challenging for anyone to communicate precisely about phylogenies, when our every day language doesn’t have to deal with similar phenomena (see also Crisp and Cook 2005; Gregory 2008; O’Hara 1997).

The gradual-morphological model of eye evolution falls into exactly the same trap as the graphical progression of primate through proto-humans to human: these iconographies re-enforce the notion of evolution as a linear, progressive, and goal-oriented series of biological entities of increased complexity. In the case of eye evolution, Salvini-Plawen and Mayr’s figure of gastropod eyes provides an outstanding example of the linear model of evolution, especially since the figure has been copied, and elaborated. The progressive series of gastropod eyes begins with a light sensitive patch (usually with no mention of its origin), followed by eyes with an increasingly deep cup. Next, a simple lens arises, usually with no mention of how the variation originates, beyond the notion that possessing a lens is a continuous extension of morphological variation. Finally, at the end of the morphological sequence, an eye with a fully formed complex lens is illustrated (Figure 1a).

It is important to think clearly about what such models can and cannot tell us. Darwin defended his model very well, writing:

In looking for the gradations by which an organ in any species has been perfected, we ought to look exclusively to its lineal ancestors; but this is scarcely ever possible, and we are forced in each case to look to species of the same group, that is to the collateral descendants from the same original parent-form, in order to see what gradations are possible, and for the chance of some gradations having been transmitted from the earlier stages of descent, in an unaltered or little altered condition.

In other words, Darwin was using living taxa as surrogates for unknown lineal ancestors. Darwin realized he was only testing a necessary requirement for natural selection to produce complex traits: intermediate forms of the trait must be functional and useful. Salvini-Plawen and Mayr were testing the same idea, using more closely related organisms. However, versions of Salvini-Plawen and Mayr’s gastropod eye sequence, copied later by other authors, have shoehorned additional data into a hardened gradual-linear theory. A common example of one of these extended mollusk eye series originated in Figure 3-1 (page 34) of Strickberger’s (1990) textbook Evolution. The figure uses four of the gastropods illustrated by Salvini-Plawen and Mayr, but adds two additional eyes, both from cephalopods: a nautilus and a squid (Fig. 1b). The nautilus and squid fit nicely into a gradual-linear series, intercalated between gastropods. Nautilus serves as a prime example of a pinhole eye, falling morphologically just between the pit eye of Pleurotomaria and the lens-eye of Nucella. The squid eye, often heralded as a highly complex eye, especially because of its uncanny – but convergent – similarity to vertebrate eyes, now takes the pinnacle position of the linear series. Apparently, these eyes were added to enhance the representation of intermediate stages of eye complexity. Alone, this is not a problem, as long as people remember that the goal of such collections of organs is to show that eyes of intermediate complexity are functional and useful.

Figure 1. Morphological series of eyes. A. Salvini-Plawen and Mayr (1977) illustrated five eyes of varying complexity, from different living species of gastropods. They made the implicit assumption that the eyes of living species approximate the ancestral states of a pectinate tree. This is illustrated by placing dashed lines leading to living species. B. An actual estimate of gastropod phylogeny (based on molecular data), indicates that the tree is not pectinate. Therefore, the actual history of gastropod eyes is more complicated than envisioned by a linear-gradual model of A. C. Two cephalopod eyes (*) were added to the gastropod eyes by Strickberger (1990). D. An estimate of mollusk phylogeny again shows how the actual history is more complicated that a direction march to complex eye. One interpretation is that lens eyes originated separately in gastropods and cephalopods.

However, gradual-linear series of eyes are often elevated to direct models of how evolution actually proceeded. Yet without explicit statistical phylogenetic analyses, these collections of eyes should not be taken as models of how evolution actually did happen in the groups being illustrated. Nevertheless, in some cases, the linear series are presented as actual accounts of evolution, even though they are only drawings of eyes from currently living species, unaccompanied by any phylogenetic analysis. Although the caption in Strickberger (2000) is much more balanced, the caption of Strickberger (1990) illustrates the point, calling the figure “Some stages in the evolution of eyes as found in mollusks…”. Ridley (2004) similarly, and somewhat more forcefully states “Stages in the evolution of the eye…”. The figure is again reproduced in Encyclopedia Britannica (Ayala 2008) and in Ayala (2007), with a caption stating “Steps in the evolution of eye complexity in living mollusks. The simplest eye is found in limpets (far left), consisting of only a few pigmented cells, slightly modified from typical epithelial (skin) cells…..The octopus eye (far right) is quite complex, with components similar to those of the human eye such as cornea, iris, refractive lens, and retina.”

There are at least two problems with viewing eyes of living species as an evolutionary model of how evolution actually proceeded. First, it promotes the fallacy of progress in evolution. The caption above is written as if the first mollusk eye is the same as a modern limpet, and that evolution progressed linearly to the human-like complex octopus eye. Second, a gradual-linear model ignores complexities like convergence. In particular, adding cephalopod eyes to a group of gastropod eyes ignores the phylogenetic relationships of the animals. When placed on a phylogentic tree of the animals, it becomes clear that the complex, lens eyes of gastropods and cephalopods probably originated separately (Fig. 1d)."


Francesco said...

I may have misread Dawkins' Ancestor's Tale but to me it read as a liner progessive reading of evolution (his non existant strawman of your quote), even if the direction of the narrative is flipped from cannonical direction.

Todd Oakley said...

Francesco - I can see your point. I liked the idea of Ancestor's Tale as a metaphor for understanding phylogeny. But the problem is that when the "bands" meet up at the ancestral nodes, none of the species has changed (if I remember correctly). Therefore, you are right. To a certain extent, living species are being equated with ancestors, and of course the story starts with humans, causing the (reversed) linear progression you mention.
But at least a branching pattern is conveyed, even if there is no concept of evolutionary change along the branches (if memory serves).

francesco said...

There are two things that made me unconfortable in Ancestor's Tale. The lack of any sort of detail given to the contents of the bands, usually just a list of names as opposed to much more detailed coverage of the Human to Last Commmon Ancestor line, gave me a feeling that the ghost of the great chain of being haunted the book. If a book is to be built with a structure of following a present day organism's lineage
back to the LCA why pick humans as that organism instead of something interesting like beetles or cichlids. Secondly the emphasis on nodes, which as Dawkins points out in the book nothing much actually happens as at the node the mother & daughter species are as similar it is possible for species to be, means that discussion of change within the lineage is minimised.