There once was a man named Chuck

I decided to write a quick Darwin Limerick, inspired by the contest over at Dispersal of Darwin, and by the concepts of pluralistic Darwinism and common descent:

There once was a man from Down House
Who convinced me I'm cousin to a brown mouse
I'm glad as can be
That all life is a tree
Toe fungus to red grouse to crown louse

I'm still working on "There once was a man named Chuck"

Choose your own descent

I was interested to see that over at The Tree of Life Jonathan Eisen discussed a few different metaphors for common descent, which include:

The tree of life
The coral of life
The watershed of life
Blood vessels of life
Lungs of life
Shrub of life


I think metaphors are useful for teaching the idea of common descent and tree thinking, and so I try to think of new metaphors from time to time, since I teach a course on Macroevolution and also talk a lot about phylogenies in an Invertebrate Zoology course.

One that I think is really powerful, is found in Richard Dawkins' book Ancestor's Tale. The idea there is to reverse time in common descent, and imagine humans walking on a path to join our ancestors. First chimps join the band, later other apes, then other primates, other mammals, etc. This analogy is good for understanding the history of a particular entity, like the human species. However, since it has a specific starting point, it tends to re-enforce the way people conceive of evolution as a linear series of events. This is true if we have one specific endpoint in mind (like humans), but evolution does not have one specific endpoint.


So, this morning, this was going around in my head, and a new metaphor sprung into my brain: A choose your own adventure book. These books have a section of the story, and then the reader must choose between two (or more) different paths, each with a different outcome to the story.


Congratulations, your character has just achieved bilateral symmetry! If you would like to proceed down the path of protostomy, go to page 150. If you would like to proceed down the path of deuterostomy, go to page 376. It might be fun to write an actual evolution book this way!

Box Jellies and the Red Herring of Eye Evolution

Smoking a herring turns it red and imbues the fishy flesh with an extra strong odor, which can be used to throw bloodhounds off your trail, or to train hunting dogs to stay on the main scent trail, even if far less odoriferous. Such are a few hypotheses for the origin of the term "red herring", used to denote an enticing, yet ultimately uninformative beacon.

I've come to realize that my own field of eye evolution has a resoundingly pungent red herring that continually throws scientists and non-scientists alike off the trail of a deep understanding of how eyes, and more generally, photoreception, evolved. In this case, the rank fish is the simple and seemingly harmless question, "How many times did eyes evolve?"

Quite often, when I tell a newly acquainted colleague that I study eye evolution, he or she will pull the red herring out of their back pocket, and dash across the room, dragging the fish behind them. "So how many times did eyes evolve, anyway?” they ask, hoping I have a key to this deep mystery. When I was a younger bloodhound, I often took that bait, howling and bounding enthusiastically after that trail, circling the room again and again. But I've followed that trail enough times by now to know that it circles back on itself in a confusing and illogical way.

How many times eyes evolved, though enticingly simple, is an all but unanswerable question; at least in terms of a numerical answer that people crave. Eyes did not evolve one time, nor did they evolve forty to sixty-five times. The answer is that some components of animal eyes are shared in all animal eyes we've ever examined. Yet some components are new comers to organs that probably functioned as eyes even before those components joined the party. Asking how many times eyes evolved makes the implicit assumption that every component of that eye shares a congruent history, that an eye is either all there, or all not there. But eyes are complicated structures, and the evolutionary histories of its parts are varied.

A new paper by Kozmik et al in PNAS is an interesting piece of science that breaks new ground. Although it does not detract from the scientific value of their study, the authors have been beguiled by our ripe red herring. Still, they have elegantly streamlined the malodorous query in a way that will allow me to more easily explain the stench, and so I retell their work here. First, I’ll explain some of the new data then I’ll point out where I think they, like many others before them, lost the main scent trail.

The authors examined a box jellyfish, a group that includes the infamous sea wasp, the deadly nemesis of Australian beach-goers. Box jellies are cnidarians, the group including corals, anemones, and hydras, which besides sponges, may be the most distant animal relatives of humans. As such, the biology that is shared between humans and cnidarians likely originated very early in the history of animals, hundreds of millions of years ago. With this logic in mind, Kozmik et al focused on elucidating some of the components of the eyes of one particular box jelly that lives near Puerto Rico, a far less noxious variety than the Australian sea wasp. In particular, they elucidated some photoreceptor genes and some genes that produce a shading pigment in the box jelly’s eyes. Choosing these two particular components was no accident. They were chosen because they represent a common minimum definition of “an eye”. Some scientists argue that minimally, an eye must contain photoreceptor cells to register the presence of photons, and some type of shielding pigment to allow the owner to determine the direction of light. By focusing on these two components, Kozmik and colleagues were streamlining the question of whether eyes evolved more than once, asking whether we could find evidence that photoreceptor plus pigment joined forces multiple separate times, or whether their partnership dates back to a singular union.


Photoreceptor components

Simplifying the real biology a bit, there are two primary classes of phototransduction, which is the cellular process sensing a photon and firing a nervous signal. One pathway that I will call “r” was first elucidated in flies, the other that I will call “c” was first elucidated in vertebrates. It was once thought that the r-pathway is an invertebrate trait, and the c-pathway is a vertebrate trait. However, the last 5 years have brought good evidence that c and r are present in both vertebrates and invertebrates, although the c-pathway is a bit cryptic in invertebrates, and the r-pathway is a bit cryptic in vertebrates. Kozmik et al investigated three components of phototransduction, all three components showed some evidence of being more similar to the “c” class of phototransduction. This primary conclusion led Kozmik et al to choose their title “Assembly of the cnidarian camera-type eye from vertebrate-like components”.

The first component they studied was opsin, a protein that binds to a light reactive chemical to initiate light perception in animals. With opsin, they argued for “c” type phototransduction, as opposed to the “r” type of fly compound eyes. Opsins form multiple sub-families, and the box-jelly opsin might be within the c-opsin subfamily based on their phylogenetic analyses. However, the analysis presented in the supplement of this paper is actually rather inconclusive about c-type status, although it is definitely not an “r”-type. We did a quick reanalysis and found the box jelly opsin might actually belong to a cnidarian-specific opsin sub-family that we discovered last year and which we named “cnidops”. None of the cnidops genes were included in the Kozmik phylogenetic analysis.

Regardless of its sub-family type, this opsin was found to be expressed specifically in the box jelly eyes. One of the most interesting new pieces of science in this paper is that Kozmik et al were able to match the function of the protein itself with that of the jellyfish eye. To do this, they functionally activated box jelly opsin in cell culture and measured the protein’s function. Cell culture is the practice of growing isolated animal cells under controlled conditions, without an animal itself. Introducing the box jelly opsin protein to cells in culture allowed the protein to fold properly and bind to a light- reactive chemical that was separately introduced into the cells. By shining lights of particular wavelengths (“colors”) on to the cells, the authors could see which colors of light activated the box jelly opsin protein. As expected if the opsin in question is used in box jelly eyes, the authors found that the opsin is sensitive to similar wavelengths as the jellyfish eye itself (light we would call blue-green, of about 465 nm wavelength). The match between the animal's light response and that of the expressed opsin protein is a very good piece of evidence supporting the idea the opsin they studied is involved in the animal's light response. This is one of the first times that an invertebrate opsin has been studied in cell culture, and certainly the first Cnidarian opsin, so this is a very exciting experiment.

Evidence for other components of the “c”-pathway being used in box jelly eyes are preliminary. The authors did find that the mRNA of genes similar to vertebrate c-pathway genes were present in the box jelly eye. However, this was as far as the present study went with the non-opsin components. Namely, they did not yet investigate protein expression, nor did they yet conduct any biochemical experiments to demonstrate the interaction of opsin and the other phototransduction components they found. Granted, these additional experiments represent a lot of work. The authors also did not report any experiments to show that r-pathway or other similar genes are not expressed in box jelly eyes (but I note they did not mention the presence of these genes in their library of genes from the box jelly eye, so perhaps they really are absent from the library). In the end, it remains a bit of a leap of faith to conclude that the opsin is actually interacting with the other genes, reported only from mRNA. Nevertheless, the authors have taken some important steps in that direction, so for this essay, we can give the authors the benefit of the doubt that a c-pathway is interestingly being used in the box jellyfish eyes. Even if our hunch is true that it is actually our new “cnidops” class of phototransduction, it doesn’t change the main message here; the herring will remain red.

The second criterion, besides the presence of photoactive cells, required to satisfy a minimal definition of an eye, is the presence of one or more pigmented cells. Here again, Kozmik et al found that pigment-components of a box jelly eye are very similar to pigment-components of vertebrate eyes. Here, the data are quite conclusive. The authors found genes homologous to vertebrate melanin genes expressed in parts of the jellyfish eyes that contain abundant pigment. In addition, there is a direct chemical test for melanin, which box jelly eyes passed. Interestingly, box jelly cells have dual function as photoreceptor and pigment-bearing cell.

The conclusion
Based on these data, Kozmik and colleagues favor the conclusion of parallel evolution – the separate assembly of homologous (“the same”) components to form eyes. In other words, they argue that box jellies and vertebrates separately evolved eyes, but happened to use the same components in each case, opsin and melanin. This conclusion is a bit of a curveball given how I introduced the study above. The conclusion is that the jelly and vertebrate eyes share the same components, so why do they not conclude a common ancestry of the eyes? They actually do point out that this is an alternative possibility. However, they favor parallel evolution based on what boils down to inferring phylogenetic history of these components and of eyes more generally. Unfortunately, they only use verbal arguments, instead of the well established statistics of phylogenetics and ancestral state reconstruction, which could’ve been used to test their claims statistically (see this paper for examples).
The main verbal argument of Kozmik for parallelism, and against common ancestry, is that many animal phyla do not have eyes. As such, it would seem that eyes would have to be lost in many phyla, if common ancestry of box jelly and vertebrate eyes holds (this is where a real phylogenetic statistical test would be nice – undergraduate honor’s project anyone?). In addition, they point out that cnidarians and vertebrates utilize different transcription factor genes in the specification of eyes during development. So basically, they are assuming a priori, based on previous evidence, that the eyes are of “independent origin”, and so based on that assumption, it is surprising to find that they use the same components.


Back to the red herring

So, both vertebrates and box jelly eyes may use c-pathway photoreception and both use melanin as a shielding pigment. You might be wondering where I stand – for parallelism or for common ancestry? Arrrooofff. Bow, wow, wow. Did I catch you? Has your mind been bounding down that pungent scent trail, sniffing and howling enthusiastically? Or have you kept the main, fainter trail, taking my advice that this question of number of origins is misplaced? Perhaps some more explanation will help you understand my point, and wizen your inner bloodhound. Let’s follow the red herring trail and see where it leads.

First, in case it’s not already clear, I’ll point out that the question of parallelism versus common ancestry of box jelly and vertebrate eyes is asking about the number of eye origins. It is our red herring question. Parallelism implies two origins, two separate unions of photoreceptor and pigment. Common ancestry implies a single merger, at least for the two eyes in question.

The reason I don’t like the red herring question, is that it puts the focus on “the eye” rather than on the components that define the eye. The Kozmik et al study is about the components – they elucidated for the first time some of the molecular components of a box jelly eye. This is noteworthy. But what do we gain from saying that eyes originated more than once? I think very little, and in fact such a conclusion masks some of the interesting biology. For example, stating that “eyes originated more than once” fails to recognize the common ancestry of some of the components, notably opsin. Any animal eye that has been examined to date, in addition to photoreceptive cells not in eyes, utilizes a homologous gene family (opsin). This point gets lost when simply stating that eyes evolved more than once.

A related difficulty is that this perspective neglects an intermediate stage in the complexity of photoreceptive structures. Someone unfamiliar (say an anti-evolutionist) who reads “eyes evolved more than once”, imagines the genesis of complex eyes from nothing. But that is not how evolution works. Evolution uses building blocks used for other functions in new combinations. This is illustrated by the conclusion of parallelism of Kozmik et at. In their model, opsin-based phototransduction existed unassociated with pigment cells. Once phototransduction and pigments became associated, perhaps separately in the box jelly and vertebrate lineages, the organisms came to posses a minimum definition of “an eye”. But this definition is just a human construct. Again, to my mind it obscures the fact that much of “the eye” was already there, it just took a new association of components (photoreception plus pigment) to evolve an eye. [For the cynics arguing that this just pushes back the real origins, I recognize this, which is why my lab is studying the origins of phototransduction – by studying the evolutionary history of each of the components separately. The bottom line – phototransduction evolved by using existing and newly duplicated and diverged components, a common theme in evolution].

You may be asking what if the opposite answer of the red herring question holds. What if we conclude “eyes share a common ancestry”, which could be concluded based on the Kozmik et al data of shared opsin and pigment components in vertebrates and box jellies. Again, I would argue that we gain little by making this statement. In this case, such a statement can only be made for the components in question. Perhaps it can serve as a sort of “null hypothesis”, where it serves as a prediction for new data that we don’t yet know about, which has some value. However, we already know that for eyes, not all components have the same history, which is implied by the statement “eyes only evolved once”. We already know that different lens proteins are used in different types of eyes. We also already know that components of phototransduction cascades differ. For example c-phototransduction utilizes the ion channel protein CNG, whereas r-pathways utilize the unrelated ion channel protein TRP. Saying that all eyes have a common ancestry does not account for these details. Again, it is about the components. Different components have different histories, and this cannot be accounted for in the simple dichotomy of “one versus more than one origin”.

The question of the number of eye origins has been around for a while now. But I argue that it is time to move beyond this question, and to recognize it for the red herring that it is. The fact of the matter is that some eye components are shared and others are not, and this varies depending on the time scale and species examined. Of value is to fill in the narrative of this amazing evolutionary story. We now have the ability to understand to the level of specific molecular changes how vastly different types of eyes evolved. This is the main – though perhaps fainter – scent trail.

Coming to grips with common descent

Talk about a bump. Since PZ mentioned this blog, visits are through the roof. I'm not sure I really ever intended to blog for the masses; I was just using a blog as a motivator to write more often, and to collect ideas for a book that I am very slowly writing. But I feel a bit obliged to have something of substance, since so many intelligent people are spending time looking at these pages.

Since I am swamped finishing up our quarter, and preparing for a family vacation to Europe, I'll pull something off my hard drive, which is pretty good. Here is a little set up: Only the most irrational and fundamental creationists object to natural selection, or "microevolution". The real barrier to the acceptance of evolution is recognizing global common descent ("macroevolution"), and especially the inference that humans are not specially created, but a twig in the tree of life. At the same time, natural selection is often directly equated with Darwinism (see this post for example) and evolution itself. I'm very slowly working on a book that explores the implications of common descent (pattern) and the processes that produce it (speciation, gene duplication, and "duplication" of biological units in between genes and species, like gene networks and traits). This post is an excerpt from a draft of one chapter. I've edited it down a bit, but it's perhaps still a little long and technical for a blog post - but pharyngulites are a smart bunch, so I have faith. For what it's worth:



Coming to grips with common descent

One of the most profound insights in the history of human thought is Darwin’s proposal that all species are united by common descent. The idea transformed the view of humankind’s place in the natural world from a self-congratulatory perch atop the Scalae Naturae to an arbitrary outpost alongside other apes as cousins to every palm, gastrotrich, and canary (note the goal here was to rhyme names of organisms with "Tom Dick and Harry", the colloquialism meaning "everyone"; not sure if anyone ever gets that though). We are still coming to grips with the idea that each and every living thing is related through a common evolutionary heritage. Despite being accepted for about a century and a half, I argue that a truly intuitive grasp of common descent remains difficult. Nevertheless, coming to grips with common descent is of fundamental importance for achieving the central goal of evolutionary biology: to understand the origins of biological diversity. Most notably, I argue that a conspicuously missing topic in evolutionary theory is the explicit comparison of hierarchical patterns and processes of lineage splitting and common descent.

[For brevity, I deleted a part describing Mayr's "horizontal processes" - like speciation; and "vertical processes", like natural selection. These are equivalent to cladogenesis and anagenesis. This horizontal is not to be confused with horizontal gene transfer. I first point out that traits, like teeth vertebrae and eyes, can duplicate within a species by mutation. Therefore, these traits can form phylogenetic trees like gene families. This is the subject of the book, so it's only briefly introduced here.]

1. Common descent is non-intuitive

The most formidable obstacle to a fully intuitive grasp of common descent may be the sheer magnitude of differences that life outwardly expresses. An oak tree and a dung beetle are cousins!? Not to mention our more obscure relatives, the oxymonads, slime molds, and E. coli’s of the world! How can it be that such enormous differences in appearance, life style, size, and behavior have evolved? The answer is not difficult to understand logically. For example, to Darwin the immensity of biotic diversity could in large part be explained by time itself. Geological time is so vast as to be practically incomprehensible. Who then could logically argue with time’s ability to allow for the origin of an equally incomprehensible biological variety – even if starting from a single point? Many of the tell tale similarities expected of cousins would logically erode away after so many millions of generations. But logic quite often does not guide intuition. Such is the case with common descent. Even after learning crystal clear indicators, we continue to be surprised by common descent.

Perhaps the clearest signal of globally shared evolutionary heritage is the genetic code. On October 12, 1962, the headlines of the New York Times read “The Genetic Code is Held Universal: All Living Things Are Said to Use Same Chemicals for Heredity Transfers”[1]. More than a full century after Darwin’s “Origin”, common descent was again in the public eye. Who could help but marvel at the thought of a universal genetic code, in place nearly since the dawn of life? A code that remained constant through the origins of cells, of gastrulation, of brains and of societies. The few exceptions now known to the universal code have not diminished the significance. The conserved hereditary machinery provides a concrete thread linking every organism in the tree of life.

One might expect that the triumphant generality of the genetic code would have precluded any more surprises relating to common descent. After discovering a “universal code”, perhaps we finally could come to grips with the universal relatedness of living things. But more surprises were in store. The advent of molecular developmental genetics would again cause biologists to re-examine their expectations regarding common descent. This time the papers read, “From worms to cows, one class of genes spells out the blueprint”. “Class of genes” of course refers to the Hox genes, the conserved transcription factors involved in body patterning. Discoveries of other genes with highly conserved roles were to follow. Of the gene Pax-6, the newspapers read, “science outdoes fiction” when a human gene was shown to step in seamlessly for a defective fly gene or to produce eyes in unnatural places.

The lesson in this tripartite sketch of the history of biology is that at three levels of organization – species, genes, and developmental processes – biologists have been surprised to discover common descent. To me, this indicates a rather fundamental lack of intuition for the process. We seem to have to relearn the lessons of common descent at every level of biological organization. In each case, we start with the assumption – or perhaps with the notion (something less formal than an assumption) – that biological entities somehow arise de novo in a massively parallel manner. In Darwin’s time very few biologists considered common descent of species (exceptions include Buffon and Erasmus Darwin), and no one else neared the sweeping generalizations for the relatedness of most or all of life that Charles Darwin suggested. Instead, Darwin’s contemporaries and predecessors believed that each species had a separate origin. Some biologists broke the strictly typological thinking common of the time – to Lamark species transformed and to Cuvier they sometimes went extinct – but no one else had formed anything near a Darwinian concept of a global tree of life. Perhaps it is the strong intellectual roots in creationism, or perhaps it is the conceptual difficulty in linking together the entire enormity of biodiversity, but even long after Darwin’s ideas of common descent have been accepted for species, a biologist’s intuition often falls back on the notion of parallel origins when considering other levels of biological organization.

Current support among biologists for common descent of genes (see also this post) is strong– but this was not always so. The 1960’s and early 1970’s was the time when biologists were learning that common descent holds for genes and proteins as well as species. One telling quote, used recently by both Gould {2002} and Carroll {1999}, can be found in an influential book by Ernst Mayr, Animal Species and Evolution.

"Much that has been learned about gene physiology
makes it evident that the search for homologous
genes is quite futile except in very close relatives. If
there is only one efficient solution for a certain
functional demand, very different gene complexes
will come up with the same solution, no matter how
different the pathway by which it is achieved.”

Ernst Mayr (1963)

I do not think that this was a particularly radical viewpoint of the time. For example, I was struck when reading Walter Fitch’s citation classic defining the terms orthology (descent by speciation) and paralogy (descent by duplication), because almost the entire paper is concerned with a topic rather different from orthology vs. paralogy. Instead, Fitch lays out statistical methods for discriminating homologous from analogous proteins. In other words, a real concern of the time was that proteins might have evolved similar sequences in parallel. This is of course a worthy consideration and is a hypothesis that is tested thousands of times per day using similarity searches of genetic databases. Nevertheless, I find comparison with biologists of Darwin’s time extremely interesting. In both cases, massively parallel and de novo evolution was taken almost as the null expectation, no matter how unparsimonious. To Darwin’s contemporaries, species were created separately and similarly. To Fitch’s biochemist and especially to Neo-Darwinist contemporaries of the 1960’s (e.g. Mayr above), the very real possibility remained that genes were “created” separately and similarly in different species. I don’t mean to imply religious-type creationist leanings in the biologists of the 1960’s. On the contrary, their “creator” was strong (Darwinian) natural selection for a particular function. They imagined natural selection to be so powerful and dominant as to allow multiple re-inventions of genes, thereby erasing most of the historical component of genes. No matter the creative force behind the purported independent origins of species or genes, the point here is that deeply rooted common descent was a hard earned realization, not an immediately obvious intuition of biologists studying the problem.

In many ways, it was history repeating in the 1980’s and 1990’s when biologists began discovering deep homologies in body patterning genes like Hox genes. The prevailing view and intuition of the time, especially of morphologists and systematists, was that many organismal structures like segments, eyes, limbs, and hearts, evolved essentially de novo, multiple times independently in various lineages. Like the pre-Darwinian concept of species or the concept of genes in the days preceding the advent of molecular biology, the dogma was that organismal structures and the developmental processes leading to them often arose separately in different lineages – especially when comparing different phyla, which are considered to have different “body plans”. But the discovery of conserved developmental genetic processes for patterning the bodies of taxonomically different organisms forced biologists again to consider common descent at new levels of biological organization.

Eye evolution provides a canonical example. A comprehensive, and well-cited paper on photoreceptors by Salvini-Plawen and Mayr from 1977 surveyed the morphological types of eyes in all animals, concluding that photoreceptors must have evolved essentially de novo 40-65 times independently. But the 1990’s saw a chain of discoveries that showed many genes involved in eye development are homologous between phyla. That visual pigment genes (opsins) are conserved was known for some time. Instead, the watershed discovery in eye evolution was that similar mutant (disease) eye phenotypes in flies, mice and humans are caused by mutations in homologous genes {Quiring, 1994}. These genes are named Pax-6 in vertebrates and eyeless in flies. Multiple other phyla were subsequently shown to utilize homologous Pax-6 proteins during in eye development. Furthermore, multiple other genes besides Pax-6 and opsin were subsequently shown to have a conserved role in eye development across phyla. Similar stories can be told for limbs, hearts, and segments. Naturally, many biologists began to question the generality of the assumption that organs and especially developmental processes evolved multiple times independently. Just as biologists recognized deep common descent of species in the 1860’s and genes in the 1960’s, they began to recognize a real possibility for deep homology at levels of biological organization in between genes and species. These conserved developmental processes and the genes that code for them have been termed “the genetic toolkit for development”.

There are many possible reasons why common descent is hard to intuit, deep-seated intuitions about ladders of progress, and an overemphasis on natural selection seem to be especially important. Yet once we start thinking about genes, traits, and species in terms of common descent, we can be greatly enlightened about how evolution has produced the enormous complexity that we see every day.

---

[1] The Genetic Code is Held Universal. Robert K. Plumb New York Times Octocter 12, 1962.

Pluralistic Darwinism

Every year I ask the undergraduate students in my course EEMB 102-Macroevolution the same question. "What do you think of when I say 'Darwin'"?

Some answers are jokes, like "The Darwin Awards". One common answer, "Darwin's Finches", was unexpected to me. But given the ubiquity of these birds in textbooks, perhaps I should not be surprised. Of course the most common answer is "survival of the fittest".

The reason I ask every year is that I like to point out two things in the first lecture of the course. First, that Darwin was not the only one to think up natural selection. A.R. Wallace famously scooped Darwin, and they published their ideas in 1858 at the urging of Charles Lyell. Two others had actually published the idea of natural selection, long before Darwin and Wallace thought of the idea, but in obscure places. I like to cite this when arguing for science as deterministic - a process unto itself that does not depend much on the individuals who practice it.

The second reason I ask is to highlight that Darwinism is so much more than just natural selection - despite most people's tendency to equate the two. All of the above answers (Darwin Awards, Finches, and survival of the fittest) re-enforce the idea that people often equate Darwinism and natural selection. The Darwin Awards go to people who do stupid things, and thereby "improve the species...by accidentally removing themselves from it". The implicit idea is that removing people with genes coding for stupidity will improve the human species. Much research on Darwin's Finches is about documenting natural selection in the wild. And "survival of the fittest" is the most common buzz phrase referring to natural selection.

Never in my (admittedly small number of) years teaching the course has anyone mentioned any of the other Darwinian Theories (see Mayr's classification), besides natural selection. Why should this be, when global common descent is the most profound idea in all of biology? Global common descent provides explanatory power in biology. It is what scares anti-evolutionists the most, as the realization has transformed the view of humankind's place in the natural world from a self-congratulatory perch atop a Scalae Naturae, to an arbitrary outpost alongside other apes as cousins to every conceivable organism, from slime mold to germ.

It makes me wonder. Why is it that global common descent so often gets such a distant second billing to natural selection? Why do people so often equate Darwinism with natural selection, despite the fact that others conceived of natural selection (unlike global common descent, where as far as I know Darwin was the first) and despite the profound implications of common descent?

One idea is that humans - sometimes even practicing evolutionary biologists - have a difficult time coming to grips with global common descent. Perhaps a branching view of time is difficult to internalize compared to linear time. Perhaps it's all just a vestige of the great chain of being concept.

Questions and realizations about common descent are precisely what I want to contemplate in future posts.