One of the common principles used to characterize the rules of growth in developing systems is the idea of allometry. Here, I’ll briefly summarize the concept with a few clear illustrations and a tiny amount of simple math.
I can’t help it—everything I read only makes sense in the light of evolution. Here, for instance, is a story about the popularity of the AK-47 assault rifle:
The AK-47’s popularity is generally attributed to its functional characteristics; ease of operation, robustness to mistreatment and negligible failure rate. The weapon’s weaknesses — it is considerably less accurate, less safe for users, and has a smaller range than equivalently calibrated weapons — are usually overlooked, or considered to be less important than the benefits of its simplicity. But other assault rifles are approximately as simple to manage, yet they have not experienced the soaring popularity of the Kalashnikov.
The AK-47’s ubiquity could alternatively be explained as a result of a path dependent process. Economic historians recognize that an inferior product may persist when a small but early advantage becomes large over time and builds up a legacy that makes switching costly. In the case of the AK-47 that early advantage may be that as a Soviet invention it was not subject to patent and so could be freely copied.
“Path dependency”…hey, that’s another phrase for something I hammer on all the time, that you can only understand the full extent of evolution if you understand the developmental processes underlying it. Many sub-optimal solutions persist because they are part of a developmental framework that isn’t easily changed.
And speaking of suboptimal…there’s Microsoft Word, an ungainly monstrosity if ever there was one. Both Science and Nature have rejected the use of the latest version of MS Word, because it is non-standard and effectively broken.
Because of changes Microsoft has made in its recent Word release that are incompatible with our internal workflow, which was built around previous versions of the software, Science cannot at present accept any files in the new .docx format produced through Microsoft Word 2007, either for initial submission or for revision. Users of this release of Word should convert these files to a format compatible with Word 2003 or Word for Macintosh 2004 (or, for initial submission, to a PDF file) before submitting to Science.
There’s a “path dependency” for you, the ubiquity of Word. Even highly evolved, complex and otherwise necessary pathways can be replaced, though, if more effective alternative pathways acquire greater importance. If the target of selection is the production of a functional end product (a standard readable file in this case) and there are multiple paths for delivering that end product (doc or pdf), the acquisition and spread of a deleterious mutation in the dominant pathway can lead to greater importance of the alternate.
Hmm, I have to go home and start a pot of minestrone soup for dinner…somebody explain that process in evolutionary terms for me.
In chapter 14 of the Origin of Species, Darwin wondered about the whole process of metamorphosis. Some species undergo radical transformations from embryo to adult, passing through larval stages that are very different from the adult, while others proceed directly to the adult form. This process of metamorphosis is of great interest to both developmental and evolutionary biologists, because what we see are major transitions in form not over long periods of time, but within a single generation.
We are so much accustomed to see a difference in structure between
the embryo and the adult, that we are tempted to look at this
difference as in some necessary manner contingent on growth. But there
is no reason why, for instance, the wing of a bat, or the fin of a
porpoise, should not have been sketched out with all their parts in
proper proportion, as soon as any part became visible. In some whole
groups of animals and in certain members of other groups this is the
case, and the embryo does not at any period differ widely from the
adult: thus Owen has remarked in regard to cuttlefish, “There is no
metamorphosis; the cephalopodic character is manifested long before
the parts of the embryo are completed.” Landshells and fresh-water
crustaceans are born having their proper forms, whilst the marine
members of the same two great classes pass through considerable and
often great changes during their development. Spiders, again, barely
undergo any metamorphosis. The larvae of most insects pass through a
worm-like stage, whether they are active and adapted to diversified
habits, or are inactive from being placed in the midst of proper
nutriment or from being fed by their parents; but in some few cases,
as in that of Aphis, if we look to the admirable drawings of the
development of this insect, by Professor Huxley, we see hardly any
trace of the vermiform stage.
Why do some lineages undergo amazing processes of morphological change over their life histories, while others quickly settle on a single form and stick with it through their entire life? In some cases, we can even find closely related species where one goes through metamorphosis, and another doesn’t; this is clearly a relatively labile character in evolution. And one of the sharpest, clearest examples of this fascinating flexibility is found in the sea urchins.
Let’s encourage this trend of scientific societies coming out with unambiguous statements of support for good science. The latest addition is the Royal Astronomical Society of Canada-Ottawa Centre statement on evolution: it’s short and to the point.
The RASC Ottawa Centre supports high standards of scientific integrity, academic freedom and the free exchange of ideas. It also respects the scientific method and recognizes that the validity of any scientific model comes only as a result of rational hypotheses, sound experimentation, and findings that can be replicated by others.
The RASC Ottawa Centre, then, is unequivocal in its support of contemporary evolutionary theory that has its roots in the seminal work of Charles Darwin and has been refined by findings accumulated over 140 years.
Some dissenters from this position are proponents of non-scientific explanations of the nature of the universe. These may include “creation science”, “creationism”, “intelligent design” or other non-scientific “alternatives to evolution”. While we respect the dissenters’ right to express their views, these views are theirs alone and are in no way endorsed by the RASC Ottawa Centre. It is our collective position that these explanations do not meet the characteristics and rigour of scientific empiricism.
Therefore the science agenda of the RASC Ottawa Centre and its publications will not promote any non-scientific explanations of the nature of the universe.
We’ve often heard this claim from creationists: “there is no way for genetics to cause an increase in complexity without a designer!”. A recent example has been Michael Egnor’s obtuse caterwauling about it. We, including myself, usually respond in the same way: of course it can. And then we list examples of observations that support the obviously true conclusion that you can get increases in genetic information over time: we talk about gene duplication, gene families, pseudogenes, etc., all well-documented manifestations of natural processes that increase the genetic content of the organism. It happens, it’s clear and simple, get over it, creationists.
Maybe we’ve been missing the point all along, though. The premise of that question from the creationists is what they consider a self-evident fact: that evolution posits a steady increase in complexity from bacteria to Homo sapiens, the deep-rooted idea of the scala natura, a ladder of complexity from simple to complex. Their argument is that the ladder cannot be climbed, and our response is usually, “sure it can, watch!” when perhaps a better answer, one that is even more damaging to their ideology, is that there is no ladder to climb.
That’s a tougher answer to explain, though, and what makes it even more difficult is that there is a long scientific tradition of pretending the ladder is there. Larry Moran has an excellent article on this problem (Alex has a different perspective), and I want to expand on it a little more.
Here are three animals. If you had to classify them on the basis of this superficial glimpse, which two would you guess were most closely related to each other, and which one would be most distant from the others?
On the left is a urochordate, an ascidian, a sessile, filter-feeding blob that is anchored to rocks or pilings and sucks in sea water to extract microorganismal meals. In the middle is a cephalochordate, Amphioxus, also a filter feeder, but capable of free swimming. On the right are some fish larvae. All are members of the chordata, the deuterostomes with notochords. If you’d asked me some years ago, I would have said it’s obvious: vertebrates must be more closely related to the cephalochordates—they have such similar post-cranial anatomies—while the urochordates are the weirdos, the most distant cousins of the group. Recent developments in molecular phylogenies, though, strongly suggest that appearances are deceiving and we vertebrates are more closely related to the urochordates than to the cephalochordates, implying that some interesting evolutionary phenomena must have been going on in the urochordates. We’d expect to see some conservation of developmental mechanisms because of their common ancestry, but the radical reorganization of their morphology suggests that there ought also be some significant divergence at a deep level. That makes the urochordates a particularly interesting group to examine.
Hey, you want some science? My latest Seed column on battling beetle balls is online.
(And I’ve just arrived in Ann Arbor after a long travel day!)
Didn’t I just say “Woo hoo” yesterday? False alarm. Scarcely do I clear one set of major tasks away than another set rise up. I already mentioned that I was going to be the speaker at the Humanists of Minnesota banquet on Saturday evening. I neglected to tell you all that I’m leaving for the University of Michigan tomorrow to give the keynote at the Genetic Programming Theory and Practice Workshop.
I know virtually nothing about genetic programming, so this is a wonderful opportunity to learn something about it.
Since I’m certainly not going to be able to tell them a thing about genetic programming, I’m planning to tell them a little about my own skewed perspective as one of those metazoan-centric fans of developmental processes. I’m hoping they might learn a little something from me, and that we’ll all have some fun with ideas about embryos. Here’s my very brief abstract:
A developmental biologist’s view of evolution
The ongoing integration of molecular genetics, developmental biology, and evolution (the field of evo-devo) is stirring up new ideas and new questions. I will tell a few stories from the evo-devo literature that illustrate the importance of the principles of developmental plasticity and developmental constraint on evolutionary trajectories — showing that these are two competing and complementary forces operating on multicellular organisms. My argument is that the contingencies of developmental architectures may well be as significant a force on evolutionary histories as selection.
Next week I get to slack off. No, wait, there’s also…
My previous contributions to the basic concepts in science collection were on gastrulation and neurulation, so let’s add the next stage, and the one I named the blog after: the pharyngula.
Greg Laden puts an interesting twist on the question of how many hominin fossils we have: the question should be, “how did they die?”. We seem to have evolved from a species that was primarily a prey item on predators’ grocery list, to one that succumbed most often to disease, to one where mortality was driven by violence (and now, at least in our prosperous corner of the world, where senescence exacerbated by sloth and gluttony is the common cause of death.)
He’s right. The cool questions our students ought to be getting excited about have nothing to do with the nonsense the Discovery Institute wants them to discuss.
