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.
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.
The Discovery Institute is so relieved — they finally found a textbook that includes a reworked version of Haeckel’s figure. Casey Luskin is very excited. I’m a little disappointed, though: apparently, nobody at the Discovery Institute reads Pharyngula. I posted a quick summary in September of 2003 that went through several textbooks, and showed a couple of examples where redrawn versions of Haeckel’s diagram were used. More recently, I posted a fairly exhaustive survey by Patrick Frank of the use of that diagram since 1923, which showed that it was rare, and that the concept of recapitulation was uniformly criticized. Really, guys, the horse of recapitulationism is dead. Biologists riddled it with bullets in the 19th century, and have periodically kicked it a few times to be sure. For Intelligent Design creationists to show up over a century later and flog the crumbling bones of a long extinguished horse and crow victory is awfully silly.
So how can you still find any vestiges of Haeckel’s work in textbooks?
The Newsweek cover story is on recent efforts to create life in the laboratory, and of course they call this “playing God”. Haven’t they got the message yet? “Playing God” is where you do absolutely nothing, take credit for other entities’ work, and don’t even exist — scientists don’t aspire to such a useless status. Besides, creating life is mundane chemistry, no supernatural powers required.
There is a must-read article at Edge by Paul Bloom and Deena Skolnick Weisberg—it’s an attempt to explain why people resist scientific knowledge that takes a psychological view of the phenomenon. The premise is that our brains have in-built simplifications and assumptions about how the world works that often conflict with how it really works—there is, for instance, an intuitive physics and a real physics that are not entirely in agreement, and that we bring our understanding into alignment with reality through education and experience. The naive assumptions of the young brain contribute to ideas like dualism and creationism. For example:
Not mine—the weirder and more peculiar the food, the more likely I am to snarf it down—but those of certain other members of the Myers clan whose identities I will abstain from mentioning, lest they decide to add some really interesting ingredients to my next meal. Anyway, it’s an interesting study that explains why some people get queasy at the thought of food “touching”—it’s a common response to fear of contamination. It’s basically documenting the psychological reality of cooties.
Now if only he had provided an explanation for how to overcome it — the prohibition on mixing too many flavors in our meals is constraining the menu too much around here.
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.
Sad news: Stanley Miller died on Sunday. If you don’t know who he was, go read this interview.
(via Evolucionarios)
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.
Yesterday, the Wall Street Journal ran a piece on their opinion page (yeah, boo, crap so runny and putrid you can’t even get rid of it with a shovel, I know) that claims biologists have been actively inflating species numbers as a cheap ploy to gain better support for conservation efforts. I really can’t rebut that any better than Loren Coleman does here:
What is occurring is a classic theoretical battle between lumpers and splitters, not a fight of conservationists vs non-conservationists, not a war of Greens vs non-Greens, although “Species Inflation May Infect Over-Eager Conservationists” appears eager to convince you of that. The splitters are making their points lately, with more scientific evidence for a diversity of species.
“Species” are not like “dollars”—they are far more fluid, and to the biologist, there is no way to rank the value of a beetle against a monkey. This is a classic case of concepts in one discipline being applied in a grossly inappropriate manner to the concepts of another.
I’ll also add that, while it may not be true of some groups that focus on charismatic furry beasts to win popular support, among the scientists who do the work of taxonomy, I don’t see the individual species being used as the coinage of environmental conservation. I see much more focus on habitat preservation, which is really where the action is at if you want to save species.
Ah, but the WSJ editorial page is more likely the province of baraminologists than credible authorities.
