Archive for: July, 2006

Why Spiders Aren't Insects IV: Molecular Homology

Photo by: Gerald Yuvailos


This is the fourth part in an ongoing series discussing the distinction and evolution of spiders and other arthropods.
Part I, part II and part III have led up to this point:

In the last post of this series, we established that spiders descended from marine arthropods called the eurypterids, distinct and separate from insects, appearing in the fossil record in the late Silurian/early Devonian, about 425 million years ago.

The cladogram we used to analyze the spider's history was based on the organism's morphological characteristics, that is, visible structures like chelicerae and book lungs that can be tied to other organisms that possess the same structures. Limulus (the extant horseshoe crab) has both of these structures and predates the spiders, placing them further back in the chelicerates' evolutionary history.

Homologous bones from human (I), dog (II), pig (III), cow (IV), tapir (V) and horse (VI):
r — Radius, u — Ulna, a — Scaphoid, b — Lunare, c — Triquetrum, d — Trapezium,
e — Trapezoid, f — Capitatum, g — Hamatum, p — Pisiforme

Paleontologists call this comparison of physical characteristics homology (coined by Richard Owen an anatomist and, ironically an opponent of Darwin). The mouth parts of a spider are homologous to the mouth parts of Limulus because of the cherlicae's exact form and function. This is a different designation than analogy; analogous structures may function in the same way, but they are different in form because of their different lineage. For this reason, scientists call analogy an artificial classification system.

A good example of analogous structures are wings from bat and bird. They perform the same function in varying degrees, but they have evolved very different forms. A bat's wing is basically a modified mammal hand, while the bird wing is a modified tetrapod arm.

Homology is essential in building an organism's phylogeny (evolutionary history). More recently, geneticists have employed this classification technique to analyze and find similarities among the less visible traits in life, RNA (ribonucleic acid) and the building blocks of proteins, amino acids.

Think of these cellular chemicals this way: If DNA is the blueprint of life, RNA is the builder and its materials are amino acids. When these amino acids are placed in the correct sequence by RNA, they become proteins, the framework of our body. And, since the genetic code for protein constructions is nearly universal*, geneticists can compare entire swaths of RNA from one organism to those of another and find homology at the molecular level.

Here's an example (sequences are greatly abbreviated for the sake of our sanity):

Organism 1: ACGC-CCCCC
Organism 2: ACGC-CCCUC
Organism 3: ACGU-CUCUC

Basically, from noting the differences in each RNA sequence, and determining the homologous sequences (such as the ACGU sequence above), a cladogram can be constructed that shows common ancestry without the murky distinctions that sometimes cloud the comparison of bones to bones, or mouth parts to mouth parts.

The problem with this molecular system of analysis is that it often provides vastly different cladograms than the ones crafted through morphological analysis. This is not necessarily the case between the spiders and and Limulus, the molecular evidence supports the fossil record's interpretation of ancestry, but it calls into question the descent of insects from chelicerates like spiders.

In short, the molecular evidence agrees with the morphological evidence: spiders are more closely related to horseshoe crabs than insects. But where and when did the insects arise?

Next time we'll tackle the more recent movements to elucidate the phylogeny of arthropods, including a discussion on the significance Hox genes and evolutionary-developmental biology (evo-devo).

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Why Spiders Aren't Insects III: The Evolutionary Advantages of Mass Extinction

So far we have established that spiders are distinct from insects for two reasons: physiology (mouth parts, body plan, respiratory structures) and more importantly, evolutionary history (or phylogeny, as scientists call it).

But where did spider's come from? How did they crawl out of the water as euryterids and speciate (become a distinct organism that cannot interbreed)?

The answer, like many in invertebrate paleontology, is cloudy. Organisms without hard, thick shells rarely become fossilized. In fact, for any organism's parts to become fossilized, even vertebrates, is a profound rarity, as Bill Bryson illustrates in A Short History of Nearly Everything:

Only about one bone in a billion, it is thought, ever becomes fossilized. If that is so, it means that the complete fossil legacy of all the Americans alive today - that's 270 million people with 206 bones each - will only be about fifty bones, one quarter of a complete skeleton.

Needless to say, invertebrate paleontologists are having a heck of a time piecing things together from such a paltry fossil record. But that doesn't mean there's no evidence.

According to morphological and geological evidence, and therefore directly observable comparison, spiders and their brethren descended from the eurypterids, many of which were sea-going creatures. The eurypterids arose in the Ordovician, a period that began with the decimation of perhaps 60% of all marine life, and consequently ended with another more devastating cataclysm, which which some paleontologists rank as the second most destructive extinction event in the history of the world (by extinction of family). This has become known, quite appropriately, as the end-Ordovician event.

Mass extinctions make room for the evolution of unique characteristics as dictated by an organism's environment, and the environment changed drastically for the eurypterids at the end of the Ordovician. Glaciers began to creep down from the upper latitude, as the greenhouse gas carbon dioxide was depleted from the atmosphere, reducing the Earth's ability to trap the sun's heat energy. As the glaciers encroached, sea levels dropped and global temperatures cooled. This rapid progression decimated habitats, and destroyed a species' equilibrium with its environment.

But the end-Ordovician event was comprised of two parts: glaciation and then a period melting, an interglacial. Temperatures warmed once more, glaciers melted, flooding the land, and raising sea levels once more. The world had completely lost almost 50 percent of the families of life, but the ancestors of the spiders had survived. The Silurian period had begun, and new ecological niches were available for exploitation, a habitat opportunity that eventually would produce the spider.

That's about how it stands from a morphological perspective. But more recently scientists have been delving into molecular evidence and crafting very different explanations of not only the rise of the spider, but the vast diversification of arthropods in general.

Next time we'll address the new cladograms produce by this molecular evidence, and what ramifications it might have in interpreting the diaspora of the most abundant creatures on the planet.

*Interestingly enough, we are in the middle of an interglacial right now, the Holocene. Much like the success of the spider, our current interglacial, which began about 16,000 years ago, may have contributed to the ultimate "success" of Homo sapiens.

Resources:

Pechenik, J. A. (2000). Biology of the Invertebrates. : McGraw Hill Companies.

Gradstein, Felix, James Ogg, and Alan Smith, eds., 2004. A Geologic Time Scale 2004 (Cambridge University Press)

Baez, J. (2005). Temperature. Retrieved July 18, 2006, from http://www.math.ucr.edu/home/baez/temperature/

Webby, Barry D. and Mary L. Droser, eds., 2004. The Great Ordovician Biodiversification Event

University of Bristol. (2004). Fossil chelicerates and evolution. Retrieved July 18, 2006, from http://palaeo.gly.bris.ac.uk/Palaeofiles/Fossilgroups/Chelicerata/fossils.html

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