Evolution Along the Branches of the Tree of Life
Our conclusions about phylogenetic relationships rely on what Darwin called “descent with modification.” As time passes, the traits of species undergo evolutionary change along the branches of the phylogenetic tree. We use these changes to understand how species are related. All else being equal, species that share a trait that evolved within a lineage will be more closely related to one another than they are to species that lack this derived trait.
Going Back In Time
We can also use knowledge of phylogenetic relationships to work backward to infer where in the tree various evolutionary changes occurred. Knowing the relationships and the traits of the species at the tips of the tree, we can reconstruct what ancestral species looked like.
For example, the tree to the right implies that the ancestor of species B and C probably had the evolutionarily derived character (represented by black boxes) shared by B and C today. That is, the change to this derived condition (represented by the short black bar) probably occurred along the branch just below the node (at Time 2) that connects B and C. The ancestor of species A, B, and C, had the ancestral character (represented by the white box).
We can make such inferences one at a time (character by character), or we can consider entire suites of characteristics, which provide us with a unique perspective on the evolution of a lineage through time.
Click here to view an animation of morphing arachnids!
The animation linked above shows how this technique of ancestral character state reconstruction is used to infer the evolution of the shape, size and number of body parts of the arachnids (the branch of the Tree of Life that includes spiders, scorpions, mites and ticks) and their relatives such as the horseshoe crabs. To see how their different body forms evolved, we can trace up the branches from an ancestor at the base to any tip of the tree and watch the evolutionary changes that are reconstructed along the way.
Notice that the ancestor of two descendants seldom looks like either descendant, because change usually happens along both paths. In fact, phylogenetic trees show that it is misguided to say, for example, that “humans evolved from chimps;” rather, both humans and chimpanzees evolved from a common ancestor that differed from both.
The common ancestor of all arachnids probably resembled the now extinct eurypterids or sea scorpions, fearsome aquatic predators that lived some 250 to 500 million years ago.
While arachnids share basic body parts, these have diverged in fascinating ways.
The chelicerae in spiders, for example, are fangs that evolved the ability to inject venom; in pseudoscorpions they evolved to produce silk; and in solpugids they became massive jaws that rip apart prey.
In scorpions, the rear of the abdomen evolved into a poisonous stinger; in spiders, into a silk-producing organ for making webs; and in whip scorpions, a feeler for targeting a cannon that fires nasty chemicals at its enemies.
Giant Whip Scorpion
Also known as uropygids or vinegaroons, whip scorpions dig burrows in the moist soil of tropical and subtropical areas in the New World and Asia. Whip scorpions have no venom, but when disturbed spray a noxious chemical that smells like vinegar. They leave their burrows at night to feed on insects, spiders, and other invertebrates. The giant whip scorpion (Mastigoproctus giganteus) is one of the largest species, and is found in southwestern North America.
Flat Rock Scorpion
The flat rock scorpion (Hadogenes troglodytes) comes from dry, rocky habitats in South Africa. Like other scorpions, its stinger is venomous, but this species relies more on its powerful pincers for hunting and defense. The bodies of these scorpions are adapted to fit in narrow rocky crevices, where they capture and eat snails and other invertebrates.