Piltdown Blog Post Assignment

 

Analogy & Homology Blog Post Assignment

For your homologus traits provide the following information (25 pts):

1. Describe (not just identify) the two different species that possess the homologous trait. The point is to provide your reader (and grader) some background on the species in question so we can understand the selection pressures that produced these traits. Don’t short-change this.  This should be a paragraph of information, not just a few words and it will be graded as such.  (5 pts) The American lobster and blue crab are both Decapoda, Crustacea invertebrates. Both share the same class but live entirely different lives based on their habitat. Lobsters inhabit deeper ocean bottoms and seawater and rocky or muddy ocean bottoms where they excavate burrows, scavenge, and forage using large claws and powerful tails. Long, tube-like bodies have evolved for quick backward movement. Blue crabs live in shallower coastal systems such as estuaries and tidal flats. Blue crabs are broad and rounded, with a sideways moving adaptation. Their environments are competitive and speed-, agility-, and coordination-dependent with other crabs through visual cues. These habits and environments have led the traits that they inherited from the common ancestor to develop differently.
2. Describe the homologus trait of each species, focusing on the differences in structure and function of the trait. Why do these homologus traits exhibit differences between the two species?  Connect the differences in structure to differences in function to show how they are related.   Make sure your explanation is clear and complete. (10 pts) Both crabs and lobsters possess chelae or claws, which are homologous structures. The claws are modified versions of their front pair of legs and are made up of corresponding jointed components with exoskeleton coverings. The structure and function of the claws in both species are different due to divergent evolution. Lobsters have one massive crusher claw to crush the prey shells and a more slender cutter claw, which is used in cutting or clamping. They are powerful and specially adapted for such feeding on diverse prey and protection. Crabs have two more general and about the same-sized claws. They use them for digging sand, scavenging, defense of territory, and even for signaling other crabs. These are indicative of the manner in which pressures from the environment have influenced their anatomy: lobsters, which live in dark, isolated environments, are equipped with powerful, specialized claws, while crabs, which live in crowded coastlines, need versatile devices for survival and communication. Although the roles are unique, the inner structure and origin of the claws are clearly the same, making them homologous.
3. Who was (generally, not specifically) the common ancestor of these two species and how do you know that ancestor possessed this homologous trait? How does this support your claim of homology?  (5 pts) From what I could find, the crab and the lobster had a shared ancestor, a primitive decapod crustacean from the Mesozoic Era more than 200 million years ago. The ancestor would have had a hard exoskeleton, jointed legs, and clawed appendages close to the head. We can be confident that this ancient animal had claws since almost all extant decapods, ranging from shrimp to crayfish to crabs and lobsters, possess claws, and the fossil record shows ancient decapods already had clawed limbs. That contemporary claws have the same internal anatomy and are derived from the same body segment gives evidence that the character was due to common ancestry and not parallelism. Because both the lobsters and the crabs developed their claws from a common ancestor and because the claws developed by way of the same genetic mechanisms, we can be sure that the claws are homologous structures, although now differentiated in shape and function.
4. Provide an image of each species in this comparison. (5 pts)

 

 1. Describe the two different species that possess the analogous trait. This should meet the same amount of detail requested in the homologous section.  (5 pts)

The American eel is a limbless, long, thin freshwater ray-finned fish of the North American Atlantic coastline which lives in freshwater estuaries and rivers. It uses an undulating motion to push itself forward. American eels are famous for the unbelievable migrations they make between freshwater habitats within and the Sargasso Sea of the Atlantic Ocean, where they lay eggs. Their elongated body structure allows them to slide into rocks, sediment, and dense aquatic vegetation. The eastern rat snake, on the other hand, is a non-venomous reptile found in forests, grasslands, and farms in the eastern United States. Like all snakes, it lacks limbs, is cold-blooded, and moves by slithering in an S-pattern. Rat snakes are excellent climbers and use their body shape to move in narrow passages, under logs, and even trees. One is a fish, while the other is a reptile, but both are convergent in the sense that they have adapted to similar streamlined body shapes, which allow them to move in complex environments where flexibility and stealth are common.

2. Describe the analogous trait of each species, focusing on the similarities in structure and function of the trait. Clearly explain why these analogous traits exhibit similarities between the two species, tying in environmental similarities that drove the process of convergent evolution.  (10 pts) Both eels and snakes have evolved long, limbless, flexible bodies, the major analogous structure in these two animals. Both creatures use this ability to effectively navigate through tight, narrow spaces. In eels, this body plan helps them to swim easily in water using side-to-side undulations that can be used to escape predators and seize prey within tight underwater spaces such as between rocks or inside aquatic vegetation. For snakes, the elongated limbless body offers slithering along the ground, burrowing into soil, and climbing. This is achieved by powerful muscles and scales griping the surfaces. While their environments are diverse—one is aquatic, while the other is terrestrial—both provide the same obstacle: having to move silently and flexibly through obstacles or sites of camouflage. These similar environmental pressures were the cause of the evolution of similarly shaped bodies and modes of locomotion, even though the two animals are not closely related. This is the textbook definition of convergent evolution, where natural selection creates analogous solutions to analogous environmental problems in unrelated groups.
3. All pairs of organisms share some common ancestor if you go back far enough in time. Could the common ancestor of these two species have possessed this analogous trait?  How do we know these traits are analogous and not genetically related from common descent?  Use your knowledge of this ancestry and the evolutionary process to provide evidence that these traits do not share a genetic lineage and are therefore analogous.     (5 pts) Both eels and snakes are vertebrates but share a distant common ancestor who existed many hundreds of millions of years ago and likely was an early four-limbed fish-like vertebrate. The original creature would have had fins or limbs, not the fully limbless elongate body. Because the modern snake and eel each evolved their limbless, elongated body shape independently of each other after they diverged from this common ancestor, their similarity in the body does not result from a common ancestor. Instead, they evolved separately as an adaptation to the same set of functional requirements—stealth and agility in confined spaces. In addition, the covering of the body, muscular, and bone anatomy of these two organisms is quite dissimilar, and they use different systems in order to power their devices of locomotion (fins and eel tails as opposed to belly scales and snake muscles). All these variations establish that the comparable shape is the result of convergent evolution and not gene similarity and hence the character is analogous.
4. Provide an image of each species in this comparison. (5 pts)