Analogy vs Homology

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

a. Briefly 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)

Whales and bats are two species that share the homologous trait of the presence of a forelimb. Previously whales resided on land and used their forelimbs for ground travel. At this point in their evolution. whales had small forelimbs since they were much smaller animals that could move freely on the ground. These early ancestors of the whale are believed to have lived a more aquatic lifestyle and were mainly found in the bay type areas between freshwater and the ocean. It is believed that these early species whales were able to drink both freshwater and saltwater. As the whale’s species continued to evolve they became more commonplace in primarily marine areas, and the ability to drink saltwater became more prevalent. On the other hand, bats are the only mammals capable of flying. Bats do not fossilize well so it has proven challenging to find definitive information on their evolution. It is believed that the ancient ancestors of the bat resided in trees and that they may have most likely had smaller forelimbs which allowed them to glide from tree to tree. Recent findings seem to point out that they most likely traveled in this manner during the day using visual flight cues, since it has been determined that their ability to fly preceded echolocation which guides the flight of current bats.

b. 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)

The presence of a forelimb, or a five-digit limb is a trait that is not only common for these two species, but in other mammals which leads to an assumption that all species with this trait share a common ancestor. The forelimb for both the whales and bats are comprised of a humerus, ulna and radius bones that end in the carpus that ultimately links to their five digits. The forelimbs of the whale now serve as flippers for swimming and navigating swiftly through the water. The flippers have increased in size over time to allow the whale to increase its mobility in pursuit of rapidly evolving prey and other predators. As the size of the whales have increased their flippers have undergone a corresponding increase in size to support the dispersion of the body weight of the whale. The whale can generate increased rate of speed within the water by faster movement of the flippers but is also able to use the flippers to make more precise, controlled navigation through the water. The forelimbs of the bat now serve as wings for flying in the air. The increase in the size of the wings over time facilitated the change from gliding to flying in bats. Through this process four very long bones formed that were supported by a small bone at the top of the wing which keeps the wing shape during flight. This uniquely structured wing keeps the bat in the air, allows the ability to travel through the air at a rapid rate and make very precise changes in direction while in the air.

c. 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)

The whale and bat share the common ancestor of a mammal that was a land-dwelling species, which had a similar tetrapod structure. This common ancestor most likely utilized its forelimbs for land travel based on its ancestral living habitat.  In addition to the similarities of the skeletal structure of the forelimbs between whales and bats, both species are also believed to have been land dwelling at one point in their evolution. Thus, it is presumable that the difference in function of their forelimbs is the result of adaptation resulting from the selective pressures experienced in their different environments. This point supports homology of the trait because the adaptations in the whales and bats reflect descent with modification from the common ancestor.

d. Provide an image of each species in this comparison.  (5 pts)

Bat

Whale

2. For your analogous traits provide the following information (25 pts):

a. 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)

Dolphins and sharks are two species that share the analogous trait of a tail fin. Dolphins are mammals with skeleton made of bones and their ancestors are believed to have been land dwelling at one point in their existence. During that time of their existence the species mainly lived near marshlands. Speciation resulted in more aquatic traits developing in the ancient dolphin species as over time they became more comfortable underwater and spending less time on land. This evolutionary trend was evident in traits developed by the species that allowed dolphins to hear, see and drink underwater while their bodies became able to keep the water of out of their airway. Sharks are believed to be the descendants of a much smaller fish species since they have always been water dwelling beings. The skeleton of sharks is comprised of cartilage instead of bone, and only their teeth are not made of this soft material. Over the course of time sharks evolved and their jaws became flexible allowing them the ability to eat much larger prey. Sharks have been one of the primary predators of the ocean through their existence, so their feeding habits led to increased size and the evolution of traits which have enhanced their speed, smell and vision.

b. 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 sharks and dolphins have tail fins that are used to help them swim and navigate through the water, which demonstrates that the tail fin trait is analogous in that it serves the same purpose for both species. In present times, both species are water dwelling beings thus they require the ability to navigate efficiently through the water for feeding and survival purposes.  The tail fins provide both species with unlimited movement possibilities within the water that allows them to adapt to any situation. Although the tail fins serve the same purpose for sharks and dolphins the appendages do have some distinct structural differences that make this trait analogous. The tails of dolphins and sharks have distinct differences in their positional orientation and in their primary movement pattern. Dolphins have wide horizontal tails that they move in and up and down motion to swim. On the other hand, sharks have tall vertical tails and move in a side to side motion. The movement pattern of the dolphin and sharks are tied to their skeletal structures. As mammals, the structure of the dolphin’s spine provides greater range of motion with the up and down movement. Whereas the shark as a fish have a notochord which requires side to side motion to work more efficiently with the muscle structure.

c. 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)

The evolutionary evidence that I have presented provides several clear indications that development of the tail fin in sharks and dolphins are the result of convergent evolution as opposed to any type of genetic change in a common ancestor.  I have pointed out that the skeletal structure between sharks and dolphins is different as evidenced by the presence of a cartilage skeleton in sharks which is consistent with fish ancestry vs a bone skeleton in dolphins which exist in mammals. I have noted that the dolphins ancestor was land dwelling at one point, whereas the sharks ancestor was always water dwelling. I have also noted that the movement patterns of the tail fin are consistent with the primary movements that are aligned with either a mammal (up and down, front to back) for dolphin and a fish (side to side) for the shark.

d. Provide an image of each species in this comparison.  (5 pts)

Shark

Dolphin