Homologous and Analogous Traits

Homologous Traits:

            If two species have homologous traits or structures, they are considered to share a common ancestor. Two species that have homologous structures are the frog and rabbit. Frogs are amphibians that live both on land and in fresh water. There exists thousands of different species of frogs, but all have similar anatomies which include lack of tail, long hind legs with webbed toes, large eyes, and smooth or warty skin. Rabbits are mammals that mainly live in North America along with some places in Europe, Southeast Asia, Africa and South America. Rabbits have long ears and large hind legs. Their paws lack pads, but have strong nails that are useful for digging.

            The forelimbs of the frog and rabbit are homologous structures. Frogs mainly use their forelimbs to balance themselves and for movement. Anatomically, it has about 15 different bones. It looks like an “L” with 4 fingers at the end. Like frogs, rabbits also use their forelimbs for balance and movement. Unlike frogs, they also use their forelimbs to grasp things. The forelimbs of both rabbits and frogs are shorter than their hind legs. Although their forelimbs are structurally similar, there are a few differences. Frogs have less space between the ulna and radius bones. The angle between the Humerus and Ulna and Radius is more obtuse than that of the rabbit’s. There exists these differences between the species because of the anatomy of the rest of their bodies. The wider angle allows the frog to balance at a wider stance.

            The common ancestor of frogs and rabbits is the Eusthenopteron which is a genus of lobe-finned fish that lived over 385 million years ago, but are now extinct. It is debated whether or not these fish lived strictly in the water or also spent time on land. Fossils from transitional species like these suggest similar structures in their fins to a humerus, ulna and radius. It is these similar structures that suggest that it is the common ancestor to both frogs and rabbits.

           

Analogous Traits:

            An analogous trait is the result of two species having similar environmental pressure that caused them to develop similar structures, rather than having a common ancestor that produced the similar structures. Two species that share an analogous trait are dolphins and sharks. Dolphins are aquatic mammals. Although they are found in oceans worldwide, they are descendants of land mammals. Sharks are fish that have over 465 different known species. The earliest known shark dates back over 420 million years ago. 

            The fins and flippers of dolphins and sharks are analogous structures. Even though these structures are anatomically different, they serve similar functions to each animal. Both the shark and dolphin's dorsal fin serves to balance the animal and keep it upright. The tail fin helps to propel them forward. The pectoral fins both help to balance them and help them turn.
            Although these structures are not a result of having a common ancestor that possessed these traits and passed them down, the common ancestor of the dolphin and shark which is an aquatic vertebrate most likely possessed a similar body shape, fin and flippers. Even though the common ancestor possessed these structures, the traits are still analogous because they were acquired due to environmental stressors rather than heredity. Because the dolphin evolved from a land mammal, it didn't acquire its fin and flippers until it entered the water.