Homologous Trait: Forelimbs

The Dolphin

Dolphins are large mammals who are closely related to whales and porpoises. They can grow between 4-30 ft and can weigh between 90 lbs and 1 ton. Dolphins are found worldwide mostly in the shallower seas of the continental shelves and they are carnivorous mammals who eat fish and squid.  They are typically a grayish color although amazon river dolphins can be pink. Dolphins bodies are adapted for fast swimming. Their tail fin are used to propel them through the water while their pectoral fins are used to provide directional control. The dorsal fin of the dolphin provides stability while they swim. Dolphins are very vocal and playful animals and communicate through signature whistling noises. They have very few natural predators outside of specific sharks. 

The Cat

The domestic cat is a small, usually furry, and carnivorous domestic animal. Cats are often valued for their companionship and ability to hunt vermin. They were considered cult animals in ancient Egypt where they are thought to have first been domesticated. Cats have strong flexible bodies, quick reflexes, sharp retractable claws, and teeth adapted for killing small prey. Cats can hear sounds to high or too faint for humans to pick up on and can see in near darkness. Despite being hunters, cats are a very social species. Cats are similar to other members of the genus Felis weighing between 8-25 pounds. 

Homologous Trait and Common Ancestor: Both cats and dolphins share a similar bone structure in their forelimbs. Both of these animals have  humerus, ulna, radius, and wrist and finger bones. Although they share similar bones their functions are different for each animal. Cat forelimbs are used for mobility and pouncing and capturing prey while a dolphins flipper is used to propel it through the water and keep it stable while it swims. Both Cats and Dolphin most likely share a common mammalian ancestor, possibly of the Artiodactyl order.

Analogous Trait: Fins

The Penguin

Penguins are aquatic flightless birds who typically live in the southern hemisphere of the world. On average adult penguins are around 4 ft and weigh about 77lbs. These animals are typically black with white stomachs and are highly adapted for life in the water because their wings have evolved into flippers. Most penguins feed on fish, krill, squid, and other forms of sea life while swimming under water. They spend about half their lives in the water and half on land. Although all penguin species are native to the Southern hemisphere they are not found in only cold climates. In fact a certain breed of penguin, the Galapagos Penguin is found near the equator.

The Fish

A fish is any member of a paraphyletic group of organisms that consist of gill bearing animals that lack limbs with digits. Most fish are cold blooded animals and are abundant in most bodies of water, they can be found in nearly every aquatic environment. The earliest organisms to be classified as fish were found in the Cambrian period. 

Analogous Traits: The analogous trait shared between penguins and fish are fins. A fin is a flattened appendage on various parts of the body of many aquatic vertebrates and some invertebrates used for propelling, steering, and balancing. Despite the fact that both of these animals share a similar trait I do not believe that their common ancestor shared this trait and since the penguin is a bird and the fish is a fish it is likely that the fin evolved in the penguin species because it was needed in the environment penguins were living in at the time.